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Fusion of the mandibular symphysis and cranial evolution in mammals

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Title:
Fusion of the mandibular symphysis and cranial evolution in mammals
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Hogard, Rachel A
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Denver, CO
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University of Colorado Denver
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94 leaves : illustrations ; 28 cm

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Subjects / Keywords:
Jaws ( lcsh )
Mastication ( lcsh )
Mammals -- Morphology ( lcsh )
Jaws ( fast )
Mammals -- Morphology ( fast )
Mastication ( fast )
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bibliography ( marcgt )
theses ( marcgt )
non-fiction ( marcgt )

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Includes bibliographical references (leaves 90-94).
Thesis:
Anthropology
General Note:
Department of Anthropology
Statement of Responsibility:
by Rachel . Hogard.

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Full Text
FUSION OF THE MANDIBULAR SYMPHYSIS
AND CRANIAL EVOLUTION IN MAMMALS
by
Rachel A. Hogard
B.A., Washington University in St. Louis, 1999
A thesis submitted to the
University of Colorado at Denver
in partial fulfillment
of the requirements for the degree of
Master of Arts
Anthropology
2003


This thesis for the Master of Arts
degree by
Rachel A. Hogard
has been approved
by
iX>W 21,2^33
Date
David Tracer


Hogard, Rachel A. (M.A, Anthropology)
Fusion of the Mandibular Symphysis and Cranial Evolution in Mammals
Thesis directed by Assistant Professor Mark A. Spencer
ABSTRACT
Hypotheses regarding masticatory force production and the distribution of
craniofacial variables are based on biomechanical models. One such model, the
constrained model, has been particularly influential within the field of jaw
biomechanics. This model is tested on a wide range of mammals, both with and
without mandibular symphyseal fusion, to assess systematic differences in
craniofacial configuration that may exist between these two groups. These groups
are expected to differ in systematic ways due to differing muscle recruitment
patterns. Mammals with an unfused symphysis recruit less balancing-side muscle
force. This has important implications for the distribution of craniofacial variables
within this model. Of particular interest is the distribution of the molars. The molars
are expected to fit into the area of the cranium where highest magnitude bite forces
can be produced. Results indicate that for both groups of mammals, the molars are
indeed maintained within that region However, there are other configurations of the
masticatory system that appear to differ between mammals with an unfused
symphysis and those with symphyseal fusion. Therefore, these two groups do appear
to differ in systematic ways. This has important implications for the evolution of
symphseal fusion and the role fusion may play in dietary adaptation.
This abstract accurately represents the content of the candidates thesis. I
recommend its publication.
Signed
Mark A. Spencer
111


DEDICATION
I dedicate this thesis to my mother and father for their unfaltering support while I
attended graduate school. I also dedicate this thesis to Karl Kesti for providing me
with generous understanding and support throughout this process.


ACKNOWLEDGEMENT
My thanks and gratitude to my advisor, Mark Spencer, for his help and dedication to
me during this process. I would also like to thank Cheri Jones of the Denver
Museum of Nature and Science and Rosanne Humphrey of the University of
Colorado at Boulder Natural History Museum.


CONTENTS
Figures ...............................................viii
Tables ...................................................x
CHAPTER
1. INTRODUCTION..........................................1
2. BIOMECHANICAL MODELS OF THE
MASTICATORY SYSTEM....................................7
The Mandible as a Lever...........................7
The Constrained Model............................10
Modifications to the Constrained Model...........19
Muscle Force Recruitment.........................25
Why Fuse the Mandibular Symphysis?...............29
Stiffness Hypothesis.........................30
Strength Hypothesis..........................33
3. HYPOTHESES AND STUDY DESIGN..............................37
The Effects of an Unfused Symphysis on the Model.....39
Specific Predictions.................................42
Study Design.........................................42
Tests ...............................................43
4. MATERIALS AND METHODS....................................45
vi


Sample .................................................45
Data Collection Process.................................48
Measurements............................................49
Size ...................................................52
5. RESULTS .....................................................56
Fused vs. Unfused.......................................70
Allometric Analysis.....................................70
6. DISCUSSION...................................................78
Support for Previous Research...........................79
Allometric Analysis.....................................81
Biomechanical Implications for Diet.....................84
Conclusion..............................................88
LITERATURE CITED................................................90
vii


FIGURES
Figure
1.1 Phylogenetic hypothesis concerning the relationships between the
orders within eutherian mammals.......................................4
2.1 Occlusal view of the mandible with a midline muscle resultant force....13
2.2 For more posterior bite points, the triangle of support may not
envelope a midline muscle resultant force producing tensile forces
in the working-side joint.............................................14
2.3 Occlusal view of the mandible showing the distributions of Regions
I, II, and HI for a mammal with a fused mandibular symphysis..........16
2.4 Predicted bite force and joint reaction force values using the
assumptions of the constrained model..................................18
2.5 Occlusal view of the mandible demonstrating how bicondylar
breadth affects the length of Region II...............................23
2.6 An additional variable that alters Region II length is the antero-
posterior position of the midline muscle resultant force...............24
2.7 Occlusal view of mandible showing how palatal breadth affects
the length of Region II...............................................26
3.1 Occlusal views of mandible showing the effects of changes in the
position of the muscle resultant force on the distribution of Region II 40
4.1 Illustration of five variables used in the calculation of Region II length ..51
4.2 Equation for estimating the predicted length of Region II...............53
5.1 Plot comparing postcanine dimensions to predicted Region II length.....69
viii


5.2 Box plot representing the means of molar length divided by the
geometric mean for mammals with a fused and unfused symphysis.........71
5.3 Bivariate plots of log-transformed data showing the relationship
between cranial size and other masticatory variables relevant to
the distribution of Region II.........................................73
IX


TABLES
Table
4.1 Taxa indicated in study with a fused symphysis..........................46
4.2 Taxa indicated in study with an unfused symphysis.......................47
5.1 Mean and standard deviation of tooth row length........................57
5.2 Mean and standard deviation of variables used to calculate
predicted Region II length..............................................60
5.3 Mean and standard deviation of the geometric mean and one
variable used to calculate it...........................................65
5.4 Variables regressed on the geometric mean to assess allometic
relationships ..........................................................76
x


CHAPTER 1
INTRODUCTION
Diet plays an influential role in almost all aspects of an organisms life.
The selection pressures associated with the acquisition and processing of food
have an influence on many aspects of a species anatomy and behavior (Cartmill,
1975; Clutton-Brock, 1974; Hylander, 1979a; Hiiemae, 1984; Fleagle, 1988;
Smith, 1993). Selection pressures range from the processes involved in finding
food to being able to efficiently process this food once ingested. These pressures
can change as a result of a changing environment. The ability of a species to
adapt to these changing selection pressures is essential to its survival. The study
of the masticatory system is therefore crucial to our understanding of the
evolutionary relationship between diet and morphology as it is this structure that
enables food acquisition and processing. This is because the masticatory system,
including the jaws, teeth, and craniofacial muscles, is a direct link between an
animals external environment and its internal requirements.
Previous studies have established that features of masticatory morphology
are functionally correlated with dietary pattern (Kay, 1975; Kay and Covert,
1984; Lucas et. al., 1986; Fleagle, 1988). This association between morphology
1


and diet in extant species is often used to guide our explanations of the fossil
record (Hiiemae, 1984). Therefore, studying this system can lead to a greater
understanding of the functional role morphology plays in the life of the animal
and the evolution of a species.
Forces are generated within the masticatory system to break down food
objects. Cranial configuration (i.e., the interaction between morphological
variables of the cranium) is known to influence force production (Greaves, 1978;
Smith, 1978; Spencer, 1998,1999).
Hypotheses regarding force production and the factors that relate to this
are based on biomechanical models (Roberts and Tattersall, 1974; Greaves, 1978;
Smith 1978; Spencer, 1999). From these biomechanical models functional
predictions are generated about ways in which the masticatory system should be
configured. Testing these predictions helps to increase our understanding of
morphological design and change over time.
While biomechanical models are needed to understand masticatory system
function and evolution, we frequently do not know how widely applicable they
are. It is important, therefore, to look at a wide range of mammalian taxa in order
to understand the broad applicability of these models to the masticatory system.
Mammals, in general, and primates in particular, differ drastically in the
morphological structure and configuration of their masticatory system. One such
structure that has marked effects on this system is the mandibular symphysis. The
2


mandibular symphysis is the structure that separates the two dentaries in the
sagittal plane. An unfused mandibular symphysis, in which the two dentaries are
joined by non-ossified connective tissue, is the ancestral condition among
mammals. However, symphyseal fusion has evolved independently in several
mammalian lineages (Greaves, 1988). Within primates, extant prosimians retain
the primitive condition of having an unfused mandibular symphysis, though they
vary in symphyseal morphology, while anthropoid primates have evolved
symphyseal fusion (Beecher, 1977; Hylander, 1979b; Ravosa, 1991). Other
groups of mammals that have evolved the derived condition of fusion include
perissodactyls, hyracoids, families of chiropterans and several artiodactyl taxa
(Suidae, Tayassuidae, Hippoptomidae, and Camelidae) (Beecher, 1977). Figure
1.1 outlines the relationships between the orders within eutherian mammals. This
phylogenetic hypothesis shows that fusion has evolved multiple times within the
order Mammalia in several different lineages (Allard et. al., 1996). Some orders
that contain families with symphyseal fusion also include families without fusion.
The repeated evolution of symphyseal fusion within mammals has
generated substantial interest concerning the adaptive significance (and
biomechanical consequences) of this feature (Beecher, 1979; Hylander, 1979a,
1979b, 1984; Scapino, 1981; Greaves, 1988, 1993; Ravosa, 1991, 1993; Hylander
and Johnson, 1994; Ravosa and Hylander, 1994; Hylander et. al., 1998; Ravosa
et. al., 2000). The biomechanical consequences of symphyseal fusion within the
3


6
41
I uerm
i Chir
k
-e:
i- c
k:
Lagomorpha
Rodantla
Macroscelidea
Artiodactyla #
Cetacea
Perissodactyla *
Hyracoidea .
Sirenia
Proboscides
Dermoptera
optera #
Primates *
Scandenti a
Edentata *
Pholidota
Carnivora *
Tublidentata
nsectivora
Metatheria *
Monotremata
Outgroup
Fig. 1.1 Phylogenetic hypothesis concerning the relationship between the orders
within eutherian mammals. .This strict consensus tree was obtained from 88
morphological characters from fossils and recent evidence (Allard et, al., 1996).
* indicates orders containing some families with symphyseal fusion.
4
\


various mammalian taxa can help provide a greater understanding of the adaptive
significance of this feature.
Experimental data indicate that there are differences in masticatory muscle
recruitment patterns between animals with a fused and unfused symphysis.
Regardless of how symphyseal fusion leads to these differences, they have
important theoretical implications because it is unknown how these differences
may affect other aspects of masticatory function or whether they lead to
systematic differences in overall cranial morphology.
The goal of this study is to test predictions regarding differences in
masticatory system configuration and how this differs between mammals with a
fused and unfused symphysis. These groups are known to differ in systematic
ways in masticatory system morphology (Beecher, 1977; Hiiemae, 1984; Greaves,
1988; Spencer, 1999). An analysis of the functional significance of these
differences should improve our understanding of the adaptive basis of cranial
form in primates, including humans.
Adaptive hypotheses must be tested using the comparative method
(Clutton-Brock and Harvey, 1979). In this method, features are inferred to be
associated if they evolved together repeatedly in several independent lineages.
An understanding of the relationship between symphyseal fusion and masticatory
system configuration must be based, therefore, on identifying commonalities
among multiple lineages that have evolved symphyseal fusion. This study
5


quantifies cranial form in a wide range of mammals with fused and unfused
mandibular symphyses in order to assess the biomechanical consequences of
symphyseal fusion.
6


CHAPTER 2
BIOMECHANICAL MODELS OF THE
MASTICATORY SYSTEM
The functional significance of masticatory system morphology has long
been of great interest within the field of biological anthropology. Descriptions of
this system often rely on simplified models. The earliest studies (Gysi, 1921,
Maynard-Smith and Savage, 1959) argued that the mandible acts as a lever. This
model was challenged in the early 1970s (e.g., Roberts and Tattersall, 1974) but
is now the most generally accepted biomechanical analogy for the mandible
(Hylander, 1975;DuBrul, 1977; Greaves, 1978; Smith, 1978). Other models,
such as the constrained model, have expanded upon the principles of the lever
model to include more complex factors.
The Mandible as a Lever
The mammalian masticatory system has historically been modeled as a
lever (Gysi, 1921; Maynard-Smith and Savage, 1959; DuBrul, 1977; Greaves,
1978,1982, 1988; Hylander, 1975, 1977; Smith, 1978, Demes and Creel, 1988).
In this model, the skull is analyzed in the sagittal plane. The condyle acts as the
fulcrum, the masticatory muscles are the applied force, and the bite point is the
7


resistance. The masticatory muscles apply an adducting force to the mandible,
which serves to close the jaw. This is resisted by reaction forces at the
temporomandibular joints (TMJ) and the bite point. The muscles responsible for
closing the jaw are the masseter, medial pterygoid, and temporalis. All of the
muscle, TMJ, and bite forces are typically simplified into individual force vectors
(Spencer, 1995). The vector representing the sum of the individual muscle force
vectors from both sides of the head is termed the muscle resultant force (MRF).
During the 1950s and 1960s this model was generally well accepted
(Maynard-Smith and Savage, 1959) and was used in evolutionary interpretations
of cranial form in various taxa. However, some argued that this is a mechanically
inefficient system and proposed alternative models (Gingerich, 1971; Roberts and
Tattersall, 1974). Those who opposed the lever model assumed that the process
of adaptation should result in greater efficiency of the masticatory system than
they believed the lever model allowed. Smith (1978) points out, however, that the
process of evolution is not necessarily a process of optimal design, rather, it
probably results in species that are just a little better than their competitors.
The alternative models centered around the idea that during biting, the jaw
functions as a link between the adductor muscle force and the bite force
(Gingerich, 1971). The argument that the mandible does not function as a lever
and instead acts as a link was based on two assertions: (1) the resultant of the
forces produced by the masticatory muscles (i.e., the muscle resultant force)
8


always passes through the bite point; (2) the condylar neck and/or TMJ is poorly
suited to withstand reaction forces.
These two assertions have since been disproved. The muscle resultant
force (MRF) does not always pass through the bite point (Greaves, 1978). It has
been proposed that the MRF actually lies in the midline between the last molars
during equal working side and balancing side muscle activity and that differential
activity between the muscles produces mediolateral movement of the MRF
(Hylander, 1985; Spencer, 1998).
Also in opposition to the link models, electromyographic data have
demonstrated that large condylar reaction forces do exist and that the condyle is
strong enough to withstand reaction forces during lever action (Hylander, 1975).
Early work within the field of biomechanics suggested that the strongest
force is exerted at the balancing condyle (i.e., the TMJ on the side opposite from
the bite point) (Gysi, 1921). It was theorized that the balancing-side muscle force,
which gets transmitted through the mandibular symphysis to the working side,
reduces the force at the working-side condyle. Modelling data showed a
reduction in working-side joint reaction force as the bite point moved distally
along the tooth row (Gysi, 1921). When food is crushed at the premolars, the
force at the working-side condyle is slightly reduced compared to more anterior
bite points. The working-side joint reaction force gets neutralized as the bite
point moves distally to the second molar. It was also hypothesized that hard
9


foods cannot be crushed on the third molars because the downward pull of the
working-side TMJ could lead to joint distraction. Later studies have indeed
confirmed that condylar forces are greater on the balancing side than the working
side (Hylander, 1979a).
This work is particularly important to later research for two reasons. First,
it supports the hypothesis that the mandible can be modeled as a lever. Although
alternative models have been offered to explain the biomechanical actions of the
mandible, the lever is still upheld as the dominant model today. Second, it shows
that balancing-side muscles contribute to the muscle resultant force, which gets
transmitted through the symphysis to the working side. These two points have
had important implications in the development of biomechanical models of the
masticatory system and explaining the functional significance of the mandibular
symphysis.
The Constrained Model
Walter Greaves constrained model of the jaw lever system of ungulates
has been particularly influential in the field of jaw biomechanics. This model was
developed based on the view that in Homo muscle forces on both sides of the
head are transmitted to the working tooth row and that there are reaction forces at
both TMJs (Greaves 1978). This model is applied, however, to selenodont
artiodactyls, which differ drastically in their mandibular morphologies. Homo, as
10


an anthropoid primate, has a fused mandibular symphysis. Selenodont
artiodactyls, with the exception of the camelids, have an unfused mandibular
symphysis (Greaves, 1978). Although Greaves recognized this, the model is
based on an assumption that applies only to those mammals with a fused
symphysis. It was not known at the time that differences in muscle recruitment
patterns between fused and unfused species could have an effect on the model,
which will be discussed later.
In Greaves model, masticatory forces are examined in the occlusal view
(Fig. 2.1). During mastication the mandible is pulled toward the skull by the
adductor muscles, the masseter, temporalis, and medial pterygoid. These muscle
forces are resisted by reaction forces at three regions of the cranium: the bite
point, the working side TMJ, and the balancing side TMJ (Greaves, 1978). It is
these three points that form what has been termed the triangle of support.
Because there are multiple muscles on both sides of the head working to adduct
the mandible, these muscle forces can be combined into a single vector termed the
muscle resultant force (MRF) through simple vector addition as is the case with
the lever model.
Greaves argued that distraction of the temporomandibular joint (i.e., forces
that can separate the mandibular condyle from the articular eminence) could lead
to potentially serious injury. He therefore proposed that natural selection should
favor a morphology that limits TMJ distraction. This is the fundamental
11


assumption of the constrained model and is predicted to cause limitations on the
evolution of masticatory morphology in mammals. There should be limitations
on masticatory morphology because jaw distraction is avoided only if the MRF
lies within the triangle of support (Fig. 2.1) (Adapted from Spencer, 1995)
(Greaves, 1978).
The mediolateral location of the MRF is determined by the positions and
relative force contributions of the adductor muscles from both the working and
balancing sides (Spencer, 1999). The muscle resultant will lie in the midline
when the balancing and working side muscles are equally active. However, this is
not always the case. Differential activity between these muscles produces
mediolateral movement of the MRF (Hylander, 1985; Spencer, 1998).
The MRF will also lie at different positions relative to the triangle of
support depending on the bite point. Biting on more anterior teeth creates a
relatively large triangle of support that will enclose a midline MRF (see Fig. 2.1).
However, during biting on more posterior teeth, the triangle of support is smaller
and gets shifted laterally toward the working side (Spencer, 1999). This smaller
triangle may not encompass a midline MRF. If the midline MRF falls outside of
the triangle of support, the mandible could potentially rotate around the bite point
and the balancing-side joint causing distraction of the working-side mandibular
condyle (Fig. 2.2).
12


Balancing Side
Joint Reaction Force
Working Side
Joint Reaction Force
Fig. 2.1 Occlusal view of the mandible with a midline muscle resultant force (B).
During biting on more anterior teeth, this midline muscle resultant force passes
through the triangle of support (shaded zone). The comers of this triangle are
positioned at the bite force (F), the balancing-side joint reaction force (J), and the
working-side joint reaction force (O).
13


Balancing Side Working Side
Joint Reaction Force Joint Reaction Force
Fig. 2.2 For more posterior bite points, the triangle of support may not envelope a
midline muscle resultant force producing tensile forces in the working-side joint.
Greaves (1978) suggested that the muscle resultant could be moved back into the
triangle of support (arrow) through a reduction in balancing-side muscle activity.
14


As stated earlier, the main assumption of the constrained model is that the
TMJ should not be subjected to distraction. Therefore, the muscle resultant force
must move so that it will always lie within the triangle of support. Differential
muscle activity between the working side and balancing side (i.e. less balancing
side muscle activity) will enable the MRF to shift laterally toward the working
side. This means that a smaller triangle of support characteristic of the more
posterior dentition will still encompass the MRF.
Changes in muscle activity and joint loading have lead to the division of
three zones of potential bite points termed Regions I, II, and III (Spencer and
Demes, 1993; Spencer, 1995,1998,1999) (Fig. 2.3). Region I encompasses the
anterior dentition. It is separated from Region II by an oblique line, which passes
through the balancing-side joint reaction force and a midline MRF. The triangle
of support for this region is able to enclose a midline MRF; it is unnecessary for
the MRF to shift toward the working side.
Regions II and El are separated by a transverse line passing through the
muscle resultant force. Region II is characterized by a relatively small triangle of
support through which a midline MRF will not pass if the adductor muscles on
both sides of the head are equally active. The MRF must shift toward the working
side in order to avoid distraction of the working-side TMJ. This is done through a
reduction in balancing-side muscle activity.
15


Region l
Balancing Side
Joint Reaction Force
Working Side
Joint Reaction Force
Fig. 2.3 Occlusal view of the mandible showing the distributions of Regions I, II,
and III for a mammal with a fused mandibular symphysis. Any bite point located
in the anterior region (Region I) will produce a triangle of support that envelopes
a midline muscle resultant force; no reduction in balancing side activity must
therefore occur. In Region II, the muscle resultant must shift toward the working
side through a reduction in balancing side activity or tension will be produced in
the working side joint. In the most posterior region (Region III) the muscle resultant
cannot be repositioned so that tensile forces are avoided.
16


A triangle of support in Region III will not be able to encompass a MRF
even if it were to move laterally. Biting in this region will be unavoidably
associated with TMJ distraction because the MRF cannot fall within the triangle
of support (Spencer, 1999). Therefore, no teeth should lie here.
Bite force and joint reaction force values differ between Regions I, II, and
HI. These provide evidence for demarcating the boundaries of these regions.
This has been demonstrated in a theoretical model of reaction force values based
on the predictions of the constrained model (Spencer, 1998) (Fig. 2.4). Region I
is characterized by bite force values that are the lowest in magnitude yet increase
as the bite point moves distally. Region II maintains the highest magnitude bite
forces, however, all values are equal within this region. Region HI does not have
any bite force values as biting in this region should be avoided due to the
possibility of joint distraction.
The joint reaction forces show a different pattern of magnitude from the
bite forces. The working-side joint reaction force decreases as the bite point
moves distally through Region I. In Region II, the working-side joint reaction
force maintains a value of zero. The balancing-side joint reaction force values
decrease slightly within Region I and then drastically in Region II as the bite point
moves distally (Spencer, 1998).
The constrained model has been very influential within the field of
functional morphology (Greaves, 1982; Dessem and Druzinsky, 1992; Spencer
17


Fig 2.4 Predicted bite force and joint reaction force values using the assumptions of
the constrained model. The magnitude of the bite force (F), the working-side.joint
reaction force (O), and the balancing-side joint reaction force (J) is predicted under
the conditions of maximum bite force production and the avoidance of tensile
working-side joint reaction forces. Bite force values in Region I are lowest in
magnitude but increase as the bite point moves posteriorly. Region II bite force
values are highest in magnitude but are all equal. Joint reaction force values show a
different pattern. Within Region I, the working-side joint reaction force values
decrease as the bite point moves posteriorly.. In Region II, the working-side joint
reaction force values are all zero. The balancing-side joint reaction force values
decrease slightly within Region I and then drastically in Region II as the bite point
moves distally.
\
18


andDemes, 1993; Spencer, 1995,1998,1999; Dumont andHerrel, 2003;
Thompson, et. al., 2003). Predictions concerning masticatory configuration have
been generated using this theoretical model. Subsequent testing of this model has
led to the modification of the constrained model to include variables and ideas
that are discussed in the next section.
Modifications to the Constrained Model
Subsequent studies have provided support for Greaves fundamental
assumption that the TMJ should not be loaded by distractive forces. A
morphometric analysis of anthropoid masticatory system configurations suggests
that the phenotypic diversity in cranial morphology in this group is limited by the
need to avoid TMJ distraction (Spencer, 1999). However, this study also
highlights some discrepancies between the constrained model and observed
anthropoid cranial morphology. One such discrepancy involves the observation
that the masticatory adductor muscles are positioned more posteriorly than
proposed in the constrained model. This means that the MRF may not be
produced directly at the posterior end of the tooth row during forceful isometric
biting as Greaves had assumed.
Spencer (1999) proposes a more posterior position of the MRF than
originally conceived of by Greaves (1978). A more posteriorly oriented MRF
means that even if the MRF were to migrate forward at larger gapes, it would still
19


be maintained within the triangle of support. This creates a more conservative
masticatory system configuration in which combined muscle force is positioned
more posteriorly than predicted by Greaves.
The constrained model has been tested through studies of force production
in the primate, canid, and opossum masticatory system. If this model is correct,
muscle activity should change with bite point position. This must occur in order
to maintain the basic assumption of this model, which is that the TMJ should not
be loaded with distractive forces. The TMJ will not be subjected to distractive
forces as long as the MRF is maintained within the triangle of support. Biting on
more posterior teeth will, therefore, cause the MRF to shift laterally in order to be
maintained within the relatively smaller triangle of support. This is done through
a reduction of balancing-side muscle activity.
Electromyographic data have shown that in humans the activity of the
largest masticatory adductor muscles, the superficial masseter and anterior
temporalis, changes with bite point (Spencer, 1998). Maximum muscle force
magnitudes were found to be greatest for the first molar, decreasing both anterior
and posterior to this bite point. Balancing-side to working-side muscle force
ratios were also found to differ by bite point. Balancing-side muscle activity was
found to be lowest during biting on the third molars. This decreased balancing
side muscle activity may serve as a mechanism for avoiding TMJ distraction by
enabling the MRF to be maintained within the triangle of support (Spencer, 1998).
20


An electromyographic study of Canis familiaris also supports the
constrained model (Dessem, 1989). This study found greater working-side than
balancing-side muscle activity which supports the notion that the MRF should
always fall within the triangle of support in order to maintain jaw-joint stability.
The constrained model also enables specific predictions to be made about
bite forces in Regions I and II. Those predictions are that bite force increases as
the bite point moves posteriorly within Region I and then reaches its highest
magnitude within Region II. All bite points within Region II are of equal value.
A study involving Monodelphis domestica, an opossum, supports these
predictions (Thompson et. al., 2003). They found that within this species, both
juveniles and adults, maintain at least three molariform teeth within Region II and
that it is within this region where highest magnitude bite forces are produced.
Also, within Region I, the bite force generated at the premolars was stronger than
that generated at the incisors or canines and forces were equivalently strong
within Region II.
Other studies (Spencer, 1999) have also tested the constrained model and
found that there are many variables related to the configuration of the masticatory
system that interact to determine the distribution of Regions I, II, and III. These
variables include bicondylar breadth, the anteroposterior position of the MRF,
palatal breadth, and height of the TMJ.
21


Changes in bicondylar breadth can alter the anterior border of Region II (Fig.
2.5). The border this Region II moves posteriorly as the distance between the two
condyles increases, causing less of the molar tooth row to fall within Region II
(Spencer, 1999). Therefore, increasing bicondylar breadth decreases the length of
Region II.
Another variable relating to the distribution of Region II is the anteroposterior
(A-P) position of the MRF. In the constrained model the MRF is located at the
midline between the last molars. Bite points posterior to the MRF will not allow
it to pass through the triangle of support even if it were to shift laterally toward
the working side (Spencer, 1999). However, a MRF that is positioned posterior to
the last molars will create a more posterior, and shorter, Region II (Fig. 2.6).
The height of the TMJ above the occlusal plane is another variable that affects
the distribution of Region II. TMJs that are positioned above the occlusal plane
will cause the MRF vector to pass more anteriorly through the triangle of support
(Spencer, 1999). This is because the triangle of support will become inclined as
the TMJ gets raised above the occlusal plane. The taller the TMJ, the more
inclined the triangle of support will be. This inclination may cause the MRF to
pass anterior to the triangle of support. Therefore, mammals with taller TMJs
may have a more anteriorly positioned Region H This has the effect of
increasing the length of Region II.
22


Balancing Side Working Side
Joint Reaction Force Joint Reaction Force
Fig. 2.5 Occlusal view of the mandible demonstrating how bicondylar breadth affects
the length of Region II. A decrease in bicondylar breadth, from positions A through
C, causes an increase in the length of Region II. This enables more of the molar row
to fall within this region.
23


-1
I
I
Balancing Side
Joint Reaction Force
Working Side
Joint Reaction Force
Fig. 2.6 An additional variable that alters Region II length is the anteroposterior
position of the midline muscle resultant force (B). As the muscle resultant force is
moved from positions A through C, the boundary line between Region II and HI (bold
transverse lines passing through the muscle resultant force) is moved posteriorly. As
the muscle resultant force is reoriented posteriorly, the length of Region II decreases
(shaded boxes).
24


Palatal width also affects the distribution of Region II (Fig. 2.7). The
mediolateral position of the tooth row determines the length of the postcanine
dentition that will fall within Region II. Thus, movement of the palate laterally
increases the length of Region II.
These variables all interact with one another to produce a masticatory
system configuration unique to each species. Studying the configuration of this
system and how these variables interact to determine the distribution of Region II
is crucial to testing the broad scale applicability of the constrained model.
Muscle Force Recruitment
The purpose of this study is to assess the differences in masticatory system
configuration between mammals with an unfused mandibular symphysis and
those with a fused one. The justification for studying these differences lies in the
finding that animals with a fused symphysis differ in muscle recruitment patterns
from those with an unfused symphysis (Hylander, 1979b; Dessem, 1989;
Hylander et. al., 1998).
It has been shown that primates with a fused symphysis recruit more
balancing-side muscle force during powerful mastication than those with an
unfused symphysis. In 1979, Hylander reported results from an experiment that
showed that only a small percentage of the bite force of Galago crassicaudatus,
which has an unfused symphysis, is due to balancing-side muscle force during
25


I
I
Balancing Side Working Side
Joint Reaction Force Joint Reaotlon Force
Fig. 2.7 Occlusal view of mandible showing how palatal breadth affects the length
of Region II, The boundary line between Regions I and II is represented as a thin
diagonal line and Regions II and III are separated by a bolder horizontal line. The.
mediolateral position of the tooth row relative to these lines will determine the length
of postcanine dentition that will fall within Region II (boxes). Thus, movement of
the tooth row laterally (i.e., increasing palatal breadth) from positions A through C
results in an increase in Region II length.
26


isometric unilateral molar biting. This suggests that the working-side jaw
musculature of galagos is much more active than the balancing side. Bone strain
data and moment arm calculations demonstrated that the working-side muscles
generate at least four to five times more force than the balancing side (Hylander,
1979b).
This decreased emphasis on balancing-side musculature for galagos
contrasts with electromyographic results for humans. Results show that the
balancing-side muscles of humans are only slightly less active than the working-
side masticatory muscles during powerful unilateral biting (Meller, 1966 as
discussed in Hylander, 1979b). One reason this may be the case can be seen in
the mandibular morphology of these two groups. Galagos have an unfused
mandibular symphysis whereas humans, including all anthropoids, have fusion.
The fused mandibular symphysis of this group of primates has been suggested to
be an adaptation to counter increased symphyseal stress due to increases in
balancing-side muscle force during powerful unilateral biting (Hylander, 1979b).
This is because symphyseal fusion functions to prevent structural failure of the
symphysis by strengthening it. This becomes important as more balancing-side
muscle force is recruited during mastication. The bone strain data of galagos
discussed above support this suggestion.
Data comparing long-tailed macaques (Macaca fascicularis) and thick-
tailed galagos (Otolemur crassicaudatus) also support the symphyseal fusion-
27


muscle recruitment hypothesis. Advocates for this hypothesis argue that
symphyseal fusion and balancing-side jaw adductor muscle force are functionally
linked (Hylander, 1979a). Long-tailed macaques, which have a fused mandibular
symphysis, have about 1.5 to 2 times more bone strain along the working-side
corpus than along the balancing-side corpus. Thick-tailed galagos, whose
symphysis is unfused, have about 7 times more bone strain along the working-
side corpus. This shows that the macaques recruit more balancing-side adductor
muscles during isometric molar biting, whereas the galagos rely more exclusively
on the working-side jaw muscles.
Lemurs, which have an unfused symphysis, have also been found to
recruit relatively little force from their balancing-side deep masseter. The
recruitment and firing pattern of this muscle is much more similar to that of the
galagos than anthropoids (Hylander et. al., 2002).
Owl monkey data show a pattern of strain that is similar to that recorded
for the macaques (Hylander et. al., 1998). This pattern differs markedly from the
galagos strain pattern. This is expected considering owl monkeys and macaques
both have a fused symphysis whereas galagos do not. Owl monkeys^ which are
smaller than galagos, show that differences in muscle recruitment patterns are
related to symphyseal fusion rather than allometric constraints.
A study of electromyographic activity from jaw-adductor muscles of
Cants familiaris (domestic dog) also supports the idea that animals with an
28


unfused symphysis recruit less balancing-side muscle force. During mastication
and bone crushing working-side muscle activity was reported to be greater than
balancing-side muscle activity in this species, which has an unfused symphysis
(Leibman and Kussick, 1965; Dessem, 1989). It was found that even during the
production of the largest bite forces, balancing-side muscle activity was never
maximally recruited. This finding supports the previously discussed studies
involving primates.
Much evidence demonstrates that there are marked differences in muscle
recruitment patterns between mammals with a fused and unfused symphysis.
These muscle recruitment patterns may cause systematic differences in the
configuration of the masticatory system between mammals with a fused and
unfused symphysis.
Why Fuse the Mandibular Svmnhvsis?
Symphyseal fusion enables those animals with this morphological trait to
recruit more balancing side muscle force as discussed in the previous section.
However, this does not explain the convergent acquisition of this feature in a
number of mammalian taxa. During the past thirty years, morphological and
experimental analyses of primates have been done in an attempt to answer this
question. Although it has been confirmed through histological studies (Beecher,
1977) that a fused symphysis is stronger than an unfused one (because bone is
29


stronger than the fibrocartilage and ligaments of an unfused joint) it is unclear
exactly why fusion has occurred. Two groups of hypotheses have been offered to
explain the functional and evolutionary significance of symphyseal fusion. These
hypotheses involve stiffness and strength in the mandibular symphysis.
Stiffness Hypotheses
One group of hypotheses involves the idea that fusion of the symphysis
occurred within anthropoid primates as an adaptation to stiffen the symphysis.
Stiffness is defined as the ability to resist deformation in response to applied
forces and is the primary mechanical property of bone that enables force transfer
(Lieberman and Crompton, 2000).
One idea that has been suggested is that symphyseal fusion occurred
within anthropoids to stiffen the symphysis in order to better resist the forces
associated with the crushing of small, hard objects, such as seeds, along the
incisors (Kay and Hiiemae, 1974; Greaves, 1988). This would prevent an
inefficient dissipation of dorsally-directed force across the symphysis. This is
because an unfused symphysis allows some independent movement between the
two dentaries during incision. Theoretically, this independent movement enables
the balancing side incisors to contact one another during the crushing of small,
hard objects. Therefore, the balancing-side muscle force gets dissipated along the
incisors on the balancing side rather than being transmitted across the symphysis
30


(Ravosa and Hylander, 1994). The muscle force that was generated on the
balancing side gets wasted on the balancing-side incisors rather than contributing
to the force production on the working side.
This scenario differs as the size of the food object gets larger. As the
diameter of a food object increases the balancing-side upper and lower incisors
will not come into contact. The balancing-side muscle force is then able to get
transmitted across the symphysis and contribute to the crushing of the food object.
This argument is, therefore, relevant only for the crushing of small, hard food
objects, such as seeds.
In conclusion, according to this hypothesis, the function of a fused
symphysis is to stiffen the symphyseal joint in order to reduce independent
movement of the two dentaries during the crushing of small, hard objects.
This hypothesis is faulty for a number of reasons. The majority of
anthropoid primates do not include small, hard objects, such as seeds, as a major
component of their diet. Also, according to Ravosa and Hylander (1993), the
literature provides no evidence for the processing of small, hard objects on the
incisors when objects such as seeds are ingested. The apparent discrepancy
between observed behaviors in anthropoid primates and this argument has led
others to modify the stiffness hypothesis.
Another problem with this argument is that it does not address the extent
to which independent movement between the two dentaries occurs. Greaves
31


(1988) schematic drawings make it look as though each dentary moves
completely independent of one another. This is not the case; the ligaments of an
unfused symphysis do not allow such an amount of independent movement
between the two dentaries. In fact, Beecher (1977) has shown that the pattern of
fibrocartilage and ligaments of prosimians are arranged to resist movement during
mastication.
The functional association between symphyseal fusion and increased
stiffness has been hypothesized to be important for molar biting rather than for the
incisors (Ravosa and Hylander, 1994). Symphyseal fusion could be an adaptation
to reduce the amount of balancing-side tooth contacts during unilateral
mastication on the molars. This is similar to the proposition that balancing side
tooth contact should be avoided so that balancing-side muscle force can
contribute to the crushing of the food object on the working side.
This idea is also problematic because anisognathic jaws and an unfused
symphysis probably represent the primitive condition for primates (Ravosa and
Hylander, 1994). Anisognathy refers to the condition of having different widths
of the upper and lower dental arches. Therefore, these teeth are not likely to come
into contact anyway even with an unfused symphysis. Fusion of the mandibular
symphysis does not appear to be necessary to prevent balancing-side tooth
contacts during either incision or mastication.
32


An alternative hypothesis focuses on the orientation of the occlusal wear
facets. The main idea is that fusion functions to stiffen the mandible in taxa with
more horizontally oriented occlusal wear facets. Both fused and unfused
symphyses are effective at transferring dorsally-oriented force. However, a fused
symphysis, because of its stiffness in all planes, is likely to be more effective at
transferring force in the transverse plane. Taxa with a strong transverse
component to chewing will benefit more from a fused symphysis.
This idea has been supported in studies of mammals; symphyseal fusion
does in fact correlate with transversely oriented occlusal planes (Lieberman and
Crompton, 2000). It can be concluded, therefore, that fusion is most likely an
adaptation for increasing the efficiency of transversely-oriented occlusal forces.
Strength Hypotheses
Another set of hypotheses regarding the functional significance of
symphyseal fusion is that fusion strengthens the symphysis to help counter
symphyseal stress during incisal biting and unilateral mastication (Ravosa and
Hylander, 1994). Hypotheses that have been offered in support of the
strengthening hypothesis involve the idea that symphyseal fusion occurred to
counter wishboning stress and/or dorsoventral shear.
It has been found that during incision and mastication, the mandibles of
macaques are twisted about their long axes (Hylander, 1979a). This twisting
33


everts the lower borders of the mandible so that the symphysis gets bent
vertically. Compression occurs along the upper or alveolar surface of the
symphysis and tension is present along the inferior surface of the symphysis. This
stress is most effectively countered by complete symphyseal fusion.
Two loading patterns are also present among anthropoid primates during
unilateral mastication: wishboning and dorsoventral shear of the symphysis.
Wishboning stress results from the laterally directed component of muscle force
on the balancing side and a laterally directed component of bite force, and perhaps
working-side muscle force. The symphysis therefore experiences high stress
concentrations and high strain magnitudes, especially along the lingual surface.
This is best countered by fusion because bone is much stronger than ligaments
(Ravosa and Hylander, 1994).
Dorsoventral shear is another explanation concerning symphyseal fusion.
In an experimental study with macaques it was found that the symphysis is
sheared dorsoventrally during mastication and incision (Hylander, 1984). The
amount of dorsoventral shear stress is directly related to the amount of vertically-
oriented muscle force transmitted across the symphysis from the balancing to the
working side. As balancing-side muscle recruitment increases so does the amount
of dorsoventral shear. This is also best countered by complete fusion of this joint
(Ravosa and Hylander, 1994).
34


Wishboning and dorsoventral shear are both present during mastication.
However, according to Ravosa and Hylander (1994), it appears that wishboning is
the most important determinant of symphyseal fusion in primates because the
stresses associated with this loading regime are considerably higher than those
associated with dorsoventral shear. It has been suggested, however, that fusion in
early anthropoids may be due to increased dorsoventral shear resulting from
increased recruitment of vertically directed balancing-side muscle force.
Evidence for this can be seen in prosimians that seem to have evolved
morphologies to resist dorsoventral shear such as having calcified or ossified
ligaments. Also, wishboning stress appears to occur only in taxa who have
already evolved complete fusion. Therefore, fusion in early anthropoids may
have followed from increased dorsoventral shear and after fusion was attained,
wishboning stress increased as greater amounts of horizontally-directed forces
could be recruited.
Both stiffness and strength hypotheses have been offered to explain the
functional and evolutionary significance of symphyseal fusion. Histological and
experimental evidence provide greater support for the strength hypothesis.
Whether fusion occurred due to wishboning or dorsoventral shear stresses
associated with the strength hypothesis is not entirely clear. It seems as though
these loading patterns may both be involved in the evolution of symphyseal
35


fusion. Experimental studies including other mammalian taxa need to be
performed in order to clarify this issue.
36


CHAPTER 3
HYPOTHESES AND STUDY DESIGN
The main goal of this study is to determine how applicable the constrained
model, discussed in the previous chapter, is for understanding masticatory form
and evolution in a wide range of mammals. The particular topic of concern here
is whether the configuration of the masticatory system differs in a patterned way
between mammals with symphyseal fusion and those without it and if this can be
explained using the biomechanical principles underlying the constrained model.
Of substantial interest is the distribution of Region II, the region where highest
magnitude bite forces occur. Because of this the grinding dentition, particularly
the molars, should be located here. This hypothesis has been tested and supported
for anthropoid primates (Spencer, 1999), but it is not clear whether it holds true
for other groups of mammals, particularly those without symphyseal fusion. If
there is an association between the length of Region II and molar length within
the mammals studied here, it will imply that masticatory system configuration,
although highly diverse, can be explained through general principles of the
constrained model. If such an association does not exist, it will suggest that the
model is incorrect.
37


The main prediction of this study is that observed molar length will be shorter
than the predicted Region II length in all taxa. This is because according to the
constrained model, the molars must lie within Region H Maintaining the molars
within this region can be accomplished by either decreasing overall molar length
or increasing the length of Region II through the interaction of other craniofacial
variables. This is because the distribution of Region II is determined by a number
of variables, including bicondylar breadth, height of the TMJ, palatal breadth, and
the anteroposterior position of the muscle resultant force. Determining the length
of Region II is essential to understanding how these craniofacial variables interact
within this model.
Predicting the position of the premolars within Region II is problematic. The
constrained model maintains that the grinding dentition should he within this
region. When applied to selenodont artiodactyls this may not be an issue because
the molars and premolars are functionally similar and consequently labeled
together as grinding dentition. However, other mammals, such as primates, have
premolars that are distinct from the molars and may be functionally different. The
diversity of premolar form within primates suggests functions including:
puncturing, slicing, and perhaps molar-like crushing (Spencer, 1995). Therefore,
many of these teeth cannot be defined as grinding dentition. Because of this
diversity in premolar function, this study will focus on the molars. The model is
accepted if at least all of the molars are located within Region II.
38


The Effects of an Unfused Symphysis on the Model
The constrained model was developed based on a mammal with a fused
mandibular symphysis. Although this model was applied to selenodont
artiodactyls, a mammal with an unfused symphysis, Greaves (1978) did not
address how lack of fusion may affect the configuration of this model.
Mammals with an unfused symphysis have a MRF that is located closer to
the working side. This has the effect of shortening the length of Region II (Fig.
3.1). However, the constrained model assumes equal balancing side (B/S) to
working side (W/S) muscle force ratio, which places the MRF in the midline. The
length of Region II will, therefore, be calculated under the assumption of a
midline MRF.
This assumption is problematic, however, for mammals with an unfused
mandibular symphysis as it is essential to know the location of the MRF in order
to calculate an exact length for Region n. This is because we know that the MRF
in mammals with an unfused symphysis does not he in the midline during
mastication. These animals recruit relatively less balancing side muscle force
than those with a fused symphysis, therefore, the MRF will be located more
laterally. A laterally located MRF would cause the length of Region II to be
shorter than if the MRF passed through the midline (Fig. 3.1).
Although I assume that it is more laterally located, the exact location cannot
be determined. This is because we do not know the working side/balancing side
39


Region I
b Midline muscle resultant force for
mammals with a fused symphysis
Region I
Balancing Side Working Side
Joint Reaction Force Join! Reaction Force
Laterally positioned muscle resultant
force for mammals with an unfused ,
symphysis
Fig 3.1 Occlusal view of the mandible showing the effects of changes in the position
of the muscle resultant force on the distribution of Region II. (a) The distribution of
Region II with a midline muscle resultant force, (b) A laterally positioned muscle
resultant force will create a shorter Region II length, therefore, less of the molar tooth
row will fall within this region.
40


muscle force ratios for all species of mammals with an unfused symphysis.
Therefore, an actual Region II length cannot be calculated for this group of
mammals.
This dilemma can be overcome by estimating the length of Region II for both
groups of mammals assuming a midline MRF. This predicted length can then be
compared to actual molar length in both groups. It is necessary to use this
calculated length for mammals with an unfused symphysis because it is the only
estimate of Region II length that can be attained. It is predicted that the molar
row will not fall outside of Region II for both groups.
Mammals without fusion can compensate for having a theoretically
shorter Region II through cranial variables. The variables that impact the
distribution of this region are bicondylar breadth, palatal breadth, height of the
TMJ, and the A-P position of the MRF. All of these variables can increase the
length of Region II regardless of whether an animal has a fused or unfused
symphysis. The model demonstrates, however, that a lack of fusion creates a
theoretically shorter Region II length in this group. These variables provide
mammals with an unfused symphysis multiple ways to overcome this constraint.
The variables discussed above provide mammals with a way to increase
the length of Region II. These variables, however, are not independent of one
another. Change in one variable is likely to covary with other variables (for
example, an animal with a wide palatal length could have a decreased bicondylar
41


breadth) perhaps still resulting in an increased Region II length. The main
prediction will be upheld as long as the molars are maintained within the
estimated Region II length, whether this is accomplished through a decrease in
molar size/number or by increasing Region II length through these other
craniofacial variables.
The second prediction of this study is that the ratio between predicted
Region II length and observed molar length will be higher in mammals with an
unfused symphysis. This is because mammals with an unfused symphysis have a
theoretically shorter Region II length due to the MRF being located laterally.
Specific Predictions
1. Molar length will be shorter than the predicted length of Region II for both
groups of mammals.
2. The ratio between predicted Region II length and observed molar length is
predicted to be higher in mammals with an unfused symphysis because of
the theoretically shorter Region II length.
Study Design
The variables related to the distributions of Region II were quantified so that
comparisons between the two groups could be performed. A broad-scale
comparative test was carried out on these hypotheses to determine the degree to
42


which overall masticatory form in mammals is consistent with the principles of
the constrained model.
It is expected that the development of fusion and the changes it brought
about in muscle force ratios, also brought about changes in cranial configuration.
This was assessed through the principle of comparison (Fleagle, 1988). The
comparative approach enables us to identify morphological trends of the
masticatory system between taxa with symphyseal fusion and those without it.
Identifying these trends is essential to uncovering the functional and evolutionary
explanation for the change in morphological structure from an unfused
mandibular symphysis to a fused one.
Tests
The main prediction of this study (Hypothesis 1) is that the molar tooth
row will lie within Region II for all taxa. This hypothesis will be rejected if the
observed length of the molars is greater than the predicted Region II length in any
taxa. This test requires that the length of Region II be predicted as it cannot be
directly measured. This will be done under the assumption of a midline MRF for
all mammals as previously discussed. The predicted length of Region II can then
be compared to actual molar length in all taxa.
Hypothesis 2 states that the ratio between predicted Region II length and
molar row length will be higher in mammals with an unfused symphysis because
43


their actual Region II length (although not able to be calculated) should be
shorter. This hypothesis will be rejected if mammals with an unfused symphysis
do not have a higher ratio of predicted Region II length to molar length.
An allometric analysis is also important to explore patterns of variation in
individual dimensions. An allometric analysis enables us to understand how
changes in shape and size allow different taxa to accomplish the same behavior,
which in this case is the maximization of force production in the masticatory
system (Clutton-Brock and Harvey, 1979; Fleagle, 1988).
44


CHAPTER 4
MATERIALS AND METHODS
Sample
Measurements were taken on a total of 135 individuals representing 15
species with symphyseal fusion and 16 species without fusion (Table 4.1 and 4.2).
Sample size ranges from 1 to 12 for the different species. Representative species
from the orders Primates, Perissodactyla, Carnivora, and Artiodactyla were
included in order to obtain a large sampling of taxa in which to compare those
with symphyseal fusion to those without fusion. It is important to include a
variety of mammalian taxa in order to assess the generalizability of this model
within the class Mammalia. Four species of artiodactyls, 12 carnivore species, 3
perissodactyl species, and 15 primate species are represented in this sample. Only
adult crania (all teeth fully erupted) and their associated mandibles were used.
Specimens were obtained from the Denver Museum of Nature and Science
and the University of Colorado at Boulder Museum of Natural History. Although
some species are represented by only a minimal number of specimens they were
included in the study as my purpose is to sample a broad range of species within
the mammalian order.
45


Table 4.1 Taxa included in study with a fused symphysis
Taxa with a Fused Symphysis Sample size
Order- Cebus capuchinus 3
Primate Pithecia pithecia 2
A teles geoffroyi 3
Aotus lemurinus 1
Alouatta palliata 4
Callithrix argentata 1
Saguinus geoffroyi 3
Cercopithecus diana 1
Cacajao calvus 1
Gorilla gorilla 2
Pongo pygmaeus 1
Order- Equus grevyi 2
Perissodactyla Equus equus 3
Tapirus terrestrius 2
Total = 29
46


Table 4.2 Taxa included in study with an unfused symphysis
Taxa with an Unfused Symphysis Sample size
Order- Galago demidoff 1
Primate Nycticebus coucang 1
Order- Canis latrans 12
Carnivora Canis lupis 9
Urocyon cinereoargenteus 8
Vulpus velox 11
Vulpus vulpus 11
Lynx canadensis 3
Lynx rufus 10
Puma concolor 10
Procyon lotor 10
Ursus americanus 9
Ursus arctos 2
Order- Cervus elaphus 2
Artiodactyla Odocoileus hemionus 2
Odocoileus virginianus 3
Mazama americana 2
Total = 106
47


Data Collection Process
The following steps were taken in the data collection process to ensure
that accurate measurements of the specimens were obtained:
Step 1- A Sony digital camera was used to photograph each specimen.
Each specimen was placed on a level work surface. Two photographs were taken
of each individual including both a transverse and occlusal view. When
photographing the occlusal surface, each specimen was held in place with
modeling clay. The modeling clay allowed the specimens to be positioned at a 90
degree angle to the table.
The camera was placed on a tripod and aimed at the specimen. The
camera was positioned as far from the specimen as possible while still filling the
viewfinder with the skull. Filling the viewfinder with as much of the image as
possible maximizes screen resolution which allows measurements to done on a
computer with finer detail, thereby, reducing measurement error (Spencer, 1995).
Step 2- A calibration grid was set up and photographed before pictures of
the specimens were taken. Any time the camera was moved, particularly upon
completion of a species, the calibration grid was repositioned and re-
photographed. This image was used during analysis to determine the size of the
space in which landmarks were located.
48


Step 3- Some of the landmarks that were measured were not clearly visible
(such as the glenoid fossa). These were highlighted with small black dots, which
helped with identification on the computer images.
Step 4- Images were downloaded into a Macintosh computer and
measured within the MacMorph data acquisition package. Each image was
calibrated using the corresponding calibration grid. Measurements were then
taken of the variables for all images.
Step 5- The statistical packages JMP and Statview were used for all
analyses.
Measurements
Three sets of measurements were taken for this study: (1) distances that
represent the observed length of the postcanine dentition, (2) dimensions for
calculating a predicted Region II length, and (3) size adjustment measurements.
Measuring the observed length of the postcanine dentition is necessary for
Hypothesis 1 and 2. The observed length of the postcanine dentition was
measured from the trigon of the maxillary last molar to the trigon of the first
molar and from the trigon of last molar to the trigon of the first premolar. The
trigon of each tooth was chosen because of the biomechanical role it plays; this
feature experiences much of the force that is produced during mastication when it
comes into direct contact with the teeth of the mandible.
49


The main hypothesis of this study is that the molars must lie within
Region II. However, the length of both the premolars and molars were measured
in order to assess the extent to which both types of teeth lie within Region II.
Calculating an estimated Region II length is necessary for Hypothesis 1
and 2. There are five variables that can influence the location of Region II and
must be quantified or estimated in order to calculate a predicted Region II length.
These variables include bicondylar breadth, palatal breadth at Ml, height of the
TMJ relative to the occlusal plane, distance from the TMJ to point of intersection
of the muscle resultant vector and occlusal plane, and the angle of the muscle
resultant vector to the occlusal plane (Fig. 4.1). The first three variables can be
directly quantified. However, it is difficult to quantify muscle resultant position
and orientation due to limited knowledge of the comparative myology and
function of the masticatory muscles among mammals (Throckmorton, 1989;
Spencer, 1999). This study will therefore assume the MRF vector intersects the
occlusal plane directly at the posterior end of the tooth row. This can be directly
quantified as the distance from the TMJ (defined here as the center of the articular
eminence) to the trigon of the last molar. This is consistent with the assumptions
of the constrained model.
A prior study involving muscle resultant force orientation for the
masticatory adductor muscles of anthropoid primates estimated a fixed orientation
of 80 degrees relative to the occlusal plane based on quantified orientations of the
50



rw o1 /
V I
ic
r
w
/ A
\ \
V. ;
."i (
\\
i-.. >
! //
\ //
/




A = Biarticular breadth
B = Palate Breadth at M1
C = Distance from TMJ to point of
intersection of muscle resultant
vector and occlusal plane
D = Height of articular eminence
above occlusal plane
E = Angle of muscle resultant
vector to occlusal plane
(90 equals perpendicular)
Fig. 4.1 Illustration of five variables used in the calculation of Region II length.
51


anterior temporalis, superficial masseter, and medial pterygoid muscles (Spencer,
1999). Muscle resultant orientation cannot be calculated in the present study
since it is likely to be highly dependent on individual muscle force magnitudes.
Collecting these data is beyond the scope of this study.
The length of the molar row is defined as the distance between the trigon
of the right maxillary last molar to the trigon of the first molar. Bicondylar
breadth is the distance between the central articular eminence landmarks. Height
of the TMJ is the perpendicular distance of the right central articular eminence
landmark from the occlusal plane. The occlusal plane is projected onto the
sagittal plane and is defined by the horizontal line connecting the distal end of the
maxillary last molar to the mesial border of the maxillary last molar. Palatal
breadth is the distance between the maxillary tooth rows at the trigons of Ml.
The five variables were placed into an algorithm that calculates expected
Region II length. The equation is shown in Figure 4.2 (Obtained from Spencer,
1995).
Size
Size and shape of the cranium are known to differ drastically among the
mammals included in this study. Only when shape differences are teased apart
from size differences can any meaningful comparison between these groups be
52


/

= arclan
A -
D
/ >
p.^JpJ-p,
o
Ps
= arctan
l^+cj
Pt =
Pi'
sHPl),
Pi *!?<,-Pi
Effective Region II Length = P7 + P4
Fig. 4.2 Equation for estimating the Predicted length of Region II.
53


made (Spencer, 1995). The calculation of the ratios of each dimension divided by
the geometric mean of the cranium allows comparisons of masticatory system
configurations between groups exhibiting different cranial sizes (Darroch and
Mosimann, 1985). These size adjusted shape variables indicate relative
proportions among the groups in this study.
In this study the geometric mean involves four distances within the facial
skeleton in order to get an accurate representation of overall cranial size. This
serves as a size summary by combining multiple size dimensions into a single
value. The equation for the geometric mean is:
GM = (D!*D2...Dn)(1/N)
where D = distance value andN = number of distance values included in the
summary of size.
1 Distances used to assess the geometric mean were chosen as representative
of overall masticatory system size as this is the system of concern to this study.
The distances used are bicondylar breadth, palatal breadth, temporal foramen
length, and molar row length. The landmarks used for measuring bicondylar
breadth, palatal breadth, and molar row length have been defined above.
Temporal foramen length is defined as the distance between the right central
articular eminence landmark and the inferior edge of the zygomatic arch.
An allometric analysis was also done to assess how changes in shape with
size affect the masticatory system. This is necessary because such a wide range
54


of sizes are being sampled. The geometric mean was also used for allometric
analyses.
55


CHAPTER 5
RESULTS
The parameters measured for this study are shown in Tables 5.1 5.3. All
values are represented as sex-specific means and standard deviations (in mm) of
all individuals. Table 5.1 displays mean values for molar row length, the length
of the postcanine dentition, and the length of the molars to the incisors. Table 5.2
lists mean values for the parameters used to calculate predicted Region II length.
These parameters include: bicondylar breadth, palatal breadth, height ofTMJ,
distance from the TMJ to the last molar. Table 5.3 reports the geometric mean, as
well as one additional variable used in this calculation. The three other variables
used in the calculation of the geometric mean, bicondylar breadth, height of the
TMJ, and molar length, were reported in Tables 5.1 and 5.2.
Figure 5.1 shows a box plot comparing postcanine dimensions to predicted
Region II length. These dimensions include the observed length of the molar
dentition, which is expected to be shorter than the predicted length of Region II,
and the observed distance between the most mesial premolar to the most distal
molar. Also included in the plot is the calculated predicted length of Region II.
56


Table 5.1 Mean (x) and standard deviation (sd) of tooth row length. (Molar length =
length of all molars; M-P length = the last molar to the first premolar; M-I length =
last molar to first incisor)
Taxon Sex n Molar Length M-P Length M-I Length
Primates x (mm) sd x (mm) sd x (mm) sd
Cacajao. culvus F 1 5.17 0 9.84 0 27.03 0
Cercoplthecus diana F 1 7.32 . 0 12.82 0 27.85 0
Pongo pygmaeus F 1 23.8 0 39.88 0 75.05 0
Alouaita palliata F 2 32.9 1.27 64.81 5.54 93.14 0
Aotus lemurinus F 1 9.55 0 21.75 0 39.34 0
Gorilla gorilla F 2 33.1 2.68 55.37 0.14 101.2 2.98
Cebus capucbinus F 3 8.64 0.23 19.76 0.41 34.79 1.18
Nycticebus coucang F 1 7.01 0 16.52 0 -
Ateles geoffroyi F 3 10.28 0.48 21.9 0.66 36.73 1.2
Calllthrix argentata F 1 4.09 0 7.36 0 14.66 0
Galago demldoff ? 1 3.76 0 6.93 0 - -
Saguinus geoffroyi M 2 4.61 0.13 8.58 0.12 17.45 0.44
F 2 4.51 0 8.64 0.22 17.39 0.19
Pithecia plthecia F 2 6.88 0.67 14.83 0.81 27.49 1.58
Carnivores
Lynx canadensis M 1 7.81 0 18.47 0 47.39 0
F 1 7.15 0 18.03 0 42.68 0
? 1 7.77 0 17.48 0 46.05 0
57


Table 5.1 (cont.)
Taxon Sex n Molar Length M P Length M 1 Length
Carnivores (Cont.) x (mm) sd x (mm) sd x (mm) sd
Lynx rufus M 4 10.46 1.73 20.5 1.81 47.53 1.55
F 4 9.21 0.83 19.37 1.57 45.03 2.38
? 2 9.25 1.25 19.14 0.33 45.13 0.49
Puma concolor M 2 14.99 0.77 33.37 2.31 62.1 3.53
F 6 17.09 * 1.8 37.56 1.68 66.75 2.14
? 2 15.34 0.77 32.18 2.5 59.23 3.78
Procyon lotor M 9 7.75 0.59 32.8 1.19 52.13 2.04
F 1 7.82 0 32.26 0 51.07 0
Ursus arctos M 1 48.98 0 66.32 0 154.1 0
? 1 31.23 0 58.14 0 125.79 0
Ursus americanus M 4 23.72 1.29 60.85 4.46 100.86 7.56
F 1 22.64 0 62.94 0 107.34 0
7 4 27.37 2.81 65.64 3.10 105.97 3.40
Canis latrans M 6 10 0.47 64.72 3.53 96.23 5.77
F 6 9.54 0.55 62.13 3.9 93 4.87
Canis lupis M 8 13.24 1.42 78.79 5.33 121.9 8.94
F 1 12.26 0 75.72 0 116.7 0
Vulpes vulpes M 7 7.99 2.55 45.42 4.30 69.14 4.94
F 4 * 8.81 2.57 44.74 4.50 68.57 4.97
Vulpes velox M 6 5.81 0.44 40.68 2.13 59.95 3.36
F 5 6.46 0.09 41.67 2.81 60.80 3.73
Urocyon M 5 6.95 0.45 39.59 1.23 58.17 3.77
clnereoargenteus
? 2 6.93 0.07 35.84 2.01 54.51 2.11
58


Table 5.1 (cont.)
Taxon Sex n Molar Length M P Length M 1 Length
Artlodactyls x (mm) sd x (mm) sd x (mm) sd
Cervus elephus M 1 57.38 0 121.53 0 - -
F 1 63.20 0 118.86 0 - -
Odocolleus vlrginlanus M 1 33.52 0 61.59 0 - -
F 2 30.31 . 0.09 67.27 1.65 -
Odocoileus hemionus F 2 35.06 5.66 69.76 3.15
Mazama americana F 2 21.91 2.11 47.25 7.24

Perissodactyls
Equus equus ? 3 45.97 2.54 106.89 5.83 232.17 1.77
Equus greyvi ? 2 46.05 2.00 111.45 6.17 228.14 2.47
Tapims terrestrius F 1 45.59 0 120.82 0 217.59 0
? 1 45.93 0 113.7 0 - -
59


Table 5.2 Mean (x) and standard deviation (sd) of variables used to calculate
Predicted Region II length.
Taxon Sex n Blcondylar Breadth Palatal B readth Height of TMJ
Primates x (mm) sd x (mm) sd x (mm) sd
Cacajao culvus F 1 31.41 0 18.81 0 13.8 0
Cercopithecus dlana F 1 30.14 0 19.69 0 5.5 0
Pongo pygmaeus F 1 84.01 0 51.64 0 27.7 0
Alouatta palliata F 2 128.88 0.91 79.79 3.44 84.8 1.7
Aotus lemurinus F 1 51.83 0 29.56 29.56 7.8 0
Gorilla gorilla F 2 107.86 0.36 63.51 0.25 55.45 9.4
Cebus capuchinus F 3 44.27 3.7 26.29 0.39 6.13 2.15
Nyctlcebus coucang F 1 28.92 0 17.57 0 1.9 0
Ateles geoffroyi F 3 46.65 3.64 26.06 2.03 12.6 2.55
Callithrix argentata F 1 20.29 0 11.48 0 2.76 0
Galago demidoff ? 1 13.74 0 9.02 0 2.65 0
Sagulnus geoffroyi M 2 23.26 0.44 13.74 0.29 3.52 1.65
F 2 23.47 0.44 13.94 0.58 5.31 1.44
Pitheda pitheda F 2 34.91 1.67 17.94 0.33 8.75 3.04
Carnivores
Lynx canadensis M 1 65.76 0 44.83 0 2.2 0
F 1 53.33 0 39.8 0 0.5 0
? 1. 67.24 0 43.61 .0 1 0
Lynx rufus M 4 65.49 3.76 39.32 1.5 1.28 0.92
F 4 64.81 3.13 40.62 4.11 0.8 0.63
? 2 60.83 2.95 38.15 2.01 0.6 0.14
60'


Table 5.2 (cont.)
Taxon Sex n Bicondylar Breadth Palatal 8 readth Height of TMJ
Carnivores (cont.) x (mm) sd x (mm) sd x (mm) sd
Puma concolor M 2 82.78 7.81 54.81 3.97 -0.15 0.21
F 6 87.74 3.94 57.23 2.84 -2.12 0.58
? 2 79.42 4.38 51.86 3.27 -6.55 0.07
Procyon lotor M 9 52.51 1.72 1 33.58 1.16 1.11 1.06
F 1 54.29 0 34.29 0 1.4 0
Ursus arvtos M 1 156.48 0 81.89 0 -18.9 0
? 1 107.31 0 59.36 0 -16.9 0
Ursus americanus M 4 109.43 6.34 52.61 3.54 -8.53 21.75
F 1 115.57 0 52.06 0 -24.2 0
? 4 109.13 5.73 53.82 0.72 5.6 10.87
Canis latrans M 6 64.68 ?-12 44.41 3.02 11.85 3.26
F 6 63.86 4.5 43.53 1.12 11.6 3.08
Canis lupis M 8 89.41 5.93 68.24 4.6 7.39 13.62
F 1 80.74 0 62.17 0 2.2 0
Vulpes vulpes M 7 48.91 3.7 32.02 2.68 0.03 1.71
F 4 46.76 2.63 32.33 2.38 0.58 2.92
Vulpes velox M 6 41.98 1.99 28.98 1.55 1.04 0.5
F 5 43.87 2.21 29.81 1.03 1.75 1.47
Urocyon cinereoargenteus M 5 45.27 1.95 28.18 0.92 6.8 1.61
? 2 41.48 0.64 27.31 1.07 5.3 0.16
Artiodactyls
Cervus elaphus M 1 112.19 0 99.36 0 34 0
F 1 101.69 0 94.92 0 20.4 0
Odocoileus vlrglnlanus M 1 68.74 0 58.8 0 27.5 0
F' 2 71.89 2.6 59.13 5.15 22.45 3.75
61
I


Table 5.2 (cont)
Taxon Sex n Blcondylar Breadth Palatal Breadth Height of TMJ
Artlodactyls (cont.) x (mm) sd x (mm) sd x (mm) sd
Odocoileus hemlonus F 2 77.62 4.75 68.43 4.01 25.5 2.83
Mazama americana F 2 54.15 2.45 9 52.46 3.68 21.1 6.22
Perissodactyls
Equus equus ? 3 105.66 8.33 82.33 3.14 89.65 2.61
Equus greyvl ? 2 103.71 3.56 83.35 2.0 81.8 4.38
Tapirus terrestrlus F 1 123.08 0 89.12 0 51.3 0
? 1 119.88 0 86.54 0 57.1 0
Taxon Sex n Molar to TMJ Est. Reg. II Length
Primates x (mm) sd x (mm) sd
Cacajao culvus F 1 21.15 0 14.13 0
Cercopithecus dlana F 1 20.73 0 14.18 0
Pongo pygmaeus F 1 49.09 0 33.18 0
Alouatta palliate F 2 96.32 2.6 68.87 0.69
Aotus lemurinus F 1 23.64 0 14.27 0
Gorilla gorilla F 2 55.68 8.34 38.55 5.86
Cebus capuchlnus F 3 23.34 3.13 14.47 1.1
Nyctlcebus coucang F 1 17.52 0 10.85 0
Ateles geoffroyl F 3 28.11 3.07 16.95 1.72
Calllthrix argentata F 1 11.43 0 6.74 0
Galago demidoff ? 1 7.55 0 5.26 0
62


Table 5.2 (cont.)
Taxon Sex n Molar to TMJ Est. Reg. II Length
Primates (cont.) x (mm) 9d x (mm) sd
Saguinus geoffroyl M 2 12.76 0.43 7.9 0.41
F 2 12.76 0.15 8.14 0.25
Pitheda pithecla F 2 20.5 0.5 11.33 0.35
Carnivores
Lynx canadensis M 1 38.57 ' 0 26.56 0
F 1 31.7 0 23.72 0
? 1 37.1 0 24.18 0
Lynx rufus M 4 32.32 2.75 21.71 2.48
F 4 34.16 3.06 20.4 1.8
7 2 30.82 0.84 18.32 0.48
Puma concolor M 2 52.25 6.89 37.3 0
F 6 48.47 3.14 31.86 2.18
? 2 47.39 0.87 31.7 0.81
Procyon lotor M 9 31.41 2.76 20.0 0.88
F 1 29.38 0 18,71 0
Ursus arctos M 1 83.94 0 45.67 0
? 1 79.64 0 45.7 0
Ursus americanus M 4 70.2 6 35.41 2.93
F 1 77.83 0 36.98 0
? 4 71.21 5.31 35.99 1.2
Canis latrans M 6 45.2 2.44 33.15 2.58
F 6 46.52 2.55 33.17 1.9
Canls lupis M 8 60.88 7.37 48.13 7.2
F 1 56.57 0 44.09 0
Vulpes vulpes M 7 30.81 1.16 20.3 0.55
F 4 28.95 1.24 20.23 0.94
Vulpes velox M 6 25.95 0.9 18.04 0.62
F 5 25.85 1.22 17.79 0.82
63


Table 5.2 (cont)
Taxon Sex n Molar to TMJ Est. Reg. II Length
Carnivores (cont.) x (mm) sd x (mm) sd
Urocyon clnereoargenteus M 5 30.54 3.28 19.75 2.01
? 2 29.02 0.63 19.74 1.51
Artlodactyls f
Cervus elaphus M 1 93.84 0 88.43 0
F 1 71.75 0 70.34 0
Odocolleus M 1 54.94 0 51.15 0
virglnianus F 2 56.65 3.71 49.91 6.13
Odocoileus hemionus F 2 57.37 10.12 54.23 1.96
Mazama americana F 2 43.31 8.32 45.66 10.25
Perissodactyls
Equus equus ? 3 108.28 4.94 96.85 3.11
Equus greyvi ? 2 105.93 2.8 96.75 0.63
Taplrus terrestrlus F 1 66.88 0 54.99 0
? 1 74.52 0 61.07 0
64


Table 5.3 Mean (x) and standard deviation (sd) of the geometric mean and one
variable used to calculate it The other variables used in this calculation are listed in
Tables 5.1 and 5.2.
Taxon Sex n Geometric Mean Temporal Leno Foramen th
Primates x (mm) sd x (mm) sd
Cacajao culvus F 1 15.60 0 19.38 0
Cercoplthecus diana F 1 16.80 t 0 18.35 0
Pongo pygmaeus F 1 45.02 0 39.84 0
Alouatta palliata F 2 72.11 1.71 79.95 0.49
Aotus lemurinus F 1 20.8 0 12.8 0
Gorilla gorilla F 2 59.51 2.29 55.42 3.63
Cebus capucbinus F 3 22.27 0.94 24.47 1.24
Nycticebus coucang F 1 15.85 0 17.73 0
Ateles geoffroyi F 3 22.94 1.47 22.16 1.39
Callithrix argentata F. 1 9.76 0 9.53 0
Galago demldoff ? 1 7.51 0 6,84 0
Saguinus geoffroyi M 2 11.32 0.08 11.16 0.21
Saguinus geoffroyi F 2 11.11 0.18 10.33 0.04
Pithecia plthecia F 2 16.47 0.76 17.10 0.36
Carnivores
Lynx canadensis M 1 28.62 0 29.16 0
F 1 24.98 0 28.19 0
? 1 27.73 0 23.6 0
Lynx rufus M 4 29.18 3.07 30.89 1.18
F 4 29.30 0.72 26.10 2.08
? 2 26.93 2.24 25.66 0.88
65


Table 5.3 (Cont.)
Taxon Sex n Geometric Mean Temporal Len Foramen qth
Carnivores (cont.) x (mm) sd x (mm) sd
Puma concolor M 2 41.87 2.34 45.39 4.88
F 6 43.61 1.19 42.44 2.08
? 2 40.24 1.68 41.55 0.06
Procyon lotor M 9 25:03 0.63 29.48 2.80
F 1 26.22 0 27.24 0
Ursus arctos M 1 85.20 0 83.95 0
? 1 62.05 0 74.5 0
Ursus americanus M 4 54.20 3.17 63.35 5.93
F 1 55.97 0 72.04 0
? 4 56.92 0.01 65.69 3.78
Canls latrans M 6 33.61 1.14 42.63 3.57
F 6 32.01 1.05 41.53 1.92
Cants lupis M 8 45.97 2.75 55.71 5.64
F 1 42.57 0 53.34 0
Vulpes vulpes M 7 24.62 2.99 29.99 1.75
F 4 24.72 2.86 28.54 1.52
Vulpes velox M 6 20.48 0.82 24.96 0.70
F 5 21.38 0.62 24.78 1.50
U. cinereoargenteus M 5 22.38 0.97 28.35 2.55
? 2 21.37 0.55 26.61 1.82
66


Table 5.3 (cont.)
Taxon Sex n Geometric Mean Temporal Foramen Length
Artiodactyls x (mm) sd x (mm) sd
Cervus elaphus M 1 87.32 0 90.9 0
F 1 85.38 0 87.02 0
Odocoileus M 1 50.77 0 49.03 0
virglnlanus
F 2 51.53 1.05 54.88 2.14
Odocoileus hemionus F 2 55.88 2.05 52.73 0.66
Mazama americana F 2 41.27 3.47 46.68 5.78
Perissodactyls
Equus equus ? 3 67.73 1.90 52.96 5.50
Equus greyvi ? 2 68.26 3.14 54.59 4.49
Taplrus terrestrius F 1 75.89 0 66.32 0
? 1 75.49 0 68.15 0
67


In order to more easily understand these graphs, the data were standardized
against molar row length. This has the effect of reducing differences in size and
shape between the different taxa.
Figure 5.1 shows that the molar length of all species fit well within the
predicted length of Region II. Hypothesis 1, which states the molars should fit
within the predicted length of Region H, is therefore accepted.
Many species also have premolars that fall within the predicted length of
Region II. This is not surprising given the wide range of premolar function that is
represented across mammals. However, what is unusual is that those species with
the typical grinding dentition do not necessarily have premolars that fit into this
region. For example, Cervus elaphus has premolars that fall well outside of
Region II, whereas, Mazama americana's premolars all fall within this region. In
addition, a primate, Saguinus geoffroyi, has premolars that fall well within the
region. It appears that there is no apparent pattern in regard to premolar length
and the predicted length of Region n. This may be due to the wide variety of
functions that premolars serve. Perhaps there is too much variation within the
function of this type of tooth across mammals to be able to predict its location
relative to Region II length.
68


T. terrestrius
E. greyvl
E. equus
0. vlrglnlanus
O. hemlonus
M. amerlcana
C. elaphus
V. vulpes
V. velox
U. cinereoargenteus
U. arctos
U. americanus
P. lotor
P. concolor
L rufus
L. canadensis
C. lupis
C. latrans
S. geoffroyi
P. pygmaeus
P. pithecia
N. coucang
G. gorilla
G. demidoff
C. diana
C. capuchinus
C. calvus
C. argentata
A. palliata
A. lemudnus
A. geoffroyi
O1 234567B9 10
Figure 5.1 Plot comparing postcanine dimensions to predicted Region II length. All distances have
been standardized against molar length. Therefore, the molar dentition fills the distance between 0
and 1 with the first molar being located at 1. The black boxes represent the anterior end of Region
II. Region II, therefore, extends from 0 to this anterior position. The white boxes represent the
anterior end of the premolars, therefore, the total length of the postcanine dentition extends from 0
to this anterior position.
69


Fused vs. Unfused
Mammals with an unfused symphysis are expected to have a theoretically
shorter Region II length due to a laterally located muscle resultant force.
According to Hypothesis 2, it is expected that the ratio between predicted
Region II length and observed molar length will be higher in mammals with an
unfused symphysis because of their theoretically shorter Region II length. A t-
test between predicted Region II length to observed molar length by species with
a fused or unfused symphysis shows that the means for these two groups are
significantly different. The mean for the fused group is 1.75 with a standard
deviation of 0.35 and 2.54 for the unfused group with a standard deviation of 0.81
(t = -5.13; p < 0.01). This is also represented graphically in Fig. 5.2. This plot
shows that mammals with an unfused symphysis tend to have higher predicted
Region II length/observed molar length ratios. Hypothesis 2 is accepted.
Allometric Analysis
An allometric analysis was performed on the variables used to calculate the
predicted length of Region II. It is important to assess how changes in shape with
size affect these variables because of the wide range of cranial sizes represented
in this sample. Figure 5.3 displays bivariate plots of log-transformed data
showing the relationship between the geometric mean, bicondylar breadth, palatal
70


Predicted/Observed Molar Length
3.75
3.5
3.25 1
3
2.75
2.5
2.25
1.75 -
1.5 -
1.25
Fused
Unfused
Figure 5.2 Box plot representing the means of molar length divided by
the geometric mean for mammals with a fuse and unfused symphysis.
These plots show that mammals with an unfused symphysis have a higher
predicted to observed molar length ratio.
71


breadth, height of the TMJ, and molar row length. Table 5.5 lists the slope and Y-
intercept for these variables.
Molar length scales with positive allometry. Its 95% confidence intervals
range from 1.19-1.36. This means that as cranial size increases molar length
increases at an even greater rate. This is surprising considering that increasing the
length of the molars decreases the likelihood that they will fit into Region II.
Other variables must be configured in order to increase the length of this region.
Bicondylar breadth scales with negative allometry. Their 95% confidence
intervals range from 0.81-0.89 and 0.87-0.96, respectively. As cranial size
increases bicondylar breadth does not increase as rapidly. Decreasing the length
between the condyles has the effect of increasing the length of Region II.
TMJ height scales with strong positive allometry. Its 95% confidence
intervals range from 2.35-3.52. However, there is only a weak correlation
between height of the TMJ and cranial size (r2 = 0.62). This is expected
considering that some species with very large cranial sizes, such as horses, have
very tall TMJs, whereas, other species with large crania have very low TMJs,
such as bears. Height of the TMJ is extremely variable within mammals.
The anteroposterior position of the MRF is another variable that affects
the distribution of Region II. The A-P position of the MRF is assumed to be
located at the distal end of the molar tooth row, therefore, the length from the
TMJ to the last molar represents this distance. This variable scales isometrically
72


Figure 5.3 Bivariate plots of log-transformed data showing the relationship between
cranial size (represented by the geometric meap) and other masticatory variables
relevant to the distribution of Region II (+ indicate species without symphyseal
fusion, indicate species with fusion), See Table 5.8 for regression parameters.
73


tn (Height of TMJ) ln (Palatal Breadth)
Figure 5,3 (cont.)
74


Figure 5,3 (cont.)
75


Table 5.4 Variables regressed on the geometric mean to assess allometric
relationships.
Variable (Regressed on GM) Slope Y-intercept 95% Confidence intervals for Slope
Molar Length 0.88 1.28 -0.84 1.19-1.36
Bicondylar Breadth 0.93 0.85 0.51 0.81-0.89
Palatal Breadth 0.94 0.92 0.23 0.88 0.96
Height of TMJ 0.62 2.93 -3.85 2.35-3.52
A-P Position of MRF 0.93 0.99 0.10 0.95-1.04
76


with the geometric mean; its 95% confidence intervals include a slope of 1. The
length from the last molar to the TMJ and cranial size increase at the same rate.
Palatal breadth also scales very close to isometry. Its 95% confidence
intervals range from 0.88 to 0.96. As cranial size increases, palatal breadth
increases at about the same rate.
These allometric analyses have allowed us to look further into the data and
explore how these variables change in relation to cranial size. This helps us to
understand the relationship between cranial size and the masticatory system
variables that are responsible for the distribution of Region II.
77


CHAPTER 6
DISCUSSION
The goal of this study is to test the constrained model on a wide variety of
mammals in order to assess systematic differences in craniofacial configuration
that may be the result of mandibular symphyseal fusion. Of particular concern is
the distribution of Region II. This region should envelope the grinding dentition
because this is where highest magnitude bite forces are produced and these are the
teeth most suitable for these forces. The following is a summary of my
predictions:
> 1. Molar length will be shorter than the predicted length of
Region II for both groups of mammals.
> 2. The ratio between predicted Region II length and observed
molar length is predicted to be higher in mammals with an
unfused symphysis because of the theoretically shorter Region II
length.
The following is a summary of the results obtained from this study:
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> 1. The molar row of all species fit well within the predicted length
of Region II. Hypothesis 1 is accepted.
> 2. The ratio between predicted Region II length and observed
molar length is higher in mammals with an unfused symphysis.
Hypothesis 2 is accepted.
The results from this study support the general applicability of the
constrained model for both mammals with a fused and unfused symphysis. This
model is able to explain the interactions between some craniofacial variables of a
broad range of mammals including primates, carnivores, perissodactyls, and
artiodactyls. This is in accordance with results from previous studies.
Support for Previous Research
The main assumption of the constrained model is that the TMJ should not
be subjected to joint distraction (Greaves, 1978). This causes limitations on
masticatory system form and function. One such limitation is the distribution of
the Region II. According to the model, Region II should envelope at least all of
the molars. Spencers (1999) morphometric analysis of anthropoid cranial
configuration provides evidence in support of this. Anthropoids appear to exhibit
craniofacial form that is consistent with selection against TMJ distraction by
having the parameters that influence the distribution of the region be configured
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in such a way so that at least all molars are maintained within this region. This is
consistent with results from the present study. Results from the present study also
indicate that the molars as well as some of the premolars are located within
Region II.
The goal of the present study was to look at the broad-scale applicability
of the constrained model to mammals. Not only were a wider range of taxa used
than in previous studies, this study also focused on a craniofacial feature that
results in fundamental differences in muscle recruitment patterns, the mandibular
symphysis. Previous studies involving galagos, lemurs, and dogs, all of which
have an unfused symphysis, have found that these species exhibit different
working-side to balancing-side muscle force ratios (Leibman and Kussick, 1965;
Hylander, 1979; Dessem, 1989; Hylander et. al., 1998, 2002). Those species with
an unfused symphysis recruit less balancing-side muscle force. Recruiting less
balancing-side muscle force means that the muscle resultant force cannot lie in the
midline. Having a MRF more laterally located consequently reduces the length of
Region II. Because mammals with an unfused symphysis have a theoretically
shorter Region II they are expected to have a higher ratio between predicted
Region II length and observed molar length. This was indeed found to be the
case. Mammals with an unfused symphysis appear to differ in a systematic way
from those with fusion.
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There has only been a small number of mammalian species whose muscle
recruitment patterns have been studied (Leibman and Kussick, 1965; Hylander,
1979; Dessem, 1989; Hylander et. al., 1998,2002). The finding from this study
provides indirect evidence for the previous research on muscle recruitment
patterns. It can be inferred from this finding that all species with an unfiised
symphysis may recruit less balancing-side muscle force. This would cause them
to have a shorter Region II length which would create a higher ratio between
predicted Region II length and observed molar length as was found in this study.
Allometric Analyses
An allometric analysis has allowed us to understand how changes in size
affect the masticatory system configuration of such a wide range of mammals. It
was found that molar length and height of the TMJ scale with positive allometry,
bicondylar breadth scales with negative allometry, and palatal breadth and the A-
P position of the MRF scale isometrically.
Molar length was found to scale with positive allometry. This means that
as cranial size increases, the molars are increasing in length more rapidly than
expected for isometry. So as cranial size increases the molars are increasing at an
even greater rate.
One would expect that the molars would not increase at this greater rate
given the constraint of Region II length. According to the model, the molars
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should fall within this region in order to avoid TMJ distraction. However, there
are other factors that act to influence this variable, such as diet.
Diet has been shown to be closely linked to body size, particularly within
primates (Fleagle, 1988). An animals teeth are what allow it to meet its
nutritional requirements. Therefore, molar size is constrained not only by the
need to avoid TMJ distraction but also by diet. This becomes even more complex
when one considers that diet can also be a function of body size.
Within primates, the natural physiological break between insectivores and
folivores occurs at 500 grams and is known as Kays threshold (Kay, 1975;
Fleagle, 1988). In general, folivorous primates have body weights that are no less
than 500 grams and insectivores tend to weigh less than this limit. This is because
as body size increases, metabolic requirements change. A larger animal actually
has relatively lower energy requirements than a smaller one. Although leaves are
generally lower in energy yield than insects or fruit, a large animal can afford this
because they need less energy per kilogram of mass than a small animal (Fleagle,
1988). Results from the allometric analysis of this study show that as cranial size
increases (and therefore body size) molar length is increasing even more rapidly.
This may be because even though a larger animal does not have as high of energy
needs it still requires larger molars in order to process vegetation which is much
tougher than either insects or fruit.
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This idea also pertains to artiodactyls and perissodactyls. Both of these
groups have very large grinding dentition. This is thought to be an adaptation to
the tough grass material that comprises their diet (L.M. Spencer, 1995). Having
larger molars increases the surface area of the grinding dentition thereby allowing
a more efficient breakdown of the tough vegetation. Although this can explain
why molar length increases more rapidly than body size this does not explain how
the larger molars are able to fit within Region II. Other craniofacial variables
must be working to increase the length of Region H.
TMJ height may be one of those variables. It scales with positive
allometry meaning that it also increases at an even greater rate that cranial size.
Many of the larger animals such as horses, sheep, and deer have very high TMJs.
This has the effect of orienting Region II more anteriorly which increases its
length.
Bicondylar breadth also seems to be working to increase the length of
Region II as cranial size increases. This is because a decrease in bicondylar
breadth increases the length of this region. This parameter scales with negative
allometry; it does not increase as rapidly as cranial size. Therefore, having
bicondylar breadth not increase as rapidly as cranial size may help the molars to
be maintained within Region II even as cranial size increases.
Overall it appears that as cranial size increases across mammals, there are
variables that are configured in such a way to increase the length of Region n.
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This allows the molars to be maintained within this region even if they are
increasing at a rate greater than cranial size.
Biomechanical Implications for Diet
Spencer (1999) proposes the idea that morphological patterns within the
masticatory system may stem from the selective trade-off between increasing bite
force magnitudes and avoiding joint distraction. Joint distraction is unavoidable
when biting occurs in Region III (Greaves, 1978). Selection should favor a
morphology that does not allow teeth to be located within this region. This idea
has been supported by the present study and the previous research discussed
above. The molars were found to lie only within Region II. However, some
species which require high magnitude bite forces to be produced on either the
incisors or the canines would benefit from having the teeth located more
posteriorly. A more posterior position for the dentition and a relatively anterior
position of the superficial masseter and anterior temporalis muscles enable greater
force production in Region I (the region where the incisors and canines are
expected to be located according to the constrained model) (Spencer and Demes,
1993; Spencer, 1999). This could potentially cause some of the postcanine
dentition to be moved back into Region III.
Some groups that may benefit from greater force production on their
anterior dentition include callitrichids, some pitheciines, Cebus, and some of the
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carnivores (Spencer, 1999). These groups are specialized for intensive force
production on either the incisors or the canines. Carnivores require especially
high levels of force production on their canines as these are the teeth that are
involved in capturing and killing their prey. The TMJ of this group is also
particularly well-suited to capturing prey. It is locked into the glenoid fossa so
that it can withstand the high magnitude forces that are inevitable as the prey
struggles to free itself from the grip of the predator. This is likely an adaptation to
limit joint distraction.
Selection can limit teeth being located within Region HI through changes
in the configuration of other variables of the masticatory system. Some changes
that would be beneficial include decreasing bicondylar breadth or increasing
palatal breadth. Increasing the height of the TMJ would also be beneficial. All of
these have the effect of increasing the length of Region II which may enable the
molars to be maintained with this region.
The avoidance of TMJ distraction places constraints on masticatory
system morphology. Mandibular symphyseal fusion is another constraint. Fusion
constrains the location of the MRF because it allows more balancing-side muscle
force to be recruited. This has the potential effect of placing the MRF in the
midline. However, the MRF must move laterally when biting in Region II in
order to be maintained within the triangle of support (Spencer and Demes, 1993;
Spencer, 1995,1998,1999). A reduction in balancing-side muscle activity
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enables this to happen and is necessary to avoid TMJ distraction. Although
having symphyseal fusion allows more balancing-side muscle force to be
recruited, the constrained model limits this in order to maintain the resultant force
within the triangle of support.
Maintaining an equal balancing-side to working-side muscle force ratio
(which would place the MRF in the midline) is acceptable within this model when
biting in Region I. This is because Region I envelopes a midline MRF. What is
interesting, however, is that none of the carnivores, which require high magnitude
force production on their anterior dentition, have a fused symphysis. It is
expected that this group would benefit from having a fused symphysis in order to
recruit more balancing-side muscle activity.
One possible explanation can be approached from a behavioral standpoint.
When a carnivore captures its prey in its jaws it may be biting equally with all of
its anterior dentition. This would make both sides of the jaw the working side and
may allow the predator to maximize force production. They, therefore, do not
need to have symphyseal fusion because they are already maximizing the amount
of force that can be produced. Further behavioral research would serve to clarify
this issue.
Some species which need high magnitude forces produced on their molars,
such as the artiodactyls, also do not have a fused symphysis (Greaves, 1978). It is
possible that selection has favored another aspect of their craniofacial morphology
86


that enables them to efficiently process their food. One such morphology could
be their large selenodont grinding dentition. Having this larger surface area
allows them to more efficiently process the tough vegetation that is characteristic
of their diet.
Colobines, which have a fused mandibular symphysis, also have a diet that
consists of tough vegetable matter (Fleagle, 1988). Perhaps having a fused
symphysis enables them to generate a higher magnitude muscle resultant force on
their molars than the artiodactyls, which allows them to break down their food
material in just as efficient a manner. If this is the case then symphyseal fusion
would be highly advantageous for this group.
This issue is not a simple one due to the numerous confounding variables
that are possible within not only the masticatory system but also the digestive
system. Colobines also have a gut morphology that is specialized to break down
vegetation, including large complex stomachs (Fleagle, 1988). It is not only the
molars that work to break down the food, but their specialized stomachs as well.
The issue of symphyseal fusion and its impact on diet is extremely
complex. There are numerous variables within not only the masticatory system
but the body as a whole that work together to produce an animal that is finely
adapted to its diet and environment. Further research is needed before any
definitive statements can be made regarding symphyseal fusion and diet.
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Conclusion
Overall it appears that mammals with a fused symphysis do differ in
systematic ways from those with an unfrised symphysis. Both groups have molars
that fit within the estimated length of Region II. However, the fact that mammals
with an unfused symphysis have higher predicted Region II length to observed
molar length ratio shows that these groups differ in craniofacial configuration that
is most likely the result of different working- to balancing-side muscle force ratios
that are in turn the result of differing symphyseal morphologies.
This has important ramifications for the evolution of symphyseal fusion.
Variables within the masticatory system interact with one another to produce a
unique configuration. However, evolution can only be so creative due to the
constraints of these variables having to interact with one anther in order to
maintain the molars within Region II. This is in addition to numerous other
constraints that can be imposed upon this system.
The molars should always be maintained within Region II in order to
maximize masticatory force production. Extinct species both with and without
fusion, should have craniofacial variables, such as bicondylar and palatal breadth,
height of the TMJ, and the A-P position of the MRF that also work in conjunction
to increase the length of Region II. Looking to see if these features are correlated
with early symphyseal fusion would help us to understand how fusion affects the
masticatory system as a whole and why it came to be in the first place.
88


Cranial form and function are extremely important issues to biological
anthropologists. Understanding craniofacial form and the changes in morphology
that have occurred through time is dependent on this biomechanical model. This
model allows us to understand why certain variables are distributed in the way
they are, such as the placement of the molars within the cranium, or the height of
the TMJ above the occlusal plane. This can be used to explain much of the
variation we see in cranial form in both extinct and extant forms, and in both
groups with and without mandibular symphyseal fusion.
89


LITERATURE CITED
Allard, M.W., B.E. McNiff, and M.M. Miyamoto (1996). Support for interordinal
eutherian relationships with an emphasis on primates and their archontan
relatives. Molecular Phylogenetics and Evolution 5:78-88.
Beecher, R.M. (1977). Function and fusion of the mandibular symphysis.
American Journal of Physical Anthropology 47:325-336.
Beecher, R.M. (1979). Functional significance of the mandibular symphysis.
Journal of Morphology 139:117-130.
Cartmill, M. (1975). Daubentonia, Dactylopsila, woodpeckers, and
klinorhynchy. In R.D. Martin, G.A. Doyle, and A.C. Walker (eds.): Prosimian
Biology. London: Duckworth, pp. 655-670.
Clutton-Brock, T.H. (1974). Primate social organization and ecology. Nature
250:539-542.
Clutton-Brock, T.H. and P.H. Harvey (1979). Comparison and adaptation.
Proceedings of the Royal Society of London B 205:547-565.
Davis, D.D. (1955). Masticatory apparatus in the spectacled bear Tremarctos
ornatus. Fieldiana-Zoology 37:25-46.
Darroch and Mosimann (1985). Canonical and principal components of shape.
Biometrika 72:241-252.
Demes, B. and N. Creel (1988). Bite force, diet, and cranial morphology of fossil
hominoids. Journal of Human Evolution 17:657-670.
Dessem, D. (1989). Interactions between jaw-muscle recruitment and jaw-joint
forces in Canis familiaris. Journal of Anatomy 164:101-121.
Dessem, D. and Druzinsky, R.E. (1992). Jaw-muscle activity in ferrets, Mustela
putorius furo. Journal of Morphology 213:275-286.
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Full Text

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FUSION OF THE MANDffiULAR SYMPHYSIS AND CRANIAL EVOLUTION IN MAMMALS by Rachel A. Hogard B.A., Washington University in St. Louis, 1999 A thesis submitted to the University of Colorado at Denver in partial fulfillment of the requirements for the degree of Master of Arts Anthropology 2003

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This thesis for the Master of Arts degree by Rachel A. Hogard has been approved by A. Spencer David Tracer -=Date

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Hogard, Rachel A. (M.A., Anthropology) Fusion of the Mandibular Symphysis and Cranial Evolution in Mammals Thesis directed by Assistant Professor Mark A. Spencer ABSTRACT Hypotheses regarding masticatory force production and the distribution of craniofacial variables are based on biomechanical models. One such model, the constrained model, has been particularly influential within the field of jaw biomechanics. This model is tested on a wide range of mammals, both with and without mandibular symphyseal fusion, to assess systematic differences in craniofacial configuration that may exist between these two groups. These groups are expected to differ in systematic ways due to differing muscle recruitment patterns. Mammals with an unfused symphysis recruit less balancing-side muscle force. This has important implications for the distribution of craniofacial variables within this model. Of particular interest is the distribution of the molars. The molars are expected to fit into the area of the cranium where highest magnitude bite forces can be produced. Results indicate that for both groups of mammals, the molars are indeed maintained within that region However, there are other configurations of the masticatory system that appear to differ between. mammals with an unfused symphysis and those with symphyseal fusion. Therefore, these two groups do appear to differ in systematic ways. This has important implications for the evolution of symphseal fusion and the role fusion may play in dietary adaptation. This abstract accurately represents the content of the candidate's thesis. I recommend its publication. iii signed MarklrSpencer

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DEDICATION I dedicate this thesis to my mother and father for their unfaltering support while I attended graduate school. I also dedicate this thesis to Karl Kesti for providing me with generous understanding and support throughout this process.

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ACKNOWLEDGEMENT My thanks and gratitude to my advisor, Mark Spencer, for his help and dedication to me during this process. I would also like to thank Cheri Jones of the Denver Museum of Nature and Science and Rosanne Humphrey of the University of Colorado at Boulder Natural History Museum.

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CONTENTS Figures ................................................................................................. v1n Tables ..................................................................................................... x CHAPTER 1. IN"TRODUCTION ............................................................................... 1 2. BIOMECHANICAL MODELS OF THE MASTICATORY SYSTEM ................................................................ 7 The Mandible as a Lever ............................................................. 7 The Constrained Model ............................................................. 1 0 Modifications to the Constrained Model ................................... 19 Muscle Force Recruitment ......................................................... 25 Why Fuse the Mandibular Symphysis? ..................................... 29 Stiffness Hypothesis .......................................................... 30 Strength Hypothesis ........................................................... 33 3. HYPOTHESES AND STUDY DESIGN .................................................. 37 The Effects of an Unfused Symphysis on the Model ........................ 39 Specific Predictions ........................................................................... 42 Study Design ..................................................................................... 42 Tests ........................................................................................... 43 4. MATERIALS AND METHODS .............................................................. 45 Vl

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Sample ........................................................................................... 45 Data Collection Process ..................................................................... 48 Measurements .................................................................................... 49 Size .................................. ........................................................ 52 5. RESULTS .................................................................................. ........ 56 Fused vs. Unfused .............................................................................. 70 Allometric Analysis ........................................................................... 70 6. DISCUSSION ........................................................................................... 78 Support for Previous Research .......................................................... 79 Allometric Analysis ........................................................................... 81 Biomechanical Implications for Diet.. ............................................... 84 Conclusion ......................................................................................... 88 LITERATURE CITED ...................................................................................... 90 Vll

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FIGURES Figure 1.1 Phylogenetic hypothesis concerning the relationships between the orders within eutherian mammals ................................................................ 4 2.1 Occlusal view of the mandible with a midline muscle resultant force ...... 13 2.2 For more posterior bite points, the triangle of support may not envelope a midline muscle resultant force producing tensile forces in the working-side joint .............................. : ............................................. l4 2.3 Occlusal view of the mandible showing the distributions of Regions I, II, and III for a mammal with a fused mandibular symphysis ............... 16 2.4 Predicted bite force and joint reaction force values using the assumptions ofthe constrained model. ...................................................... l8 2.5 Occlusal view ofthe mandible demonstrating how bicondylar breadth affects the length ofRegion II ...................................................... 23 2.6 An additional variable that alters Region II length is the anteroposterior position of the midline muscle resultant force ........................... 24 2. 7 Occlusal view of mandible showing how palatal breadth affects the length of Region II ............................................................................... 26 3.1 Occlusal views of mandible showing the effects of changes in the position of the muscle resultant force on the distribution of Region II .... .40 4.1 Illustration of five variables used in the calculation ofRegion II length .. 51 4.2 Equation for estimating the predicted length ofRegion II ........................ 53 5.1 Plot comparing postcanine dimensions to predicted Region II length ...... 69 viii

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5.2 Box plot representing the means of molar length divided by the geometric mean for mammals with a fused and unfused symphysis ......... 71 5.3 Bivariate plots oflog-transformed data showing the relationship between cranial size and other masticatory variables relevant to the distribution of Region II ...................................................................... 73 lX

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TABLES Table 4.1 Taxa indicated in study with a fused symphysis ...................................... .46 4.2 Taxa indicated in study with an unfused symphysis ................................ .47 5.1 Mean and standard deviation of tooth row length ..................................... 57 5.2 Mean and standard deviation of variables used to calculate predicted Region II length ......................................................................... 60 5.3 Mean and standard deviation of the geometric mean and one variable used to calculate it.. ...................................................................... 65 5.4 Variables regressed on the geometric mean to assess allometic relationships ........................................................................................... 76 X

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CHAPTER 1 INTRODUCTION Diet plays an influential role in almost all aspects of an organism's life. The selection pressures associated with the acquisition and processing of food have an influence on many aspects of a species' anatomy and behavior (Cartmill, 1975; Clutton-Brock, 1974; Rylander, 1979a; Hiiemae, 1984; Fleagle, 1988; Smith, 1993). Selection pressures range from the processes involved in finding food to being able to efficiently process this food once ingested. These pressures can change as a result of a changing environment. The ability of a species to adapt to these changing selection pressures is essential to its survival. The study of the masticatory system is therefore crucial to our understanding of the evolutionary relationship between diet and morphology as it is this structure that enables food acquisition and processing. This is because the masticatory system, including the jaws, teeth, and craniofacial muscles, is a direct link between an animal's external environment and its internal requirements. Previous studies have established that features of masticatory morphology are functionally correlated with dietary pattern (Kay, 1975; Kay and Covert, 1984; Lucas et. a!., 1986; Fleagle, 1988). This association between morphology 1

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and diet in extant species is often used to guide our explanations of the fossil record (Hiiemae, 1984). Therefore, studying this system can lead to a greater understanding ofthe functional role morphology plays in the life of the animal and the evolution of a species. Forces are generated within the masticatory system to break down food objects. Cranial configuration (i.e., the interaction between morphological variables of the cranium) is known to influence force production (Greaves, 1978; Smith, 1978; Spencer, 1998, 1999). Hypotheses regarding force production and the factors that relate to this are based on biomechanical models (Roberts and Tattersall, 1974; Greaves, 1978; Smith 1978; Spencer, 1999). From these biomechanical models functional predictions are generated about ways in which the masticatory system should be configured. Testing these predictions helps to increase our understanding of morphological design and change over time . While biomechanical models are needed to understand masticatory system function and evolution, we frequently do not know how widely applicable they are. It is important, therefore, to look at a wide range of mammalian taxa in order to understand the broad applicability of these models to the masticatory system. Mammals, in general, and primates in particular, differ drastically in the morphological structure and configuration of their masticatory system. One such structure that has marked effects on this system is the mandibular symphysis. The 2

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mandibular symphysis is the structure that separates the two dentaries in the sagittal plane. An unfused mandibular symphysis, in which the two dentaries are joined by non-ossified connective tissue, is the ancestral condition among mammals. However, symphyseal fusion has evolved independently in several mammalian lineages (Greaves, 1988). Within primates, extant prosimians retain the primitive condition of having an unfused mandibular symphysis, though they vary in symphyseal morphology, while anthropoid primates have evolved symphyseal fusion (Beecher, 1977; Rylander, 1979b; Ravosa, 1991). Other groups of mammals that have evolved the derived condition of fusion include perissodactyls, hyracoids, families of chiropterans and several artiodactyl taxa (Suidae, Tayassuidae, Hippoptomidae, and Camelidae) (Beecher, 1977). Figure 1.1 outlines the relationships between the orders within eutherian mammals. This phylogenetic hypothesis shows that fusion has evolved multiple times within the order Mammalia in several different lineages (Allard et. a/., 1996). Some orders that contain families with symphyseal fusion also include families without fusion. The repeated evolution of symphyseal fusion within mammals has generated substantial interest concerning the adaptive significance (and biomechanical consequences) of this feature (Beecher, 1979; Rylander, 1979a, 1979b, 1984; Scapino, 1981; Greaves, 1988, 1993; Ravosa, 1991, 1993; Rylander and Johnson, 1994; Ravosa and Rylander, 1994; Rylander et. al., 1998; Ravosa et. a/., 2000). The biomechanical consequences of symphyseal fusion within the 3

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Lagomorpha Rodentia Macros eel idea r------Artiodactyla Cetacea Perissodactyla Hyraooidea Sirenia Proboscidea Dermoptera Chiroptera Primates Sca:nden t i a E!3entata Pholidota Carnivora Tublidentata Insectivora Out group Fig. 1.1 Phylogenetic hypothesis concerning the relationship between the orders within mammals . This strict consensus tree was obtained from 88 morphological characters from fossils and recent evidence (Allard et. al., 1996). indicates orders containing some families with symphyseal fusion. 4

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various mammalian taxa can provide a greater understanding of the adaptive significance of this feature. Experimental data indicate that there are differences in masticatory muscle recruitment patterns between animals with a fused and unfused symphysis. Regardless of how symphyseal fusion leads to these differences, they have important theoretical implications because it is unknown how these differences may affect other aspects of masticatory function or whether they lead to systematic differences in overall cranial morphology. The goal of this study is to test predictions regarding differences in masticatory system configuration and how this differs between mammals with a fused and unfused symphysis. These groups are known to differ in systematic ways in masticatory system morphology (Beecher, 1977; Hiiemae, 1984; Greaves, 1988; Spencer, 1999). An analysis ofthe functional significance ofthese differences should improve our understanding ofthe adaptive basis of cranial form in primates, including humans. Adaptive hypotheses must be tested using the comparative method (Clutton-Brock and Harvey, 1979). In this method, features are inferred to be associated if they evolved together repeatedly in several independent lineages. An understanding of the relationship between symphyseal fusion and masticatory system configuration must be based, therefore, on identifying commonalities among multiple lineages that have evolved symphyseal fusion. This study 5

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quantifies cranial form in a wide range of mammals with fused and unfused mandibular symphyses in order to assess the biomechanical consequences of symphyseal fusion. 6

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CHAPTER2 BIOMECHANICAL MODELS OF THE MASTICATORY SYSTEM The functional significance of masticatory system morphology has long been of great interest within the field of biological anthropology. Descriptions of this system often rely on simplified models. The earliest studies (Gysi, 1921, Maynard-Smith and Savage, 1959) argued that the mandible acts as a lever. This model was challenged in the early 1970's (e.g., Roberts and Tattersall, 1974) but is now the most generally accepted biomechanical analogy for the mandible (Rylander, 1975; DuBrul, 1977; Greaves, 1978; Smith, 1978). Other models, such as the constrained model, have expanded upon the principles of the lever model to include more complex factors. The Mandible as a Lever The mammalian masticatory system has historically been modeled as a lever (Gysi, 1921; Maynard-Smith and Savage, 1959; DuBrul, 1977; Greaves, 1978, 1982, 1988; Rylander, 1975, 1977; Smith, 1978, Demes and Creel, 1988). In this model, the skull is analyzed in the sagittal plane. The condyle acts as the fulcrum, the masticatory muscles are the applied force, and the bite point is the 7

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resistance. The masticatory muscles apply an adducting force to the mandible, which serves to close the jaw. This is resisted by reaction forces at the temporomandibular joints (TMJ) and the bite point. The muscles responsible for closing the jaw are the masseter, medial pterygoid, and temporalis. All of the muscle, TMJ, and bite forces are typically simplified into individual force vectors (Spencer, 1995). The vector representing the sum of the individual muscle force vectors from both sides of the head is termed the muscle resultant force (MRF). During the 1950's and 1960's this model was generally well accepted (Maynard-Smith and Savage, 1959) and was used in evolutionary interpretations of cranial form in various taxa. However, some argued that this is a mechanically inefficient system and proposed alternative models (Gingerich, 1971; Roberts and Tattersall, 1974). Those who opposed the lever model assumed that the process of adaptation should result in greater efficiency of the masticatory system than they believed the lever model allowed. Smith (1978) points out, however, that the process of evolution is not necessarily a process of optimal design, rather, it probably results in species that are just a little better than their competitors. The alternative models centered aroWld the idea that during biting, the jaw fimctions as a link between the adductor muscle force and the bite force (Gingerich, 1971). The argument that the mandible does not fimction as a lever and instead acts as a 'link' was based on two assertions: (1) the resultant ofthe forces produced by the masticatory muscles (i.e., the muscle resultant force) 8

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always passes through the bite point; (2) the condylar neck and/or TMJ is poorly suited to withstand reaction forces. These two assertions have since been disproved. The muscle resultant force (MRF) does not always pass through the bite point (Greaves, 1978). It has been proposed that the MRF actually lies in the midline between the last molars during equal working side and balancing side muscle activity and that differential activity between the muscles produces mediolateral movement ofthe MRF (Rylander, 1985; Spencer, 1998). Also in opposition to the 'link' models, electromyographic data have demonstrated that large condylar reaction forces do exist and that the condyle is strong enough to withstand reaction forces during lever action (Rylander, 1975). Early work within the field of biomechanics suggested that the strongest force is exerted at the balancing condyle (i.e., the TMJ on the side opposite from the bite point) (Gysi, 1921). It was theorized that the balancing-side muscle force, which gets transmitted through the mandibular symphysis to the working side, reduces the force at the working-side condyle. Modelling data showed a reduction in working-side joint reaction force as the bite point moved distally along the tooth row (Gysi, 1921). When food is crushed at the premolars, the force at the working-side condyle is slightly reduced compared to more anterior bite points. The working-side joint reaction force gets neutralized as the bite point moves distally to the second molar. It was also hypothesized that hard 9

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foods cannot be crushed on the third molars because the downward pull of the working-side TMJ could lead to joint distraction. Later studies have indeed confirmed that condylar forces are greater on the balancing side than the working side (Rylander, 1979a). This work is particularly important to later research for two reasons. First, it supports the hypothesis that the mandible can be modeled as a lever. Although alternative models have been offered to explain the biomechanical actions of the mandible, the lever is still upheld as the dominant model today. Second, it shows that balancing-side muscles contribute to the muscle resultant force, which gets transmitted through the symphysis to the working side. These two points have had important implications in the development ofbiomechanical models of the masticatory system and explaining the functional significance of the mandibular symphysis. The Constrained Model Walter Greaves' constrained model of the jaw lever system of ungulates has been particularly influential in the field of jaw biomechanics. This model was developed based on the view that in Homo muscle forces on both sides of the head are transmitted to the working tooth row and that there are reaction forces at both TMJs (Greaves 1978). This model is applied, however, to selenodont artiodactyls, which differ drastically in their mandibular morphologies. Homo, as 10

PAGE 21

an anthropoid primate, has a fused mandibular symphysis. Selenodont artiodactyls, with the exception of the camelids, have an unfused mandibular symphysis (Greaves, 1978). Although Greaves recognized this, the model is based on an assumption that applies only to those mammals with a fused symphysis. It was not known at the time that differences in muscle recruitment patterns between fused and unfused species could have an effect on the model, which will be discussed later. In Greaves' model, masticatory forces are examined in the occlusal view (Fig. 2.1 ). During mastication the mandible is pulled toward the skull by the adductor muscles, the masseter, temporalis, and medial pterygoid. These muscle forces are resisted by reaction forces at three regions of the craniwn: the bite point, the working side TMJ, and the balancing side TMJ (Greaves, 1978). It is these three points that form what has been termed the triangle of support. Because there are multiple muscles on both sides of the head working to adduct the mandible, these muscle forces can be combined into a single vector termed the muscle resultant force (.MRF) through simple vector addition as is the case with the lever model. Greaves argued that distraction ofthe temporomandibular joint (i.e., forces that can separate the mandibular condyle from the articular eminence) could lead to potentially serious injury. He therefore proposed that natural selection should favor a morphology that limits TMJ distraction. This is the fundamental 11

PAGE 22

assumption of the constrained model and is predicted to cause limitations on the evolution of masticatory morphology in mammals. There should be limitations on masticatory morphology because jaw distraction is avoided only if the MRF lies within the triangle of support (Fig. 2.1) (Adapted from Spencer, 1995) (Greaves, 1978). The mediolaterallocation of the .MRF is determined by the positions and relative force contributions of the adductor muscles from both the working and balancing sides (Spencer, 1999). The muscle resultant will lie in the midline when the balancing and working side muscles are equally active. However, this is not always the case. Differential activity between these muscles produces mediolateral movement of the .MRF (Rylander, 1985; Spencer, 1998). The MRF will also lie at different positions relative to the triangle of support depending on the bite point. Biting on more anterior teeth creates a relatively large triangle of support that will enclose a midline MRF (see Fig. 2.1 ). However, during biting on more posterior teeth, the triangle of support is smaller and gets shifted laterally toward the working side (Spencer, 1999). This smaller triangle may not encompass a midline .MRF. If the midline .MRF falls outside of the triangle of support, the mandible could potentially rotate around the bite point and the balancing-side joint causing distraction of the working-side mandibular condyle (Fig. 2.2). 12

PAGE 23

Balancing Side Joint Reaction Force Side Joint Reaction Force Fig. 2.1 Occlusal view of the mandible with a midline muscle resultant force (B). During biting on more anterior teeth, this midline muscle resultant force passes through the triangle of support (shaded zone). The comers of this triangle are positioned at the bite force (F), the balancing-side joint reaction force (J), and the working-side joint reaction force (0). 13

PAGE 24

Balancing Side Joint Reaction Force Working Side Joint Reaction Force Fig. 2.2 For more posterior bite points, the triangle of support may not envelope a midline muscle resultant force producing tensile forces in the working-side joint. .Greaves (1978) suggested that the muscle resultant could be moved back into the triangle of support (arrow) through a reduction in balancing-side muscle activity. 14

PAGE 25

As stated earlier, the main assumption ofthe constrained model is that the TMJ should not be subjected to distraction. Therefore, the muscle resultant force must move so that it will always lie within the triangle of support. Differential muscle activity between the working side and balancing side (i.e. less balancing side muscle activity) will enable the MRF to shift laterally toward the working side. This means that a smaller triangle of support characteristic of the more posterior dentition will still encompass the MRF. Changes in muscle activity and joint loading have lead to the division of three zones of potential bite points termed Regions I, II, and III (Spencer and Demes, 1993; Spencer, 1995, 1998, 1999) (Fig. 2.3). Region I encompasses the anterior dentition. It is separated from Region II by an oblique line, which passes through the balancing-side joint reaction force and a midline :MRF. The triangle of support for this region is able to enclose a midline MRF; it is unnecessary for the l\1RF to shift toward the working side. Regions II and ill are separated by a transverse line passing through the muscle resultant force. Region II is characterized by a relatively small triangle of support through which a midline l\1RF will not pass ifthe adductor muscles on both sides of the head are equally active. The l\1RF must shift toward the working side in order to avoid distraction of the working-side TMJ. This is done through a reduction in balancing-side muscle activity. 15

PAGE 26

Balancing Side Joint Reaction Force REGION I ........... ... :/ REGION II REGION Ill Working Side Joint Reaotlon Force Fig. 2.3 Occlusal view o( the mandible showing the distributions of Regions I, II, and III for a mammal with a fused mandibular symphysis. Any bite point located in the anterior region (Region I) will produce a triangle of support that envelopes a midline muscle resultant force; no reduction in balancing side activity must therefore occur. In Region II, the muscle resultant must shift toward the working side through a reduction in balancing side activity or tension will be produced in the working In the most P.osterior region III) the muscle resultant cannot be repositioned so that tensile forces are avmded. 16

PAGE 27

A triangle of support in Region III will not be able to encompass a MRF even if it were to move laterally. Biting in this region will be unavoidably associated with TMJ distraction because the MRF cannot fall within the triangle of support (Spencer, 1999). Therefore, no teeth should lie here. Bite force and joint reaction force values differ between Regions I, II, and III. These provide evidence for demarcating the boundaries of these regions. This has been demonstrated in a theoretical model of reaction force values based on the predictions of the constrained model (Spencer, 1998) (Fig. 2.4). Region I is characterized by bite force values that are the lowest in magnitude yet increase as the bite point moves distally. Region II maintains the highest magnitude bite forces, however, all values are equal within this region. Region Til does not have any bite force values as biting in this region should be avoided due to the possibility of joint distraction. The joint reaction forces show a different pattern of magnitude from the bite forces. The working-side joint reaction force decreases as the bite point moves distally through Region I. In Region II, the working-side joint reaction force maintains a value of zero. The balancing-side joint reaction force values decrease slightly within Region I and then drastically in Region II as the bite point moves distally (Spencer, 1998). The constrained model has been very influential within the field of functional morphology (Greaves, 1982; Dessem and Druzinsky, 1992; Spencer 17

PAGE 28

Region II Re lon I 41126 )100 i 75 :i 60 J -50 ........... 1 o 20 30 40 50 eo 10 eo Distance of Bite Point Anterior to TMJ Fig 2.4 Predicted bite force and joint reaction force values using the assumptions of the constrained model. The magnitude of the bite force (F), the working-side.joint reaction force (0), and the balancing-side joint reaction force (J) is predicted under the conditions of maximum bite force production and the avoidance of tensile working-side joint reaction forces. Bite force values in Region I are lowest in magnitude but increase as the bite point moves posteriorly. Region II bite force values are highest in magnitude but are all equal. Joint reaction force values show a different pattern. Within Region I, the working-side joint reaction force values decrease as the bite point moves posteriorly .. In Region II, the working-side joint reaction force values are all zero. The balancing-side joint reaction force values decrease slightly within Region I and then drastically in Region II as the bite point moves distally. 18

PAGE 29

and Demes, 1993; Spencer, 1995, 1998, 1999; Dumont and Herrel, 2003; Thompson, et. a/., 2003). Predictions concerning masticatory configuration have been generated using this theoretical model. Subsequent testing of this model has led to the modification of the constrained model to include variables and ideas that are discussed in the next section. Modifications to the Constrained Model Subsequent studies have provided support for Greaves' fundamental assumption that the TMJ should not be loaded by distractive forces. A morphometric analysis of anthropoid masticatory system configurations suggests that the phenotypic diversity in cranial morphology in this group is limited by the need to avoid TMJ distraction (Spencer, 1999). However, this study also highlights some discrepancies between the constrained model and observed anthropoid cranial morphology. One such discrepancy involves the observation that the masticatory adductor muscles are positioned more posteriorly than proposed in the constrained model. This means that the MRF may not be produced directly at the posterior end of the tooth row during forceful isometric biting as Greaves had assumed. Spencer (1999) proposes a more posterior position of the MRF than originally conceived ofby Greaves (1978). A more posteriorly oriented MRF means that even ifthe MRF were to migrate forward at larger gapes, it would still 19

PAGE 30

be maintained within the triangle of support. This creates a more conservative masticatory system configuration in which combined muscle force is positioned more posteriorly than predicted by Greaves. The constrained model has been tested through studies of force production in the primate, canid, and opossum masticatory system. If this model is correct, muscle activity should change with bite point position. This must occur in order to maintain the basic assumption of this model, which is that the TMJ should not be loaded with distractive forces. The TMJ will not be subjected to distractive forces as long as the MRF is maintained within the triangle of support. Biting on more posterior teeth will, therefore, cause the MRF to shift laterally in order to be maintained within the relatively smaller triangle of support. This is done through a reduction ofbalancing-side muscle activity. Electromyographic data have shown that in humans the activity of the largest masticatory adductor muscles, the superficial masseter and anterior temporalis, changes with bite point (Spencer, 1998). Maximum muscle force magnitudes were found to be greatest for the first molar, decreasing both anterior and posterior to this bite point. Balancing-side to working-side muscle force ratios were also found to differ by bite point. Balancing-side muscle activity was found to be lowest during biting on the third molars. This decreased balancing side muscle activity may serve as a mechanism for avoiding TMJ distraction by enabling the MRF to be maintained within the triangle of support (Spencer, 1998). 20

PAGE 31

An electromyographic study of Canis fami/iaris also supports the constrained model (Dessem, 1989). This study found greater working-side than balancing-side muscle activity which supports the notion that the MRF should always fall within the triangle of support in order to maintain jaw-joint stability. The constrained model also enables specific predictions to be made about bite forces in Regions I and IT. Those predictions are that bite force increases as the bite point moves posteriorly within Region I and then reaches its highest magnitude within Region II. All bite points within Region II are of equal value. A study involving Monodelphis domestica, an opossum, supports these predictions (Thompson et. al., 2003). They found that within this species, both juveniles and adults, maintain at least three molariform teeth within Region II and that it is within this region where highest magnitude bite forces are produced. Also, within Region I, the bite force generated at the premolars was stronger than that generated at the incisors or canines and forces were equivalently strong within Region II. Other studies (Spencer, 1999) have also tested the constrained model and found that there are many variables related to the configuration of the masticatory system. that interact to determine the distribution of Regions I, II, and ill. These variables include bicondylar breadth, the anteroposterior position of the :MRF, palatal breadth, and height of the TMJ. 21

PAGE 32

Changes in bicondylar breadth can alter the anterior border of Region II (Fig. 2.5). The border this Region II moves posteriorly as the distance between the two condyles increases, causing less of the molar tooth row to fall within Region II (Spencer, 1999). Therefore, increasing bicondylar breadth decreases the length of Region II. Another variable relating to the distribution of Region II is the anteroposterior (A-P) position of the MRF. In the constrained model the MRF is located at the midline between the last molars. Bite points posterior to the MRF will not allow it to pass through the triangle of support even if it were to shift laterally toward the working side (Spencer, 1999). However, a MRF that is positioned posterior to the last molars will create a more posterior, and shorter, Region II (Fig. 2.6). The height of the TMJ above the occlusal plane is another variable that affects the distribution of Region II. TMJs that are positioned above the occlusal plane will cause the MRF vector to pass more anteriorly through the triangle of support (Spencer, 1999). This is because the triangle of support will become inclined as the TMJ gets raised above the occlusal plane. The taller the TMJ, the more inclined the triangle of support will be. This inclination may cause the MRF to pass anterior to the triangle of support. Therefore, mammals with taller TMJs may have a more anteriorly positioned Region II. This has the effect of increasing the length of Region II. 22

PAGE 33

c B A Working Sld.e Joint Reaction Force Fig. 2.5 Occlusal view of the mandible demonStrating how bicondylar breadth affects the length of Region IT. A decrease in bicondylar breadth, from positions A through C, causes an inc!ease in the length of Region II. This enables more of the molar row to fall within this region. 23

PAGE 34

' ...... .. / \ I "' .... \ Midline '\""< J 1 Muscle \ '\ i 1 Resultant \ \-. 1 (.. --( 'l !. I \ \ i \ \ \ jt I \.\\\\ . --: "I i i -.. "'::l.. ,..---/ \ \. ---o -' .. ,. ___ _.// ____ Balancing Side Joint Reaction Force Working Side Joint Reaction Force Fig. 2.6 An additional varianle that alters Region II length is the anteroposterior position of the midline muscle resultant force (B). As the muscle resultant force is moved from positions A through C, the boundary line between Region II and lli (bold transverse lines passing through the muscle resultant force) is moved posteriorly. As the muscle resultant force is reoriented posteriorly, the length of Region II decreases (shaded boxes). 24

PAGE 35

Palatal width also affects the distribution of Region II (Fig. 2.7). The mediolateral position of the tooth row determines the length of the postcanine dentition that will fall within Region II. Thus, movement of the palate laterally increases the length of Region II. These variables all interact with one another to produce a masticatory system configuration unique to each species. Studying the configuration of this system and how these variables interact to determine the distribution of Region II is crucial to testing the broad scale applicability of the constrained model. Muscle Force Recruitment The purpose of this study is to assess the differences in masticatory system configuration between mammals with an unfused mandibular symphysis and those with a fused one. The justification for studying these differences lies in the finding that animals with a fused symphysis differ in muscle recruitment patterns from those with an unfused symphysis (Rylander, 1979b; Dessem, 1989; Rylander et. a/., 1998). It has been shown that primates with a fused symphysis recruit more balancing-side muscle force during powerful mastication than those with an unfused symphysis. In 1979, Rylander reported results from an experiment that showed that only a small percentage of the bite force of Galago crassicaudatus, which has an unfused symphysis, is due to balancing-side muscle force during 25

PAGE 36

. / ( "-..(_ 1 ,,, / J /_ r ', l \,, ....... -, / Balancing Side Joint Reaction Force \ \, ,,, .. .. l; '' I 1 Midline "''"t.>,_ I I Muscle \ r; 1 Resultant \ \ \ \. \ '\. \ l \. '\ \. \ \ '\ 1 -...,.r-/ . .r,(_rO,., _,/ ...... :,..---.... __ Working Side Joint Reaotlon Force Fig. 2.7 Occlusal view of mandible showing how palatal breadth affects the length of Region II. The boundary line between Regions I and II is represented as a thin diagonal line and Regions II and III are separated by a bolder horizontal line. The. rnediolateral position of the tooth row relative to these lines will determine the length of postcanine dentition that will fall within Region II (boxes). Thus, movement of the tooth row laterally (i.e., increasing palatal breadth) from positions A through C results in an increase in Region II length. 26

PAGE 37

isometric unilateral molar biting. This suggests that the working-side jaw musculature of galagos is much more active than the balancing side. Bone strain data and moment arm calculations demonstrated that the working-side muscles generate at least four to five times more force than the balancing side (Rylander, 1979b). This decreased emphasis on balancing-side musculature for galagos contrasts with electromyographic results for humans. Results show that the balancing-side muscles of humans are only slightly less active than the working side masticatory muscles during powerful unilateral biting (Meller, 1966 as discussed in Rylander, 1979b). One reason this may be the case can be seen in the mandibular morphology of these two groups. Galagos have an unfused mandibular symphysis whereas humans, including all anthropoids, have fusion. The fused mandibular symphysis of this group of primates has been suggested to be an adaptation to counter increased symphyseal stress due to increases in balancing-side muscle force during powerful unilateral biting (Rylander, 1979b ). This is because symphyseal fusion functions to prevent structural failure ofthe symphysis by strengthening it. This becomes important as more balancing-side muscle force is recruited during mastication. The bone strain data of galagos discussed above support this suggestion. Data comparing long-tailed macaques (Macacafascicularis) and thick tailed galagos (Otolemur crassicaudatus) also support the symphyseal fusion-27

PAGE 38

muscle recruitment hypothesis. Advocates for this hypothesis argue that symphyseal fusion and balancing-side jaw adductor muscle force are functionally linked (Rylander, 1979a). Long-tailed macaques, which have a fused mandibular symphysis, have about 1.5 to 2 times more bone strain along the working-side corpus than along the balancing-side corpus. Thick-tailed galagos, whose symphysis is unfused, have about 7 times more bone strain along the working side corpus. This shows that the macaques recruit more balancing-side adductor muscles during isometric molar biting, whereas the galagos rely more exclusively on the working-side jaw muscles. Lemurs, which have an unfused symphysis, have also been found to recruit relatively little force from their balancing-side deep masseter. The recruitment and firing pattern of this muscle is much more similar to that of the galagos than anthropoids (Rylander et. a/., 2002). Owl monkey data show a pattern of strain that is similar to that recorded for the macaques (Rylander et. a/., 1998). This pattern differs markedly from the galagos' strain pattern. This is expected considering owl monkeys and macaques both have a fused symphysis whereas galagos do not. Owl which are smaller than galagos, show that differences in muscle recruitment patterns are related to symphyseal fusion rather than allometric constraints. A study of electromyographic activity from jaw-adductor muscles of Canis familiaris (domestic dog) also supports the idea that animals with an 28

PAGE 39

unfused symphysis recruit less balancing-side muscle force. During mastication and bone crushing working-side muscle activity was reported to be greater than balancing-side muscle activity in this species, which has an unfused symphysis (Leibman and Kussick, 1965; Dessem, 1989). It was found that even during the production ofthe largest bite forces, balancing-side muscle activity was never maximally recruited. This finding supports the previously discussed studies involving primates. Much evidence demonstrates that there are marked differences in muscle recruitment patterns between mammals with a fused and unfused symphysis. These muscle recruitment patterns may cause systematic differences in the configuration of the masticatory system between mammals with a fused and unfused symphysis. Why Fuse the Mandibular Symphysis? Symphyseal fusion enables those animals with this morphological trait to recruit more balancing side muscle force as discussed in the previous section. However, this does not explain the convergent acquisition of this feature in a number of mammalian taxa. During the past thirty years, morphological and experimental analyses of primates have been done in an attempt to answer this question. Although it has been confirmed through histological studies (Beecher, 1977) that a fused symphysis is stronger than an unfused one (because bone is 29

PAGE 40

stronger than the fibrocartilage and ligaments of an unfused joint) it is unclear exactly why fusion has occurred. Two groups of hypotheses have been offered to explain the functional and evolutionary significance of symphyseal fusion. These hypotheses involve stiffness and strength in the mandibular symphysis. Stiffness Hypotheses One group of hypotheses involves the idea that fusion of the symphysis occurred within anthropoid primates as an adaptation to stiffen the symphysis. Stiffness is defined as the ability to resist deformation in response to applied forces and is the primary mechanical property of bone that enables force transfer (Lieberman and Crompton, 2000). One idea that has been suggested is that syniphyseal fusion occurred within anthropoids to stiffen the symphysis in order to better resist the forces associated with the crushing of small, hard objects, such as seeds, along the incisors (Kay and Hiiemae, 1974; Greaves, 1988). This would prevent an inefficient dissipation of dorsally-directed force across the symphysis. This is because an unfused symphysis allows some independent movement between the two dentaries during incision. Theoretically, this independent movement enables the balancing side incisors to contact one another during the crushing of small, hard objects. Therefore, the balancing-side muscle force gets dissipated along the incisors on the balancing side rather than being transmitted across the symphysis 30

PAGE 41

(Ravosa and Rylander, 1994). The muscle force that was generated on the balancing side gets wasted on the balancing-side incisors rather than contributing to the force production on the working side. This scenario differs as the size ofthe food object gets larger. As the diameter of a food object increases the balancing-side upper and lower incisors will not come into contact. The balancing-side muscle force is then able to get transmitted across the symphysis and contribute to the crushing of the food object. This argument is, therefore, relevant only for the crushing of small, hard food objects, such as seeds. In conclusion, according to this hypothesis, the function of a fused symphysis is to stiffen the symphyseal joint in order to reduce independent movement of the two dentaries during the crushing of small, hard objects. This hypothesis is faulty for a number of reasons. The majority of anthropoid primates do not include small, hard objects, such as seeds, as a major component of their diet. Also, according to Ravosa and Rylander (1993), the literature provides no evidence for the processing of small, hard objects on the incisors when objects such as seeds are ingested. The apparent discrepancy between observed behaviors in anthropoid primates and this argument has led others to modify the stiffuess hypothesis. Another problem with this argument is that it does not address the extent to. which independent movement between the two dentaries occurs. Greaves' 31

PAGE 42

(1988) schematic drawings make it look as though each dentary moves completely independent of one another. This is not the case; the ligaments of an unfused symphysis do not allow such an amount of independent movement between the two dentaries. In fact, Beecher (1977) has shown that the pattern of fibrocartilage and ligaments of pro simians are arranged to resist movement during mastication. The functional association between symphyseal fusion and increased stiffness has been hypothesized to be important for molar biting rather than for the incisors (Ravosa and Rylander, 1994). Symphyseal fusion could be an adaptation to reduce the amount of balancing-side tooth contacts during unilateral mastication on the molars. This is similar to the proposition that balancing side tooth contact should be avoided so that balancing-side muscle force can contribute to the crushing of the food object on the working side. This idea is also problematic because anisognathic jaws and an unfused symphysis probably represent the primitive condition for primates (Ravosa and Rylander, 1994). Anisognathy refers to the condition of having different widths of the upper and lower dental arches. Therefore, these teeth are not likely to come into contact anyway even with an unfused symphysis. Fusion of the mandibular symphysis does not appear to be necessary to prevent balancing-side tooth contacts during either incision or mastication. 32

PAGE 43

An alternative hypothesis focuses on the orientation of the occlusal wear facets. The main idea is that fusion functions to stiffen the mandible in taxa with more horizontally oriented occlusal wear facets. Both fused and unfused symphyses are effective at transferring dorsally-oriented force. However, a fused symphysis, because of its stiffness in all planes, is likely to be more effective at transferring force in the transverse plane. Taxa with a strong transverse component to chewing will benefit more from a fused symphysis. This idea has been supported in studies of mammals; symphyseal fusion does in fact correlate with transversely oriented occlusal planes (Lieberman and Crompton, 2000). It can be concluded, therefore, that fusion is most likely an adaptation for increasing the efficiency of transversely-oriented occlusal forces. Strength Hypotheses Another set of hypotheses regarding the functional significance of symphyseal fusion is that fusion strengthens the symphysis to help counter symphyseal stress during incisal biting and unilateral mastication (Ravosa and Rylander, 1994). Hypotheses that have been offered in support of the strengthening hypothesis involve the idea that symphyseal fusion occurred to counter wishboning stress and/or dorsoventral shear. It has been found that during incision and mastication, the mandibles of macaques are twisted about their long axes (Rylander, 1979a). This twisting 33

PAGE 44

everts the lower borders of the mandible so that the symphysis gets bent vertically. Compression occurs along the upper or alveolar surface of the symphysis and tension is present along the inferior surface ofthe symphysis. This stress is most effectively countered by complete symphyseal fusion. Two loading patterns are also present among anthropoid primates during unilateral mastication: wishboning and dorsoventral shear of the symphysis. Wishboning stress results from the laterally directed component of muscle force on the balancing side and a laterally directed component of bite force, and perhaps working-side muscle force. The symphysis therefore experiences high stress concentrations and high strain magnitudes, especially along the lingual surface. This is best countered by fusion because bone is much stronger than ligaments (Ravosa and Rylander, 1994). Dorsoventral shear is another explanation concerning symphyseal fusion. In an experimental study with macaques it was found that the symphysis is sheared dorsoventrally during mastication and incision (Rylander, 1984). The amount of dorsoventral shear stress is directly related to the amount of vertically oriented muscle force transmitted across the symphysis from the balancing to the working side. As balancing-side muscle recruitment increases so does the amount of dorsoventral shear. This is also best countered by complete fusion of this joint (Ravosa and Rylander, 1994). 34

PAGE 45

Wishboning and dorsoventral shear are both present during mastication. However, according to Ravosa and Rylander (1994), it appears that wishboning is the most important determinant of symphyseal fusion in primates because the stresses associated with this loading regime are considerably higher than those associated with dorsoventral shear. It has been suggested, however, that fusion in early anthropoids may be due to increased dorsoventral shear resulting from increased recruitment of vertically directed balancing-side muscle force. Evidence for this can be seen in prosimians that seem to have evolved morphologies to resist dorsoventral shear such as having calcified or ossified ligaments. Also, wishboning stress appears to occur only in taxa who have already evolved complete fusion. Therefore, fusion in early anthropoids may have followed from increased dorsoventral shear and after fusion was attained, wishboning stress increased as greater amounts of horizontally-directed forces could be recruited. Both stiffness and strength hypotheses have been offered to explain the functional and evolutionary significance of symphyseal fusion. Histological and experimental evidence provide greater support for the strength hypothesis. Whether fusion occurred due to wishboning or dorsoventral shear stresses associated with the strength hypothesis is not entirely clear. It seems as though these loading patterns may both be involved in the evolution of symphyseal 35

PAGE 46

fusion. Experimental studies including other mammalian taxa need to be performed in order to clarify this issue. 36

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CHAPTER3 HYPOTHESES AND STUDY DESIGN The main goal of this study is to determine how applicable the constrained model, discussed in the previous chapter, is for understanding masticatory form and evolution in a wide range of mammals. The particular topic of concern here is whether the configuration of the masticatory system differs in a patterned way between mammals with symphyseal fusion and those without it and if this can be explained using the biomechanical principles underlying the constrained model. Of substantial interest is the distribution of Region II, the region where highest magnitude bite forces occur. Because of this the grinding dentition, particularly the molars, should be located here. This hypothesis has been tested and supported for anthropoid primates (Spencer, 1999), but it is not clear whether it holds true for other groups of mammals, particularly those without symphyseal fusion. If there is an association between the length of Region II and molar length within the mammals studied here, it will imply that masticatory system configuration, although highly can be explained through general principles ofthe constrained model. If such an association does not exist, it will suggest that the model is incorrect. 37

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The main prediction of this study is that observed molar length will be shorter than the predicted Region II length in all taxa. This is because according to the constrained model, the molars must lie within Region II. Maintaining the molars within this region can be accomplished by either decreasing overall molar length or increasing the length of Region II through the interaction of other craniofacial variables. This is because the distribution of Region II is determined by a number of variables, including bicondylar breadth, height of the TMJ, palatal breadth, and the anteroposterior position of the muscle resultant force. Determining the length ofRegion II is essential to understanding how these craniofacial variables interact within this model. Predicting the position of the premolars within Region II is problematic. The constrained model maintains that the grinding dentition should lie within this region. When applied to selenodont artiodactyls this may not be an issue because the molars and premolars are functionally similar and consequently labeled together as grinding dentition. However, other mammals, such as primates, have premolars that are distinct from the molars and may be functionally different. The diversity of premolar form within primates suggests functions including: puncturing, slicing, and perhaps molar-like crushing (Spencer, 1995). Therefore, many of these teeth cannot be defined as grinding dentition. Because ofthis diversity in premolar function, this study will focus on the molars. The model is accepted if at least all of the molars are located within Region II. 38

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The Effects of an Unfused Symphysis on the Model The constrained model was developed based on a mammal with a fused mandibular symphysis. Although this model was applied to selenodont artiodactyls, a mammal with an unfused symphysis, Greaves (1978) did not address how lack of fusion may affect the configuration of this model. Mammals with an unfused symphysis have a MRF that is located closer to the working side. This has the effect of shortening the length of Region II (Fig. 3.1). However, the constrained model assumes equal balancing side (B/S) to working side (W/S) muscle force ratio, which places the MRF in the midline. The length of Region II will, therefore, be calculated under the assumption of a midline MRF. This assumption is problematic, however, for mammals with an unfused mandibular symphysis as .it is essential to know the location ofthe MRF in order to calculate an exact length for Region II. This is because we know that the MRF in mammals with an unfused symphysis does not lie in the midline during mastication. These animals recruit relatively less balancing side muscle force than those with a fused symphysis, therefore, the MRF will be located more laterally. A laterally located MRF would cause the length of Region II to be shorter than if the MRF passed through the midline (Fig. 3.1 ). Although I assume that it is more laterally located, the exact location cannot be determined. This is because we do not know the working side/balancing side 39

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REGION I a .. ,' .. ,' ' ' ' ReGION I / ReatoNII 8atan<:ing Side Joint Reaaion Foree Working Side Joint Aeaalon Foroe B Midline muscle resultant force for mammids with a fused symphysis t:=::==J Laterally positioned muscle resultant force for mammals with an unfused symphysis Fig.1 Occlusal view of the mandible showing the effects of changes in the position of the muscle resultant force on the distribution of Region II. (a) The distribution of Region II with a midline muscle resultant force. (b) A laterally positioned muscle resultant force will create a Region II length, therefore, less of the molar tooth row will fall within this region. 40

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muscle force ratios for all species of mammals with an unfused symphysis. Therefore, an actual Region II length cannot be calculated for this group of mammals. This dilemma can be overcome by estimating the length of Region II for both groups of mammals assuming a midline MRF. This predicted length can then be compared to actual molar length in both groups. It is necessary to use this calculated length for mammals with an unfused symphysis because it is the only estimate of Region IT length that can be attained. It is predicted that the molar row will not fall outside of Region II for both groups. Mammals without fusion can compensate for having a theoretically shorter Region TI through cranial variables. The variables that impact the distribution of this region are bicondylar breadth, palatal breadth, height of the TMJ, and the A-Pposition ofthe MRF. All of these variables can increase the length of Region II regardless ofwhether an animal has a fused or unfused symphysis. The model demonstrates, however, that a lack of fusion creates a theoretically shorter Region II length in this group. These variables provide mammals with an Wlfused symphysis multiple ways to overcome this constraint. The variables discussed above provide mammals with a way to increase the length of Region II. These variables, however, are not independent of one another. Change in one variable is likely to covary with other variables (for example, an animal with a wide palatal length could have a decreased bicondylar 41

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breadth) perhaps still resulting in an increased Region II length. The main prediction will be upheld as long as the molars are maintained within the estimated Region II length, whether this is accomplished through a decrease in molar size/number or by increasing Region II length through these other craniofacial variables. The second prediction of this study is that the ratio between predicted Region II length and observed molar length will be higher in mammals with an unfused symphysis. This is because mammals with an unfused symphysis have a theoretically shorter Region II length due to the MRF being located laterally. Specific Predictions 1. Molar length will be shorter than the predicted length ofRegion II for both groups of mammals. 2. The ratio between predicted Region II length and observed molar length is predicted to be higher in mammals with an unfused symphysis because of the theoretically shorter Region II length. Study Design The variables related to the distributions of Region II were quantified so that comparisons between the two groups could be performed. A broad-scale comparative test was carried out on these hypotheses to determine the degree to 42

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which overall masticatory form in mammals is consistent with the principles of the constrained model. It is expected that the development of fusion and the changes it brought about in muscle force ratios, also brought about changes in cranial configuration. This was assessed through the principle of comparison (Fleagle, 1988). The comparative approach enables us to identify morphological trends of the masticatory system between taxa with symphyseal fusion and those without it. Identifying these trends is essential to uncovering the functional and evolutionary explanation for the change in morphological structure from an unfused mandibular symphysis to a fused one. Tests The main prediction of this study (Hypothesis 1) is that the molar tooth row will lie within Region II for all taxa. This hypothesis will be rejected if the observed length of the molars is greater than the predicted Region II length in any taxa. This test requires that the length of Region IT be predicted as it cannot be directly measured. This will be done under the assumption of a midline MRF for all mammals as previously discussed. The predicted length of Region II can then be compared to actual molar length in all taxa. Hypothesis 2 states that the ratio between predicted Region II length and molar row length will be higher in mammals with an unfused symphysis because 43

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their actual Region II length (although not able to be calculated) should be shorter. This hypothesis will be rejected if mammals with an unfused symphysis do not have a higher ratio of predicted Region II length to molar length. An allometric analysis is also important to explore patterns of variation in individual dimensions. An allometric analysis enables us to understand how changes in shape and size allow different taxa to accomplish the same behavior, which in this case is the maximization of force production in the masticatory system (Clutton-Brock and Harvey, 1979; Fleagle, 1988). 44

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CHAPTER4 MATERIALS AND METHODS Sample Measurements were taken on a total of 135 individuals representing 15 species with symphyseal fusion and 16 species without fusion (Table 4.1 and 4.2). Sample size ranges from 1 to 12 for the different species. Representative species from the orders Primates, Perissodactyla, Carnivora, and Artiodactyla were included in order to obtain a large sampling oftaxa in which to compare those with symphyseal fusion to those without fusion. It is important to include a variety of mammalian taxa in order to assess the generalizability of this model within the class Mammalia. Four species of artiodactyls, 12 carnivore species, 3 perissodactyl species, and 15 primate species are represented in this sample. Only adult crania (all teeth fully erupted) and their associated mandibles were used. Specimens were obtained from the Denver Museum of Nature and Science and the University of Colorado at Boulder Museum of Natural History. Although some species are represented by only a minimal number of specimens they were included in the study as my purpose is to sample a broad range of species within the mammalian order. 45

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Table 4.1 Taxa included in study with a fused symphysis Taxa with a Fused Symphysis Sample size Order-Cebus capuchinus 3 Primate Pithecia pithecia 2 Ateles geoffroyi 3 Aotus lemurinus 1 Alouatta palliata 4 Callithrix argentata 1 Saguinus geoffroyi 3 Cercopithecus diana 1 Cacajao calvus 1 Gorilla gorilla 2 Pongo pygmaeus 1 Order-Equus grevyi 2 Perissodactyla Equus equus 3 Tapirus terrestrius 2 Total =29 46

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Table 4.2 Taxa included in study with an unfused symphysis Taxa with an Unfused Symphysis Sample size OrderGalago demidoff 1 Primate Nycticebus coucang 1 OrderCanis latrans 12 Carnivora Canis lupis 9 Urocyon cinereoargenteus 8 Vulpus velox 11 Vulpus vulpus 11 Lynx canadensis 3 Lynx rufus 10 Puma concolor 10 Procyon lotor 10 Ursus americanus 9 Ursus arctos 2 OrderCervus elaphus 2 Artiodactyla Odocoileus hemionus 2 Odocoileus virginianus 3 Mazama americana 2 Total= 106 47

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Data Collection Process The following steps were taken in the data collection process to ensure that accurate measurements of the specimens were obtained: Step 1A Sony digital camera was used to photograph each specimen. Each specimen was placed on a level work surface. Two photographs were taken of each individual including both a transverse and occlusal view. When photographing the occlusal surface, each specimen was held in place with modeling clay. The modeling clay allowed the specimens to be positioned at a 90 degree angle to the table. The camera was placed on a tripod and aimed at the specimen. The camera was positioned as far from the specimen as possible while still filling the viewfinder with the skull. Filling the viewfinder with as much of the image as possible maximizes screen resolution which allows measurements to done on a computer with finer detail, thereby, reducing measurement error (Spencer, 1995). Step 2A calibration grid was set up and photographed before pictures of the specimens were taken. Any time the camera was moved, particularly upon completion ofa species, the calibration grid was repositioned andrephotographed. This image was used during analysis to determine the size of the space in which landmarks were located. 48

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Step 3Some of the landmarks that were measured were not clearly visible (such as the glenoid fossa). These were highlighted with small black dots, which helped with identification on the computer images. Step 4Images were downloaded into a Macintosh computer and measured within the MacMorph data acquisition package. Each image was calibrated using the corresponding calibration grid. Measurements were then taken of the variables for all images. Step 5The statistical packages J1v1P and Statview were used for all analyses. Measurements Three sets ofmeasurements were taken for this study: (1) distances that represent the observed length of the postcanine dentition, (2) dimensions for calculating a predicted Region II length, and (3) size adjustment measurements. Measuring the observed length of the postcanine dentition is necessary for Hypothesis 1 and 2. The observed length of the postcanine dentition was measured from the trigon of the maxillary last molar to the trigon of the first molar and from the trigon oflast molar to the trigon of the first premolar. The trigon of each tooth was chosen because of the biomechanical role it plays; this feature experiences much of the force that is produced during mastication when it comes into direct contact with the teeth of the mandible. 49

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The main hypothesis of this study is that the molars must lie within Region II. However, the length of both the premolars and molars were measured in order to assess the extent to which both types of teeth lie within Region II. Calculating an estimated Region II length is necessary for Hypothesis 1 and 2. There are five variables that can influence the location of Region II and must be quantified or estimated in order to calculate a predicted Region II length. These variables include bicondylar breadth, palatal breadth at M1, height of the TMJ relative to the occlusal plane, distance from the TMJ to point of intersection of the muscle resultant vector and occlusal plane, and the angle of the muscle resultant vector to the occlusal plane (Fig. 4.1). The first three variables can be directly quantified. However, it is difficult to quantify muscle resultant position and orientation due to limited knowledge of the comparative myology and function ofthe masticatory muscles among mammals (Throckmorton, 1989; Spencer, 1999). This study will therefore assume the MRF vector intersects the occlusal plane directly at the posterior end of the tooth row. This can be directly quantified as the distance from the TMJ (defined here as the center of the articular eminence) to the trigon of the last molar. This is consistent with the assumptions of the constrained model. A prior study involving muscle resultant force orientation for the masticatory adductor muscles of anthropoid primates estimated a fixed orientation of 80 degrees relative to the occlusal plane based on quantified orientations of the 50

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Fig. 4.1 I I I I I'' : : : 1., ; ..J'I: ........ ''t. : I /0 M .. ... I I I I; I I l I '' ". :. ......... I J A A = Biarticular breadth B = Palate Breadth at M 1 C = Distance from TMJ to point of intersection of muscle resultant vector and occlusal plane D =Height of articular eminence above occlusal plane E =Angle of muscle resultant vector to occlusal plane (90 equals perpendicular) Illustration of five variables used in the calculation of Region II length. 51

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anterior temporalis, superficial masseter, and medial pterygoid muscles (Spencer, 1999). Muscle resultant orientation cannot be calculated in the present study since it is likely to be highly dependent on individual muscle force magnitudes. Collecting these data is beyond the scope of this study. The length of the molar row is defined as the distance between the trigon of the right maxillary last molar to the trigon of the first molar. Bicondylar breadth is the distance between the central articular eminence landmarks. Height of the TMJ is the perpendicular distance of the right central articular eminence landmark from the occlusal plane. The occlusal plane is projected onto the sagittal plane and is defmed by the horizontal line connecting the distal end of the maxillary last molar to the mesial border of the maxillary last molar. Palatal breadth is the distance between the maxillary tooth rows at the trigons of M 1. The five variables were placed into an algorithm that calculates expected Region II length. The equation is shown in Figure 4.2 (Obtained from Spencer, 1995). Size Size and shape of the cranium are known to differ drastically among the mammals included in this study. Only when shape differences are teased apart from size differences can any meaningful comparison between these groups be 52

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o (DP2) Ps = arctan --C Effective Re.gion ll Length::;: P7 + P4 Fig. 4.2 Equation for estimating the Predicted length of Region II. 53

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made (Spencer, 1995). The calculation of the ratios of each dimension divided by the geometric mean of the cranium allows comparisons of masticatory system configurations between groups exhibiting different cranial sizes (Darroch and Mosimann, 1985). These size adjusted "shape variables" indicate relative proportions among the groups in this study. In this study the geometric mean involves four distances within the facial skeleton in order to get an accurate representation of overall cranial size. This serves as a size summary by combining multiple size dimensions into a single value. The equation for the geometric mean is: GM = (Dl D2 ... DN)(l/N) where D = distance value and N = number of distance values included in the summary of size. Distances used to assess the geometric mean were chosen as representative of overall masticatory system size as this is the system of concern to this study. The distances used are bicondylar breadth, palatal breadth, temporal foramen length, and molar row length. The landmarks used for measuring bicondylar breadth, palatal breadth, and molar row length have been defined above. Temporal foramen length is defmed as the distance between the right central articular eminence landmark and the inferior edge of the zygomatic arch. An allometric analysis was also done to assess how changes in shape with size affect the masticatory system. This is necessary because such a wide range 54

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of sizes are being sampled. The geometric mean was also used for allometric analyses. 55

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CHAPTERS RESULTS The parameters measured for this study are shown in Tables 5.1-5.3. All values are represented as sex-specific means and standard deviations (in mm) of all individuals. Table 5.1 displays mean values for molar row length, the length of the postcanine dentition, and the length of the molars to the incisors. Table. 5.2 lists mean values for the parameters used to calculate predicted Region II length. These parameters include: bicondylar breadth, palatal breadth, height ofTMJ, distance from the TMJ to the last molar. Table 5.3 reports the geometric mean, as well as one additional variable used in this calculation. The three other variables used in the calculation of the geometric mean, bicondylar breadth, height of the TMJ, and molar length, were reported in Tables 5.1 and 5.2. Figure 5.1 shows a box plot comparing postcanine dimensions to predicted Region II length. These dimensions include the observed length of the molar dentition, which is expected to be shorter than the predicted length of Region IT, and the observed distance between the most mesial premolar to the most distal molar. Also included in the plot is the calculated predicted length of Region II. 56

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Table 5.1 Mean (x) and standard deviation (sd) of tooth row length. (Molar length= length of all molars; M-P length= the last molar to the first premolar; M-1 length= last molar to first incisor) Taxon Sex n Molar Length M -P Length M-I Length PrlmatE!S x (mm) sd x (mm) sd x (mm) sd Cacajao. cufvus F 1 5.17 0 9.84 0 27.03 0 Cercopithecus diana F 1 7.32 0 1.82 0 27.85 0 Pongo pygmaeus F 1 23.8 0 39.88 0 75.05 0 Alouatta palliata F 2 32.9 1.27 64.81 5.54 93.14 0 Aotus lemurinus F 1 9.55 0 21.75 0 39.34 0 Gorilla _gorilla F 2 33.1 2:68 55.37 0.14 101.2 2.98 Cebus capuchinus F 3 8.64 0.23 19.76 0.41 34.79 1.18 Nycticebus coucang F 1 7.01 0 16.52 0 -A te/es F 3 10.28 0.48 21.9 0.66 36:73 1.2 Callithrfx argentata F 1 4.09 0 7.36 0 14.66 0 Galago demldoff ? 1 3.76 0 6.93 0 Saguinus geoffroyi M 2 4.61 0.13 8.58 0.12 17.45 0.44 F 2 4.51 0 8.64 0.22 17.39 0.19 Pithecia plthec/a F 2 6.88 0.67 14.83 0.81 27.49 1.58 Carnivores Lynx canadensis M 1 7.81 0 18.47 0 47.39 0 F 1 7.15 0 18.03 0 42.68 0 7 1 7.77 0 17.48 0 46.05 0 57

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Table 5.1 (cont.) Taxon Sex n Molar Length M P Length M-I Length Carnivores (Cont.) x (mm) sd x (mm) sd x (mm) sd Lynx rufus M 4 10.46 1.73 20."5 1.81 47.53 1.55 F 4 9.21 0.83 19.37 1.57 45.03 2.38 7 2 9.25 1.25 19.14 0.33 45.13 0.49 Puma concolor M 2 14.99 0.77 33.37 2.31 62.1 3.53 F 6 17.09 ( 1.8 37.56 1.68 66.75 2.14 1 2 15.34 0.77 32.18 2.5 59.23 3.78 Procyon lotor M 9 7.75 0.59 32.8 1.19 52.13 2.04 F 1 7.82 0 32.26 0 51.07 0 Ursus arctos M 1 48.98 0 66.32 0 154.1 0 7 1 31.23 0 58.14 0 125.79 0 Ursus americanus M 4 23.72 1.29 60.85 4.46 100.86 7.56 F 1 22.64 0 62.94 0 107.34 0 ? 4 27.37 2.81 65.64 3.10 105.97 3.40 Canis latrans M 6 10 0.47 64.72 3.53 96.23 .77 F 6 9.54 0.55 62.13 3.9 93 4.87 Canis lupls M 8 13.24 1.42 78.79 5.33 121.9 8.94 F 1 12.26 0 75.72 0 116.7 0 Vu/pes vulpes M 7 7.99 2.55 45.42 4.30 69.14 4.94 F 4 '8.81 2.57 44.74 4.50 68.57 4.97 Vulpes velox M 6 5.81 0.44 40.68 2.13 59.95 3.36 F 5 6.46 0.09 41.6.7 2.81 60.80 3.73 Urocyon M 5 6.95 0.45 39.59 1.23 58.17 3.77 clnereoargenteqs 7 2 6.93 0.07 35.84 2.01 54.51 2.11 58

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Table 5.1 (cont.) Taxon Sex n Molar Length MP Length M-I Length Artlodactyls x (mm) sd x (mm) sd x (mm) sd Cervus elephus M 1 57.38 0 121.53 0 --F 1 63.28 0 118.86 0 --Odoco/leus virgin/anus M 1 33.52 0 61.59 0 --F 2 30.31 0.09 67.27 1.65 -Odocoileus hemionus F 2 35.06 5.66 69.76 3.15 --Mazama americana F 2 21.91 2.11 47.25 7.24 -Perissodactyls Equus equus ? 3 45.97 2.54 106.89 5.83 232.17 1.77 Equus greyvi ? 2 46.05 2.00 1 1 1.45 6.17 228.14 2.47 Tapirus terrestrius F 1 45.59 0 120.82 0 217.59 0 ? 1 45.93 0 113.7 0 --59

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Table 5.2 Mean (x) and standard deviation (sd) of variables used to calculate Predicted Region II length. Taxon Sex n Blcondylar Breadth Palatal Breadth Height of TMJ Primates x (mm) sd x (mm) sd x (mm) sd Cacajao cu/vus F 1 31.41 0 18.81 0 13.8 0 Cercopithecus diana F 1 30.14 0 19.69 0 5.5 0 Pongo pygmaeus F 1 84.01 0 51.64 0 27.7 0 Alouatta pa/liata F 2 128.88 0.91 79.79 3.44 84.8 1.7 Aotus /emurlnus F 1 51.83 0 29.56 29.56 7.8 0 Gorilla gorilla F 2 107.86 0.36 63.51 0.25 55.45 9.4 Cebus capuchinus F 3 44.27 3.7 26.29 0.39 6.13 2.15 Nyctlcebus coucang F 1 28.92 0 17.57 0 1.9 0 Ate/es geoffroyi F 3 46.65 3.64 26.06 2.03 12.6 2.55 Callfthrix argentata F 1 20.29 0 11.48 0 2.76 0 Ga/ago demidoff ? 1 13.74 0 9.02 0 2.65 0 Sagulni.Js geoffroyi M 2 23.26 0.44 13.74 0.29 3.52 1.65 F 2 23.47 0.44 13.94 0.58 5.31 1.44 Pithec/a pithecia F 2 34.91 1.67 17.94 0.33 8.75 3.04 Carnivores Lynx canadensis M 1 65.76 0 44.83 0 2.2 0 F 1 53.33 0 39.8 0 o.s 0 7 1 67.24 0 43.61 0 1 0 Lynx rufus M 4 65.49 3.76 39.32 1.5 1.28 0.92 F 4 64.81 3.13 40.62. 4.11 0.8 0.63 7 2 60.83 2.95 38.15 2.01 0.6 0.14 60'

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Table S.l (cont.) Taxon Sex n Blcondylar Breadth Palatal Breadth Height of TMJ Carnivores x (mm) sd x (mm) sd x (mm) sd !(cont.) Puma concolor M 2 82.78 7.81 54.81 3.97 -0.15 0.21 F 6 87.74 3.94 57.23 2.84 -2.12 0.58 7 2 79.42 4.38 51.86 3.27 -6.55 0.07 Procyon lotor M 9 52.51 1.72 .. 33.58 1.16 1.1 1 1.06 F 1 54.29 0 34.29 0 1.4 0 Ursus arctos M 1 156.48 0 81.89 0 -18.9 0 7 1 107.31 0 59.36 0 -16.9 0 Ursus americanus M 4 109.43 6.34 52.61 3.54 -8.53 21.75 F 1 115.57 0 52.06 0 -24.2 0 ? 4 109.13 5.73 53.82 0.72 5.6 10.87 Canis latrans M 6 64.68 12 44.41 3.02 11.85 3.26 F 6 63.86 4.5 43.53 1.12 11.6 3.08 Canis lupis M 8 89.41 5:93 68.24 4.6 7.39 13.62 F 1 80.74 0 62.17 o 2.2 0 Vulpes vulpes M 7 48.91 3.7 32.02 2.68 0.03 1.71 F 4 46.76 2.63 32.33 2.38 0.58 2.92 Vulpes velox M 6 41.98 1.99 28.98 1.55 1.04 0.5 F 5 43.87 2.21 29.81 1.03 1.75 1.47 Urocyon M 5 45.27 1.95 28.18 0.92 6.8 1.61 clnereoargenteus ? 2 41.48 0.64 27.31 1.07 5.3 0.16 Artlodactyls Cervus elaphus M 1 112.19 0 99.36 0 34 0 F 1 101.69 0 94.92 0 20.4 0 Odocoi/eus M 1 68.74 0 58.8 0 27.5 0 virgin/anus F 2 71.89 2.6 59.13 5.15 22.45 3.75 61

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Table S.2 (cant) Taxon Sex n Bicondylar Breadth Palatal Breadth Height of TMJ Artlodactyls x (mm) sd x (mm) sd x (mm) sd l(cont.) Odocoileus F 2 77.62 4.75 68.43 4.01 25.5 2.83 hem/onus Mazama americana F 2 54.15 2.45 52.46 3.68 21.1 6.22 Perlssodactyls Equus equus 7 3 105.66 8.33 82.33 3.14 89.65 2.61 Equus greyv/ 7 2 103.71 3.56 83.35 2.0 81.8 4.38 Tapirus terrestr/us F 1 123.08 0 89.12 0 51.3 0 7 1 119.88 0 86.54 0 57.1 0 Taxon Sex n Molar to TMJ Est. Reg. II Length Primates x (mm) sd x (mm) sd Caca}ao culvus F 1 21.15 0 14.13 0 Cercopithecus diana F 1 20.73 0 14.18 0 Pongo pygmaeus F 1 49.09 0 33.18 0 Alouatta palliats F 2 96.32 2.6 68.87 0.69 Aotus lemurinus F 1 23.64 0 14.27 0 Gorilla gorilla F 2 55.68 8.34 38.55 5.86 Cebus capuchlnus F 3 23.34 3.13 14.47 1.1 Nyctlcebus coucang F 1 17.52 0 10.85 0 Ate/es geoffroyl F 3 28.11 3.07 16.95 .72 Calllthrix argentata F 1 11.43 0 6.74 0 Gslago demidoff 7 7.55 0 5.26 0 62

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Table S.2 (cont.) Taxon Sex n Molar to TMJ Est. Reg. II Length Primates (cont.) x (mm) sd x (mm) sd Sagu/nus geoffroy/ M 2 12.76 0.43 7.9 0.41 F 2 12.76 0.15 8.14 0.25 P/thec/a pfthecfa F 2 20.5 0.5 11.33 0.35 Carnivores Lynx canadensis M 1 38.57 0 26.56 0 F 1 31.7 0 23.72 0 7 1 37.1 0 24.18 0 Lynx rufus M 4 32.32 2.75 21.71 2.48 F 4 34.16 3.06 20.4 1.8 7 2 30.82 0.84 18.32 0.48 Puma concolor M 2 52.25 6.89 37.3 0 F 6 48.47 3.14 31.86 2.18 ? 2 47.39 0.87 31.7 0.81 Procyon Jotor M 9 31.41 2.76 20.0 0.88 F 1 29.38 0 18 .. 71 0 Ursus arctos M 1 83.94 0 45.67 0 7 1 79.64 0 45.7 0 Ursus americanus M 4 70.2 6 35.41 2.93 F 1 77.83 0 36.98 0 7 4 71.21 5.31 35.99 1.2 Canis Jatrans M 6 45.2 2.44 33.15 2.58 F 6 46.52 2.55 33.17 1.9 Canis lupis M 8 60.88 7.37 48.13 7.2 F 1 56.57 0 44.09 0 Vulpes vulpes M 7 30.81 1.16 20.3 0.55 F 4 28.95 1.24 20.23 0.94 Vu/pes velox M 6 25.95 0.9 18.04 0.62 F 5 25.85 1.22 17.79 0.82 63

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Table 5.2 (cont) Taxon Sex n Molar to TMJ Est. Reg. II Length Carnivores x (mm) sd x (mm) sd (cont.) Urocyon M 5 30.54 3.28 19.75 2.01 clnereoargenteus 7 2 29.02 0.63 19.74 1.51 Artlodactyls Cervus e/aphus M 1 93.84 0 88.43 0 F 1 71.75 0 70.34 0 Odocolfeus M 1 54.94 0 51.15 0 virg/nianus F 2 56.65 3.71 49.91 6.13 Odocoileus F 2 57.37 10.12 54.23 1.96 hemionus Mazama americana F 2 43.31 8.32 45.66 10.25 Perissodactyls Equus equus 7 3 108.28 4.94 96.85 3.11 Equus greyvi 1 2 105.93 2.8 96.75 0.63 Taplrus terrestrlus F 1 66.88 0 54.99 0 7 1 74.52 0 61.07 0 64

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Table 5.3 Mean (x) and standard deviation (sd) of the geometric mean and one variabfe used to calculate it. The other variables used in this calculation are listed in Tables 5.1 and 5.2. Taxon Sex n Geometric Mean Temporal Foramen Length Primates x (mm) sd x (mm) sd Cacajao cu/vus F 1 15.60 0 19.38 0 Cercopithecus diana F 1 16.80 0 18.35 0 Pongo pygmaeus F 1 45.02 0 39.84 0 A/ouatta palliats F 2 72.11 1.71 79.95 0.49 Aotus lemur/nus F 1 20.8 0 12.8 0 Gorilla gorilla F 2 59.51 2.29 55.42 3.63 Cebus capuchinus F 3 22.27 0.94 24.47 1.24 Nycticebus coucang F 1 15.85 0 17.73 0 Ateles geoffroyi F 3 22.94 1.47 22.16 1.39 Calllthrix argentata F. 1 9.76 0 9.53 0 Galago demldoff 7 1 7.51 0 6 .. 84 0 Saguinus geoffroyi M 2 11.32 0.08 11.16 0.21 Sagu/nus geoffroyi F 2 11.11 0.18 10.33 0.04 Pithecia plthecia F 2 16.47 0.76 17.10 0.36 Carnivores Lynx canadensis M 1 28.62 0 29.16 0 F 1 24.98 0 28.19 0 7 27.73 0 23.6 0 Lynx rufus M 4 29.18 3.07 30.89 1.18 F 4 29.30 0.72 26.10 2.08 7 2 26.93 2.24 25.66 0.88 65

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Table S.3 (Cont.) Taxon Sex n Geometric Mean Temporal Foramen Len :Jth Carnivores (cont.) x (mm) sd x (mm) sd Puma conco/or M 2 41.87 2.34 45.39 4.88 F 6 43.61 1.19 42.44 2.08 ? 2 40.24 1.68 41.55 0.06 Procyon lotor M 9 0.63 29.48 2.80 F 1 26.22 0 27.24 0 Ursus arctos M 1 85.20 0 83.95 0 ? 1 62.05 0 74.5 0 Ursus americanus M 4 54.20 3.17 63.35 5.93 F 1 55.97 0 72.04 0 7 4 56.92 0.01 65.69 3.78 Canis latrans M 6 33.61 1.14 42.63 3.57 F 6 32.01 1.05 41.53 1.92 Canis lupis M 8 45.97 2.75 55.71 5.64 F 1 42.57 0 53.34 0 Vu/pes vulpes M 7 24.62 2.99 29.99 1.75 F 4 24.72 2.86 28.54 1.52 Vu/pes velox M 6 20.48 0.82 24.96 0.70 F 5 21.38 0.62 24.78 1.50 U. cinereoargenteus M 5 22.38 0.97 28.35 2.55 7 2 21.37 0.55 26.61 1.82 .66

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Table S.3 (cont.) Taxon Sex n Geometric Mean Temporal Foramen Length Artiodactyls x (mm) sd x (mm) sd Cervus e/aphus M 1 87.32 0 90.9 0 F 1 85.38 0 87.02 0 Odocoileus M 1 50.77 0 49.03 0 virgin/anus F 2 51.53 1.05 54.88 2.14 Odocoi/eus hemionus F 2 55.88 2.05 52.73 0.66 Mazama americana F 2 41.27 3.47 46.68 5.78 Perlssodactyls Equus equus ? 3 67.73 1.90 52.96 5.50 Equus greyvi ? 2 68.26 3.14 54.59 4.49 Taplrus terrestrlus F 1 75.89 0 66.32 0 ? 1 75.49 0 68.15 0

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In order to more easily understand these graphs, the data were standardized against molar row length. This has the effect of reducing differences in size and shape between the different taxa. Figure 5.1 shows that the molar length of all species fit well within the predicted length ofRegion II. Hypothesis 1, which states the molars should fit within the predicted length of Region II, is therefore accepted. Many species also have premolars that fall within the predicted length of Region II. This is not surprising given the wide range of premolar function that is represented across mammals. However, what is unusual is that those species with the typical "grinding dentition" do not necessarily have premolars that fit into this region. For example, Cervus elaphus has premolars that fall well outside of Region II, whereas, Mazama americana's premolars all fall within this region. In addition, a primate, Saguinus geoffroyi, has premolars that fall well within the region. It appears that there is no apparent pattern in regard to premolar length and the predicted length of Region II. This may be due to the wide variety of functions that premolars serve. Perhaps there is too much variation within the function of this type of tooth across mammals to be able to predict its location relative to Region II length. 68

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T. terrestrius I OJ E. greyvl I i E. equus 1 m 0. virgin/anus ID 0. hem/onus rn M. americana C. elaphus rn V. vulpes I f, V. velox ,._, t-Q]----; U. cinereoargenteus ...--. r-{D U. arctos U. americanus 4 {]}. P. lotor t-[]}-; P. concolor L. rufus ..... L. canadensis C. /upis C.latrans t{[}; 5. geoffroyi I P. pygmaeus I I P. pithec/a m N. coucang I I .G. gorilla [[] G. demidoff I I C. diana II C. capuchlnus I u C. calvus IiI C. argentata A. pal/lata [) A. lemur/nus I I A. geoffroy/ I m I I I I 0 2. 3 4 5 6 7 B 9 10 Figure 5.1 Plot comparing postcanine dimensions to predicted Region II length. All distances have been standardized against molar length. Therefore, the molar dentition fills the distance between 0 and I with the first molar being located at 1. The black boxes represent the anterior end of Region II. Region II, therefore, extends from 0 to this anterior position. The white boxes represent the anterior end of the premolars, therefore, the total length of the postcanine dentition extends from 0 to this anterior position. 69

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Fused vs. Unfused Mammals with an unfused symphysis are expected to have a theoretically shorter Region II length due to a laterally located muscle resultant force. According to Hypothesis 2, it is expected that the ratio between predicted Region II length and observed molar length will be higher in mammals with an unfused symphysis because of their theoretically shorter Region II length. A t test between predicted Region II length to observed molar length by species with a fused or unfused symphysis shows that the means for these two groups are significantly different. The mean for the fused group is 1. 75 with a standard deviation of0.35 and 2.54 for the unfused group with a standard deviation of0.81 (t = -5.13; p < 0.01). This is also represented graphically in Fig. 5.2. This plot shows that mammals with an unfused symphysis tend to have higher predicted Region II length/observed molar length ratios. Hypothesis 2 is accepted. Allometric Analysis An allometric analysis was performed on the variables used to calculate the predicted length of Region II. It is important to assess how changes in shape with size affect these variables because of the wide range of cranial sizes represented in this sample. Figure 5.3 displays bivariate plots of log-transformed data showing the relationship between the geometric mean, bicondylar breadth, palatal 70

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3.75 3.5 -r---' 3.25 3 c ol1l 2.75 _. 1.. .!2 0 I: 2.5 ., ol1l > 1.. ol1l 1:l 2.25 0 ........ ., ol1l .... 2 ., T f: a.. 1.75 1.5 j_ -'----1.25 Fused Unfused Figure 5.2 Box plot representing the means of molar length divided by the geometric mean for mammals with a fuse and unfused symphysis. These plots show that mammals with an unfused symphysis have a higher predicted to observed molar length ratio.

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breadth, height of the TMJ, and molar row length. Table 5.51ists the slope andYintercept for these variables. Molar length scales with positive allometry. Its 95% confidence intervals range from 1.19-1.36. This means that as cranial size increases molar length increases at an even greater rate. This is surprising considering that increasing the length ofthe molars decreases the likelihood that they will fit into Region II. Other variables must be configured in order to increase the length of this region. Bicondylar breadth scales with negative allometry. Their 95% confidence intervals range from 0.81-0.89 and 0.87-0.96, respectively. As cranial size increases bicondylar breadth does not increase as rapidly. Decreasing the length between the condyles has the effect of increasing the length ofRegion II. TMJ height scales with strong positive allometry. Its 95% confidence intervals range from 2.35-3.52. However, there is only a weak correlation between height of the TMJ and cranial size(?= 0.62). This is expected considering that some species with very large cranial sizes, such as horses, have very tall TMJ's, whereas, other species with large crania have very low TMJ's, such as bears. Height of the TMJ is extremely variable within mammals. The anteroposterior position of the MRF is another variable that affects the distribution ofRegion II. The A-Pposition of the MRF is assumed to be located at the distal end of the molar tooth row, therefore, the length from the TMJ to the last molar represents this distance. This variable scales isometrically 72

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Figure 5.3 Bivariate plots of log-transfonned data showing the relationship between cranial size (represented by the geometric mean) and other masticatory variables relevant to the distribution of Region II (+indicate species without symphyseal fusion, indicate species with fusion). See Table 5.8 for regression parameters. 1.9 1.8 1.7 .6 1.5 ,...... 1.4 :15 Q'l 1.3 c Q/ ...J 1 .2 II. 1.1 0 6 1.0 . 0.9 0.8 0.7 0.6 0.5 2.2 2.1 2".0 1.9 t 1.8 1.7 .!!! f 1.6 1.5 e 1.4 .s 1.3 1.2 .8 .9 1.0 1.1 1.3 1.5 1.7 1.9 In (Geometric Mean) y = 0.85x + 0.51 1.1 .8 .9 1.0 1.1 1.3 1.5 1.7 1.9 In (Geometric Mean)

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Figure 5.3 (cont.) 2.0 1.9 1.8 .......... 1.7 1.6 Cll 1.5 L. m 1.4 .!!1. "' 1 3 0.. '-" 1.2 1.1 1.0 + y = 0 .92x + 0.23 0.9 .8 .9 1.0 1.1 1.3 1.5 1.7 1.9 In (Geometric Mean) 2.0-r---------------,...,---. 1.5 1.0 ,._ 0 -:. 0.5 .5 0.0 -0.5 a II B y= 2.93x-3.85 .8 .9 1.0 1.1 1.3 1.5 1.7 1.9 In (Geometric Mean) 74

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Figure 5.3 (cont.) 2.1 ,...., 1.9 1.8 1.7 1.6 1.5 1.4 .k 1.3 '-" .!: 1.2 1.1 1.0 0.9 + 0.8 y = 0.99x + 0.10 .. 8 .9 1.0 1.1 1.3 1.5 1.7 1.9 In (Geometric Mean) 75

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Table 5.4 Variables regressed on the geometric mean to assess allometric relationships. Variable r:z. Slope -intercept 95% Confidence (Re2ressed on GM) intervals for Slope Molar Length 0.88 1.28 -0.84 1.19-1.36 Bicondylar Breadth 0.93 0.85 0.51 0.81-0.89 Palatal Breadth 0.94 0.92 0.23 0.88-0.96 Height of TMJ 0.62 2.93 -3.85 2.35-3.52 A-P Position of 0.93 0.99 0.10 0.95-1.04 MRF 76

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with the geometric mean; its 95% confidence intervals include a slope of I. The length from the last molar to the TMJ and cranial size increase at the same rate. Palatal breadth also scales very close to isometry. Its 95% confidence intervals range from 0.88 to 0.96. As cranial size increases, palatal breadth increases at about the same rate. These allometric analyses have allowed us to look further into the data and explore how these variables change in relation to cranial size. This helps us to understand the relationship between cranial size and the masticatory system variables that are responsible for the distribution of Region II. 77

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CHAPTER6 DISCUSSION The goal of this study is to test the constrained model on a wide variety of mammals in order to assess systematic differences in craniofacial configuration that may be the result of mandibular symphyseal fusion. Of particular concern is the distribution of Region IT. This region should envelope the grinding dentition because this is where highest magnitude bite forces are produced and these are the teeth most suitable for these forces. The following is a summary of my predictions: 1. Molar length will be shorter than the predicted length of Region II for both groups of mammals. 2. The ratio between predicted Region II length and observed molar length is predicted to be higher in mammals with an unfused symphysis because of the theoretically shorter Region II length. The following is a summary of the results obtained from this study: 78

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1. The molar row of all species fit well within the predicted length ofRegion II. Hypothesis 1 is accepted. 2. The ratio between predicted Region II length and observed molar length is higher in mammals with an unfused symphysis. Hypothesis 2 is accepted. The results from this study support the general applicability of the constrained model for both mammals with a fused and unfused symphysis. This model is able to explain the interactions between some craniofacial variables of a broad range of mammals including primates, carnivores, perissodactyls, and artiodactyls. This is in accordance with results from previous studies. Support for Previous Research The main assumption of the constrained model is that the TMJ should not be subjected to joint distraction (Greaves, 1978). This causes limitations on masticatory system form and function. One such limitation is the distribution of the Region II. According to the model, Region II should envelope at least all of the molars. Spencer's (1999) morphometric analysis of anthropoid cranial configuration provides evidence in support of this. Anthropoids appear to exhibit craniofacial form that is consistent with selection against TMJ distraction by having the parameters that influence the distribution of the region be configured 79

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in such a way so that at least all molars are maintained within this region. This is consistent with results from the present study. Results from the present study also indicate that the molars as well as some of the premolars are located within Region II. The goal ofthe present study was to look at the broad-scale applicability ofthe constrained model to mammals. Not only were a wider range oftaxa used than in previous studies, this study also focused on a craniofacial feature that results in fundamental differences in muscle recruitment patterns, the mandibular symphysis. Previous studies involving galagos, lemurs, and dogs, all ofwhich have an unfused symphysis, have found that these species exhibit different working-side to balancing-side muscle force ratios (Leibman and Kussick, 1965; Rylander, 1979; Dessem, 1989; Rylander et. a!., 1998, 2002). Those species with an unfused symphysis recruit less balancing-side muscle force. Recruiting less balancing-side muscle force means that the muscle resultant force cannot lie in the midline. Having a l\t1RF more laterally located consequently reduces the length of Region II. Because mammals with an unfused symphysis have a theoretically shorter Region II they are expected to have a higher ratio between predicted Region II length and observed molar length. This was indeed found to be the case. Mammals with an unfused symphysis appear to differ in a systematic way from those with fusion. 80

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There has only been a small number of mammalian species whose muscle recruitment patterns have been studied (Leibman and Kussick, 1965; Rylander, 1979; Dessem, 1989; Rylander et. a/., 1998, 2002). The finding from this study provides indirect evidence for the previous research on muscle recruitment patterns. It can be inferred from this fmding that all species with an unfused symphysis may recruit less balancing-side muscle force. This would cause them to have a shorter Region II length which would .create a higher ratio between predicted Region II length and observed molar length as was found in this study. Allometric Analyses An allometric analysis has allowed us to understand how changes in size affect the masticatory system configuration of such a wide range of mammals. It was found that molar length and height of the TMJ scale with positive allometry, bicondylar breadth scales with negative allometry, and palatal breadth and the Apposition of the MRF scale isometrically. Molar length was found to scale with positive allometry. This means that as cranial size increases, the molars are increasing in length more rapidly than expected for isometry. So as cranial size increases the molars are increasing at an even greater rate. One would expect that the molars would not increase at this greater rate given the constraint of Region II length. According to the model, the molars 81

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should fall within this region in order to avoid TMJ distraction. However, there are other factors that act to influence this variable, such as diet. Diet has been shown to be closely linked to body size, particularly within primates (Fleagle, 1988). An animal's teeth are what allow it to meet its nutritional requirements. Therefore, molar size is constrained not only by the need to avoid TMJ distraction but also by diet. This becomes even more complex when one considers that diet can also be a function of body size. Within primates, the natural physiological break between insectivores and folivores occurs at 500 grams and is known as Kay's threshold (Kay, 1975; Fleagle, 1988). In general, folivorous primates have body weights that are no less than 500 grams and insectivores tend to weigh less than this limit. This is because as body size increases, metabolic requirements change. A larger animal actually has relatively lower energy requirements than a smaller one. Although leaves are generally lower in energy yield than insects or fruit, a large animal can afford this because they need less energy per kilogram of mass than a small animal (Fleagle, 1988). Results from the allometric analysis of this study show that as cranial size increases (and therefore body size) molar length is increasing even more rapidly. This may be because even though a larger animal does not have as high of energy needs it still requires larger molars in order to process vegetation which is much tougher than either insects or fruit. 82

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This idea also pertains to artiodactyls and perissodactyls. Both of these groups have very large grinding dentition. This is thought to be an adaptation to the tough grass material that comprises their diet (L.M. Spencer, 1995). Having larger molars increases the surface area of the grinding dentition thereby allowing a more efficient breakdown of the tough vegetation. Although this can explain why molar length increases more rapidly than body size this does not explain how the larger molars are able to fit within Region II. Other craniofacial variables must be working to increase the length of Region II. TMJ height may be one of those variables. It scales with positive allometry meaning that it also increases at an even greater rate that cranial size. Many of the larger animals such as horses, sheep, and deer have very high TMJ's. This has the effect of orienting Region II more anteriorly which increases its length. Bicondylar breadth also seems to be working to increase the length of Region II as cranial size increases. This is becausea decrease in bicondylar breadth increases the length of this region. This parameter scales with negative allometry; it does not increase as rapidly as cranial size. Therefore, having bicondylar breadth not increase as rapidly as cranial size may help the molars to be maintained within Region II even as cranial size increases. Overall it appears that as cranial size increases across mammals, there are variables that are configured in such a way to increase the length ofRegion II. 83

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This allows the molars to be maintained within this region even if they are increasing at a rate greater than cranial size. Biomechanical hnplications for Diet Spencer (1999) proposes the idea that morphological patterns within the masticatory system may stem from the selective trade-off between increasing bite force magnitudes and avoiding joint distraction. Joint distraction is unavoidable when biting occurs in Region III (Greaves, 1978). Selection should favor a morphology that does not allow teeth to be located within this region This idea has been supported by the present study and the previous research discussed above. The molars were found to lie only within Region II. However, some species which require high magnitude bite forces to be produced on either the incisors or the canines would benefit from having the teeth located more posteriorly. A more posterior position for the dentition and a relatively anterior position ofthe superficial masseter and anterior temporalis muscles enable greater force production in Region I (the region where the incisors and canines are expected to be located according to the constrained model) (Spencer and Demes, 1993; Spencer, 1999). This could potentially cause some ofthe postcanine dentition to be moved back into Region ill. Some groups that may benefit from greater force production on their anterior dentition include callitrichids, some pitheciines, Cebus, and some of the 84

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carnivores (Spencer, 1999). These groups are specialized for intensive force production on either the incisors or the canines. Carnivores require especially high levels of force production on their canines as these are the teeth that are involved in capturing and killing their prey. The TMJ of this group is also particularly well-suited to capturing prey. It is "locked" into the glenoid fossa so that it can withstand the high magnitude forces that are inevitable as the prey struggles to free itself from the grip ofthe predator. This is likely an adaptation to limit joint distraction. Selection can limit teeth being located within Region ill through changes in the configuration of other variables of the masticatory system. Some changes that would be beneficial include decreasing bicondylar breadth or increasing palatal breadth. Increasing the height of the TMJ would also be beneficial. All of these have the effect of increasing the length ofRegion II which may enable the molars to be maintained with this region. The avoidance of TMJ distraction places constraints on masticatory system morphology. Mandibular symphyseal fusion is another constraint. Fusion constrains the location of the MRF because it allows more balancing-side muscle force to be recruited. This has the potential effect of placing the MRF in the midline. However, the MRF must move laterally when biting in Region IT in order to be maintained within the triangle of support (Spencer and Demes, 1993; Spencer, 1995, 1998, 1999). A reduction in balancing-side muscle activity 85

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enables this to happen and is necessary to avoid TMJ distraction. Although having symphyseal fusion allows more balancing-side muscle force to be recruited, the constrained model limits this in order to maintain the resultant force within the triangle of support. Maintaining an equal balancing-side to working-side muscle force ratio (which would place the MRF in the midline) is acceptable within this model when biting in Region I. This is because Region I envelopes a midline MRF. What is interesting, however, is that none of the carnivores, which require high magnitude force production on their anterior dentition, have a fused symphysis. It is expected that this group would benefit from having a fused symphysis in order to recruit more balancing-side muscle activity. One possible explanation can be approached from a behavioral standpoint. When a carnivore captures its prey in its jaws it may be biting equally with all of its anterior dentition. This would make both sides of the jaw the working side and may allow the predator to maximize force production. They, therefore, do not need to have symphyseal fusion because they are already maximizing the amount of force that can be produced. Further behavioral research would serve to clarify this issue. Some species which need high magnitude forces produced on their molars, such as the artiodactyls, also do not have a fused symphysis (Greaves, 1978). It is possible that selection has favored another aspect of their craniofacial morphology 86

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that enables them to efficiently process their food. One such morphology could be their large selenodont grinding dentition Having this larger surface area allows them to more efficiently process the tough vegetation that is characteristic of their diet. Colobines, which have a fused mandibular symphysis, also have a diet that consists of tough vegetable matter (Fleagle, 1988). Perhaps having a fused symphysis enables them to generate a higher magnitude muscle resultant force on their molars than the artiodactyls, which allows them to break down their food material in just as efficient a manner. lfthis is the case then symphyseal fusion would be highly advantageous for this group. This issue is not a simple one due to the numerous confounding variables that are possible within not only the masticatory system but also the digestive system. Colobines also have a gut morphology that is specialized to break down vegetation, including large complex stomachs (Fleagle, 1988). It is not only the molars that work to break down the food, but their specialized stomachs as well. The issue of symphyseal fusion and its impact on diet is extremely complex. There are numerous variables within not only the masticatory system but the body as a whole that work together to produce an animal that is finely adapted to its diet and environment. Further research is needed before any definitive statements can be made regarding symphyseal fusion and diet. 87

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Conclusion Overall it appears that mammals with a fused symphysis do differ in systematic ways from those with an unfused symphysis. Both groups have molars that fit within the estimated length of Region II. However, the fact that mammals with an unfused symphysis have higher predicted Region II length to observed molar length ratio shows that these groups differ in craniofacial configuration that is most likely the result of different workingto balancing-side muscle force ratios that are in turn the result of differing symphyseal morphologies. This has important ramifications for the evolution of symphyseal fusion. Variables within the masticatory system interact with one another to produce a utiique configuration. However, "evolution" can only be so creative due to the constraints of these variables having to interact with one anther in order to maintain the molars within Region II. This is in addition to numerous other constraints that can be imposed upon this system. The molars should always be maintained within Region II in order to maximize masticatory force production. Extinct species both with and without fusion, should have craniofacial variables, such as bicondylar and palatal breadth, height ofthe TMJ, and the A-Pposition of the MRF that also work in conjunction to increase the length of Region II. Looking to see if these features are correlated with early symphyseal fusion would help us to understand how fusion affects the masticatory system as a whole and why it came to be in the first place. 88

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Cranial form and function are extremely important issues to biological anthropologists. Understanding craniofacial form and the changes in morphology that have occurred through time is dependent on this biomechanical model. This model allows us to understand why certain variables are distributed in the way they are, such as the placement ofthe molars within the cranium, or the height of the TMJ above the occlusal plane. This can be used to explain much of the variation we see in cranial form in both extinct and extant forms, and in both groups with and without mandibular symphyseal fusion. 89

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LITERATURE CITED Allard, M.W., B.E. McNiff, and M.M. Miyamoto (1996). Support for interordinal eutherian relationships with an emphasis on primates and their archontan relatives. Molecular Phylogenetics and Evolution 5:78-88. Beecher, R.M. (1977). Function and fusion of the mandibular symphysis. American Journal of Physical Anthropology 47:325-336. Beecher, R.M. (1979). Functional significance of the mandibular symphysis. Journal of Morphology 139:117-130. Cartmill, M. (1975). Daubentonia, Dactylopsila, woodpeckers, and klinorhynchy. In R.D. Martin, G.A. Doyle, and A.C. Walker (eds.): Prosimian Biology. London: Duckworth, pp. 655-670. Clutton-Brock, T.H. (1974). Primate social organization and ecology. Nature 250:539-542. Clutton-Brock, T.H. and P.H. Harvey (1979). Comparison and adaptation. Proceedings of the Royal Society of London B 205:547-565. Davis, D.D. (1955). Masticatory apparatus in the spectacled bear Tremarctos ornatus. Fie/diana-Zoology 37:25-46. Darroch and Mosimann (1985). Canonical and principal components of shape. Biometrika 72:241-252. Demes, B. and N. Creel ( 1988). Bite force, diet, and cranial morphology of fossil hominoids. Journal of Human Evolution 17:657-670. Dessem, D. (1989). Interactions betweenjaw-muscle recruitment and jaw-joint forces in Canisfamiliaris. Journal of Anatomy 164:101-121. Dessem, D. and Druzinsky, R.E. (1992). Jaw-muscle activity in ferrets, Mustela putoriusfuro. Journal of Morphology 213:275-286. 90

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DuBrul, E. L. (1977). Early hominid feeding mechanisms. American Journal of Physical Anthropolology 47:305-320. Dumont, E.R. and A. Rerrel (2003). The effects of gape angle and bite point on bite force in bats. Journal of Experimental Biology 206:2117-2123. Fleagle, J.G. (1988). Primate Adaptation and Evolution. New York: Academic Press. Gingerich, P.D. (1971). Functional significance of mandibular translation in vertebrate jaws. Postilla 152:1-10. Greaves, W.S. (1978). The jaw lever system in ungulates: a new model. Journal ofZoology 184, 271-285. Greaves, W.S. (1982). A mechanical limitation on the position of the jaw muscles of mammals: the one-third rule. Journal ofMammalogy 63: 261-266. Greaves, W.S. (1988). A functional consequence of an ossified mandibular symphysis. American Journal of Physical Anthropology 77:53-56. Gysi, A. (1921). Studies on the leverage problems ofthe mandible. Dental Digest 27, 74-84, 144-150, 203-208. Riiemae, K. (1984). Functional aspects of primate jaw morphology. In D.J. Chivers, B.A. Wood, and A. Bilsborough (eds.): Food Acquisition and Processing in Primates. New York: Plenum Press, pp. 257-281. Rylander, W.L.(1975). The human mandible: lever or link? American Journal of Physical Anthropology 43, 227-242. Rylander, W.L. (1977). In vivo bone strain in the mandible of Galago crassicaudatus. American Journal of Physical Anthropology 46:309-326. Rylander, W.L.(1979a). An experimental analysis oftemporomandibular joint reaction force in macaques. American Journal of Physical Anthropology 51, 433-456. Rylander, W.L. (1979b). The functional significance of primate mandibular form. Journal of Morphology 160, 223-240. 91

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Rylander, W.L. (1984). Stress and strain in the mandibular symphysis of primates: a test of competing hypotheses. American Journal of Physical Anthropology 64:1-46. Rylander, W.L. (1985). Mandibular function and temporomandibular joint loading. In D.S. Carlson, J.A. McNamara, and K.A. Ribbens (eds.): Developmental Aspects of Temporomandibular Joint Disorders. Ann Arbor, MI: University of Michigan Rylander, W.L. and K.R. Johnson (1994). Jaw muscle function and wishboning of the mandible during mastication in macaques and baboons. American Journal of Physical Anthropology 94:523-547. Rylander, W.L., Ravosa, M.J., Ross, C.F., and K.R. Johnson (1998). Mandibular corpus strain in primates: further evidence for a functional link between symphyseal fusion and jaw-adductor muscle force. American Journal of Physical Anthropology 107:257-271. Rylander, W.L., Vinyard, C.J., Wall, C.E., Williams, S.R., and K.R. Johnson (2002). Recruitment and firing patterns of jaw muscles during mastication in ring-tailed lemurs. American Journal of Physical Anthropology Supplement 117:88. Kay, R.F. (1975). The functional adaptations ofprimate molar teeth. American Journal of Physical Anthropology 43: 195-216. Kay, R.F. and R.R. Covert (1984). Anatomy and behaviour of extinct primates. In D.J. Chivers, B.A. Wood, and A. Bilsborough (eds.): Food Acquisition and Processing in Primates. New York: Plenum Press Kay, R.F. and K.M. Riiemae (1974). Jaw movement and tooth use in recent and fossil primates. American Journal of Physical Anthropology 40:227-256. Leibman, F.M. and L. Kussick (1965). An electromyographic analysis of masticatory muscle imbalance with relation to skeletal growth in dogs. Journal of Dental Research 44:768-774. Lieberman, D.E. and A.W. Crompton (2000). Why fuse the mandibular symphysis? A comparative analysis. American Journal of Physical Anthropology 112:517-540. 92

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Lucas, P.W., R. T. Corlett, and D.A. Luke (1986). Postcanine tooth size and diet in anthropoid primates. Z. Morph. Anthrop. 76:253-276. Maynard-Smith, J. and J.G. Savage (1959). The mechanics of mammalian jaws. School Science Review 40:289-301. Ravosa, M.J. (1991) Structural allometry of the prosimian mandibular corpus and symphysis. Journal of Human Evolution 20, 3-20. Ravosa, M.J. and W.L. Rylander (1993). Functional significance of an ossified mandibular symphysis: a reply. American Journal of Physical Anthropology 90:509:512. Ravosa, M.J. and W.L. Rylander (1994). Function and fusion of the mandibular symphysis in primates: Stiffness or strength? In J.G. Fleagle and R.F. Kay (eds): Anthropoid Origins. New York: Plenum Press Roberts, D. and I. Tattersall (1974). Skull form and the mechanics of mandibular elevation in mammals. American Museum Novitates No. 2536. Scapino, R. (1981 ). Morphological investigation into functions of the jaw symphysis in carnivorans. Journal of Morphology 167:339-375. Smith, R.J. (1978). Mandibular biomechanics and temporomandibular joint function in primates. American Journal of Physical Anthropology 49:341-350. Smith, R.J. (1993). Categories of allometry: body size versus biomechanics. Journal of Human Evolution 24:173-182. Spencer, L.M. (1995). Morphological correlates of dietary resource partitioning in the African Bovidae. Journal ofMammalogy 76:448-471. Spencer, M.A. (1995). Masticatory system configuration and diet in anthropoid primates. Ph.D. Dissertation, State University of New York, Stony Brook. Spencer, M.A. (1998). Force production in the primate masticatory system: electromyographic tests ofbiomechanical hypotheses. Journal of Human Evolution 34, 25-54. Spencer, M.A. (1999). Constraints on masticatory system evolution in anthropoid primates. American Journal of Physical Anthropology 108, 483-506. 93

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M.A. and B. Demes (1993). Biomechanical analysis of masticatory system configuration in Neandertals and Inuits. American Journal of Physical Anthropology 91, 1-20. Spencer, M.A. and G.S. Spencer (1993). MacMorph Data Acquisition Package. Stony Brook, NY: State University ofNew York Department of Anthropology. Thompson, E. N., A. R. Biknevicius, and R.Z. German (2003). Ontogeny of feeding function in the gray short-tailed opossum Monodelphis domestica: empirical support for the constrained model of jaw biomechanics. Journal of Experimental Biology 206:923-932. Throckmorton, G.S. (1989). Sensitivity of temporomandibular joint force calculations to errors in muscle force measurements. Journal of Biomechanics 22:455-468. Turnbull, W.D. (1970). Mammalian masticatory apparatus. Fie/diana-Geology 18. 94



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PROSOCIAL MESSAGES IN CHILDREN'S SATURDAY MORNING TELEVISION PROGRAMS by Ferris Edward Hoover Jr. B.A., University of Colorado, 1988 A thesis submitted to the Faculty of the Graduate School of the University of Colorado in partial fulfillment of the requirements for the degree of Master of Arts Department of communication 1990

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This thesis for the Master of Arts degree by Ferris Edward Hoover Jr. has been approved for the Communication Department by D. MorlelJ V Kim B. Walker Richard L. Dukes Pamela s. J= Michael z. Hackman l-l9-9D Date

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Hoover, Ferris Edward, Jr. (M.A., Communication) Prosocial Messages in Children's Saturday Morning Television Programs Thesis directed by Associate Professor Donald D. Morley This study was done to test a methodology for measuring the existence and recognition of prosocial messages in children's saturday morning television programs. A panel of four adults was trained to count the occurrence of prosocial behaviors in sample episodes of the five top-rated Saturday morning children's programs. Per hour occurrence ranged from 22 per hour to 48.5 per hour. Fourth grade children were surveyed to determine if some degree of recognition of prosocial messages can be measured. The method used was to ask the respondent's opinion of whether or not the major character in a program would exhibit prosocial traits. The results of the study were somewhat inconsistent. The rank ordering of prosocial content of the programs by the children was sharply different from that of the adults with the children's most prosocial program being the least prosocial according to the adult panel count. The surprising results could have been due to one of several factors. Among them might be that the intended

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connotations of the words used in the questions might not be the same as those assigned by the respondents. or, the one-time count of a single sample episode of a program by an adult unfamiliar with the usual plot structure of that program would not necessarily coincide with the overall view taken by a child who chooses to watch the program regularly. The trained adult panel method seems to be a valid measurement of the occurrence of prosocial messages, but the method for determining the recognition of such messages by children needs improvement. The form and content of this abstract are approved. I recommend its publication. Signed Donald D. Morley iv

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CONTENTS Tables . . . . . . . vi Acknowledgments . . CHAPTER 1. INTRODUCTION 2. SOME HISTORICAL ASPECTS OF MEDIA EFFECTS RESEARCH . . . . . . . . . . . Social Learning Theory . . . . . The 1982 NIMH Report . . . . . Prosocial Research . . . . . Other Research . . . . . 3 RATIONALE . . . . . . 4. METHODS . . . . . . . . Procedures . . . . . . . . 1 4 11 15 18 23 31 36 36 Instruments 4 o Data Analysis . . . . . . 41 5. RESULTS . . . . . . . . 45 6. CONCLUSIONS . . . . . . . 51 APPENDIX A. ADULT RATING PANEL CODING SHEET 61 B. FOURTH GRADE STUDENT SURVEY INSTRUMENT . 62 BIBLIOGRAPHY . . . . . . . . 63

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TABLES Table 5.1. Interrater Reliability . . . . 45 5.2. Adult Rating Panel Observation of Prosocial Behaviors 4 6 5.3. Fourth Grader Ranking of Positiveness Ratios 47 vi

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ACKNOWLEDGMENTS I would like to express special appreciation to Don Morley, my thesis director, and Kim Walker, who acted as second reader, for the extra effort they put into guiding me in the development and execution of the research and the literature search. Thanks go to the Pikes Peak Broadcasting Corporation for providing me with the Nielsen ratings. Thanks to all the members of the Communication Department at uccs for the support they have given during the process of getting this thesis done. The other Graduate Teaching Assistants get special thanks for their participation in the rating panel and the suggestions they had for overcoming the innumerable obstacles that seemed to appear from nowhere. Without the support freely given by the people around me, this thesis would have never been completed. vii

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CHAPTER 1 INTRODUCTION Mass media effects have been the subject of numerous research efforts among the disciplines of psychology, sociology, political science and communication. Early research was generally done by scientists from disciplines other than communication, and a great deal of what is being done today is done by other than communication scholars. Actual internal effects are probably best studied by behavioral scientists or cognitive psychologists while the mechanisms and processes involved in the formulation and the reception of messages through a mass medium are more appropriate areas for a communication scholar. Other social science disciplines generally utilize controlled exposure and observation of behavior to determine whether or not a linkage between media consumption and behavioral change exists. This type of methodology is best suited to examining the internal processes that may be occurring in the subjects being studied. If the intent is to examine what sort of messages are discernable in a particular form of program

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and whether or not the messages are recognizable, a different methodology is needed. Although examining the internal processes which lead to behavioral changes is important, this does not lead to a single generalizable conclusion which explains all possible effects that media consumption might cause. An easily accepted idea, that might cause many people some concern about possible effects of media consumption, is that whatever effects occur are the same for everyone. This is a misconception according to Becker (1987): Too often, when people who do not understand mass communication or mass communication research think about the effects of the mass media, they think in terms of all or none. That is, they -.think if mass communication has a particular effect on one person it must have it on everyone who uses the mass media or who was exposed to that content. Or if they do not see an effect on everyone, they believe it must be due to something other than mass communication. (p. 457) The idea that mass media have different effects on different people provides many paths for a communication researcher to follow. The goal of this thesis was to find a method to explore one of the factors that might contribute to whatever effects might occur. The factor to be examined was whether or not prosocial messages can be reliably identified by adults and recognized by children in children's television programming. First, it was necessary to become familiar with what research has 2

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been done in the area of mass media effects in order to choose a useful and logical method of research. To this end, a review was done of some of the research from all the disciplines involved that has dealt with what effects the medium of television might have on children. The next step was the development of a valid measure of the prosocial message content of the programs in question and a valid method of indirectly measuring the comprehension of the messages by the intended audience. This involved finding methods that have been used in past research and adapting them to this project. The measures which were developed were then executed as a trial of their viability. The methodologies which have been used primarily to search for the adverse effects of mass media were adaptable to the search for possible prosocial effects. The criteria which were used in the examination of whether or not prosocial messages are present and recognizable in Saturday morning children's television constituted the major difference between this research and other explorations of media effects. 3

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CHAPTER 2 SOME HISTORICAL ASPECTS OF MEDIA EFFECTS RESEARCH since the 1920s popular media have been under attack by parents, educators and others for causing such antisocial outcomes as an erosion of moral standards and aggression. The fears that are the basis for such feelings are rooted in the theory of uniform influences of mass media (Lowery & DeFleur, 1988). That is, the mass media have powerful effects on their audiences and these effects are nearly the same for all audience members. The Payne Fund Studies done in 1929 and 1930 were the first attempt to assess the effects of motion pictures on children and adolescents. Some of their findings indicated that adolescents in particular used the actions, styles, and attitudes depicted in the movies as models for their own behavior and style of dress (Lowery & DeFleur, 1988). The next modern mass medium to come under fire was the comic book. Dr. Frederick Wertham, a noted New York psychiatrist published Seduction of the Innocent in 1954 after having published several articles in popular family magazines of the time. In Seduction of the Innocent,

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wertham attacked comic books for having a "bad influence" on their young audience. Although Wertham's methods were scientifically discredited according to Lowery and DeFleur, he did arouse national attention and contributed to the decline of comic book sales. Wertham was generally ignored by social scientists, but his work is an illustration of uniform influence theory in that he described his research subjects as "normal" even though virtually all of them were referred to him because of some social problem such as delinquency. Because the content analyses conducted by Wertham showed that a great deal of socially unacceptable behavior was depicted, he concluded that so called "crime comics" were severely affecting children (Lowery & DeFleur, 1988). The increasing popularity of television as the medium viewed by children again aroused concern over the effects of a mass medium during the 1960s. The first major effort to determine what effects, if any, television might have on children was published by Professors Wilbur Schramm, Jack Lyle and Edwin Parker (1961). Their series of reports investigated why children watch television and what they learn from it. The summation of the findings of this study is the often quoted: 5

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For some children, under some conditions, television is harmful. For other children under the same conditions. or for the same children under other conditions, it may be beneficial. For most children, under most conditions, most television is probably neither harmful nor particularly beneficial. (Schramm, Lyle, & Parker, 1961, p. 13) This conclusion did not satisfactorily answer the question of whether or not television was causing adverse effects on children. The debate continues today as indicated by stories of consumers pressuring advertisers to stop supporting programs that are 11obj ("Idea of boycotting," 1989). Nor has the concept of direct and uniform influences disappeared, as illustrated by retiring Surgeon General c. Everett Keep's 1989 call for restricting and modifying alcohol-related-advertising to reduce the number of teenage drunken driving accidents ( "Koop takes aim," 1989). In short, the public continues to believe that the media serves as a powerful cause of socially unacceptable behavior. The National Commission on the Causes and Prevention of Violence studied violence on television during the late 1960s. The resultant manuscript, Violence and the Media, edited by Robert K. Baker and Sandra J. Ball (1969), among its intended research purposes, analyzed the portrayals of violence in television programming, measured the violent experiences of Americans, and 6

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compared television portrayals of violence with the perception of the audience as to the existence of violence in society. This was done in order to determine the existence of longand/or short-term effects on accepted norms pertaining to violence (Lowery & DeFleur 1988). Their conclusions confirmed the findings of short-term effects that had baen measured in previous experimental studies. The report then suggested that long-term studies should be conducted to determine whether or not long-term effects existed. The call for further research was answered when the surgeon General appointed an advisory committee to supervise a massive research effort in 1969. The report that resulted included more than 40 scientific papers and an overview written by The Surgeon General's Scientific Advisory Committee in 1969. The summation of the research effort, Television and Growing Ye (The Surgeon General's Scientific Advisory Committee on Television and Social Behavior, 1971), was that viewinq television violence could be causally linked to aggressive behavior, but only in children who were inclined in that direction. It was further arqued that the context in which the violence was viewed had a larqe influence on the actual effects. In the words of the committee: 7

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Thus, the two sets of findings converge in three respects: a preliminary and tentative indication of a causal relation between viewing violence on television and aggressive behavior, an indication that any such causal relation operates only on some children (who are predisposed to be aggressive); and an indication that it operates only in some environmental contexts. such tentative and limited conclusions are not very satisfying. They represent substantially more knowledge than we had two years ago, but they leave many questions unanswered. (Surgeon General's Scientific Advisory Committee, 1971, pp. 18-19) Instead of focusing attention on which children in which contexts were most affected by viewing violence on television, the question that the public, as well as the scientific community, most wanted to answer was that of how much violence was portrayed on television. A comprehensive study conducted by George Gerbner (1971) of the Annenberg School of Communications at the University of Pennsylvania measured the number and types of violent acts, who the perpetrators were, and who the victims were in both prime time and Saturday morning programming. From this study emerged the use of the violence index that has been cited by both the popular press and the scientific community to argue that almost all of the programs viewed by children contain violence. Of note is the fact that Gerbner's study found Saturday morning cartoons to have the highest level of violent content with a violence index of 251.1 compared to the 8

PAGE 16

next highest index of 241.9 for crime, western and action adventure programs (Gerbner, 1971, p. 67). However, these findings must be tempered by the fact that the study counted violent acts without regard to context or intent. Gerbner was investigating content as being symbolic material without regard to artistic merit and without distinguishing between the relative impact of any individual action and any other action occurring within the same program (Gerbner, 1971). For example, a pie in the face was weighted equally with the gunning down of an unarmed person. Gerbner's measurement of violent content would seem to be the basis for many proposals which periodically are reported in the popular press for control of the content of children's programs by government agencies. The recurrence of these proposals indicates that popular wisdom still supports the idea of uniform and universal effects. Since most of the questions raised by Baker and Ball in 1969 about what might cause long-term effects were not answered despite the massive effort, the committee continued with a call for more research and specified some areas to be covered. Among them were such areas as television's effects in the context of other mass media, individual developmental history, and other environmental influences including the home environment. They also 9

PAGE 17

called for research on the relationship between televised violence and aggression, specifically in the areas of predispositional characteristics of individuals, age differences, effects of labeling, contextual cues and other program factors, and longitudinal influences of television. The modeling and imitation of prosocial behavior, the role of environmental factors, including the mass media, in the teaching and learning of values about violence and the effects of such learning on social development were also included as areas to be researched (Surgeon General's Scientific Advisory Committee, 1971, p. 19) The scientific community responded by multiplying their efforts during the decade that followed. According to Lowery and DeFleur (1988) "approximately 90 percent of all research publications gn television's influence on behavior appeared!" and more than 3000 titles. are included in the information that was published on television violence between 1971 an"d 1982 (p. 353). This accumulation led to the publication in 1982 of Television and Behavior: Ten Years of Scientific Progress and Implications for the Eighties by the National Institute of Mental Health (NIMH) at the direction of the Surgeon General. This report is a synthesis of the knowledge gained from the effort instigated by the 1971 Television 10

PAGE 18

and Growing Ye and included not only research done on the effects of television violence but many other areas. The areas germane to this thesis will be discussed. Social Learning Theory The studies done to determine whether or not television violence could be causally linked to aggressive behavior were generally based on social learning theories which posit that children learn by observation or incidentally. One of the foremost researchers and theoreticians in this area is Albert Bandura, who began working in the area during the early 1960s. His work is well represented in his 1977 book Social Learning Theory. It provides a framework for describing the processes experienced by children in their social development and the continuing effects of media throughout most people's lives. Bandura (1977) posits that observational learning, which is a primary factor in social learning, has four components: attention, retention, reproduction, and motivation. Social learning according to Bandura is a three-way interaction between the person, the behavior, and the situation. The person part of the interaction includes the developmental stage of the individual which affects the ability to model a behavior on a delayed 11

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basis. Another factor is the self-regulatory capacity of an individual. This falls into the motivational component of observational learning in that it affects the determination of whether or not the outcome of a behavior is considered to be valuable to an individual. The situational aspect of the interaction is influenced by reinforcement of the learning process, which Bandura describes as "facilitative" rather than necessary for learning because other influences can determine what is attended to and learning can occur without reinforcement. That is, even if the situation surrounding the observation of a behavior provides no reinforcement, the behavior may be successfully performed at a later time. Behaviors may also be duplicated in the short-term because of a situation like a laboratory experiment, but not retained in an individual's behavioral repertoire for later use. Social learning theory does recognize that the actual learning of a behavior is an individual process, but Bandura indicates that the media have broadranging effects in the population when he states: "It has been shown that both children and adults acquire attitudes, emotional responses, and new styles of conduct through filmed and televised modeling" (Bandura, 1977, p. 39). 12

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Social learninq theory has been used as a model for explaininq many findinqs of media effects research. Other research usinq the theory was beinq done concurrently with the work that Bandura was doinq that resulted in the publication of Social Learning Theory as well as after its appearance. Schramm, Lyle, and Parker (1961) theorize that most of what children learn from television falls into the category of incidental learning. As children mature, it has been demonstrated that they acquire the ability to distinquish between reality and fantasy in what they view (Corder-Bolz, 1982). Durinq the period between three and eight years, however, they identify with television characters who they think are "real" and probably learn more behaviors from them than they do as they mature and differentiate between reality and fantasy. Social learninq theory as a model would explain these findinqs in terms of increased ability to perform a behavior after a lapse of time due to maturation and the individual characteristics (i.e., the "person part" of the interaction) beinq a determininq factor in what behaviors would be perceived as havinq valued outcomes. Situational factors would include the perception of what was desired of the subjects by siqnificant social others such as the researchers in an experimental settinq and 13

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what previous learning had been acquired. As long as it is not assumed that a conscious effort is required to learn a behavior, social learning theory would seem to be a convenient and sensible model for explaining incidental learning from the media, particularly that which takes place in experimental settings. Fishbein and Ajzen (1975) theorized that attitudes and behaviors are learned on the basis of rewards and punishments. That is, what is perceived to have a valued outcome will be adopted. Whether or not the outcome of a behavior will be valued will be largely dependent on what attribution of responsibility for a behavior is assigned to the model. Five levels of attribution are described by Fishbein and Ajzen. They are: association with the behavior, commission of the behavior, foreseeability of the result, the intentionality of the model and intentionality with justification. Fishbein and Ajzen also describe three factors which bear on what attribution will be assigned to a model. These are: the consistency of the behavior, the distinctiveness of the behavior and consensus with other behaviors of the model. Rosenthal and Zimmerman (1978) include cognition, defined as the covert processing of information, and abstraction, defined as going beyond discrete stimulus-response associations, in their expansion of social learning 14

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theory. They explain four specific behaviors as a refinement of social learning theory. The first is inhibition which is the result of learning what behaviors result in punishment. The second is disinhibition which results from learning what behaviors will not result in immediate punishment. The third is facilitation which results when an observed behavior serves as a reminder of things learned at an earlier time. The fourth is novel behavior which is something not previously modeled. Rosenthal and Zimmerman (1978) posit that enculturation is the result of modeling parental behaviors and learning group standards and mores. This process could include behaviors learned from modeling on television. They also state that "the social context factors in implementing learning have received limited attention" (p. 267). This relates to the "situation part" of the social learning interaction as described by Bandura (1977) and reiterates a call for research in an area that was specifically mentioned by the: Surgeon General's Scientific Advisory Committee in 1971. The 1982 H1!1H Report Among the areas explored in the 1982 National Institute of Mental Health Report were factors believed to determine children's attention to television, how much 15

PAGE 23

and what type of content was retained, and what processes might explain the relationship between violence viewing and behavior. Collins (1982) postulated three factors for determining children's attention to and comprehension of television content. The factors relating to attention were: viewer characteristics (e.g., mostly age-related), content attributes (e.g., auditory and visual cues), and comprehensibility (which again is a function of the child's age and development). Research reviewed by Collins suggested that older children have a more extensive background making more material understandable and have a longer attention span. Factors relating to processing content were also found to be age related. They were: knowledge of common formats such as narrative stories and commercials, knowledge of commonly portrayed situations and event sequences, and knowledge of the forms and conventions such as the minor climax just before a commercial break. Collins also posited that younger children would be less likely to have the ability to link actions with their consequences. Huesmann-(1982), in his contribution to the NIMH report, describes five processes that might be used to explain the relationship between violence viewing and behavior. The first is observational learning that he 16

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states has been verified in laboratory settings. The persistence, or retention of observational learning is dependent on whether or not it is reinforced in some manner. The second is attitude change which can alter the acceptance of violence by an individual. This process of attitude change can be modified by prosocial reinforcement or outside attitude training. The third is the arousal process which is somewhat of a dichotomy. That is, viewing violence could cause an "overload" leading to hyperactivity or could have an anesthetizing effect due to sensory "overload" and thereby lead to the need for increased aggressive behaviors to achieve normal arousal levels. The fourth is catharsis, that is, watching violence reduces the need for aggressive behavior because normal arousal levels are reached by the act of viewing. Huesmann states that the catharsis postulation has been rejected by research. The fifth is justification. Aggressive individuals watch violent programming in order to justify their own behavior as "normal" and reduce guilt levels. Freedman (1984, 1986) challenged the conclusions of several articles in the 1982 NIMH report that causally linked viewing television violence with aggressive behavior. He found in reviewing the research for the NIMH report that "The bulk of the correlations fall 17

PAGE 25

between .10 and .20." (1984, p. 237). He described these correlations which were found in both laboratory studies and field research as "mild" and judged them insufficient to extrapolate to a cause and effect determination. He concluded that: "(a) exposure to and preference for violent programming correlates with aggressive behavior and (b) there is little convincing evidence that viewing violence on television in natural settings causes an increase in subsequent aggressiveness" (1984, p. 244). This position would also dispute any argument for the possibility of television viewing having beneficial effects on the audience. Cook, Kendzierski and Thomas (1983) also argued that the 1982 NIMH report failed to establish the causal link between viewing television violence and aggressive behavior. Additionally, they criticized the report for focusing on behavioral studies only and its failure to discuss aspects of television programming other than its possible influence on children. Prosocial Research Bryan and Walbek (1970a, 1970b) in studies with second through fifth grade children found that prosocial behavior modeled by both live and videotaped models affected the propensity to share game winnings from a 18

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subsequent session. The actions of the model seemed to have more effect than what was said while the game to be played was demonstrated. Inconsistency on the part of the model was found to have no significant effect. Stein and Bryan (1972), however, found that preaching about following rules while practicing violation of those rules led to an increase in cheating by their third and fourth grade female subjects. When the model preached and practiced rule compliance, cheating was nearly eliminated. In a study of three-to five-year-old children who were tested before and after being exposed to violent, neutral or prosocial programming, Stein, Friedrich and Vondracek (1972) found that those who were most aggressive according to the baseline test exhibited the greatest increase in aggressive behaviors after exposure to the violent programming. They also found that the group receiving the prosocial treatment exhibited a higher level of self controlling behaviors than the other groups. In an experimental study on three-to six-year-old children, Friedrich and stein (1975) found that prosocial behavior could be learned from television and that the subjects applied the behaviors. They also found that verbal labeling training and role playing exercises 19

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enhanced the learning experience. Gorn, Goldberg, and Kanungo (1976) also found that prosocial television could have clearcut short-term effects on the attitude of three-to five-year-old children toward members of other racial and ethnic groups. Singer and Singer (1976) in a study of three-and four-year-old children in a day care center over a twoweek period used a prosocial program along with structured play sessions to determine the effect of adult mediation on the level of imaginativeness showed by the children. The findings indicated that imaginativeness of play could be increased by the presence of an adult mediator. Poulos, Harvey and Liebert (1976) in their c'ontent analysis of Saturday morning television found that prosocial acts were being depicted on children's programs. In the fifty programs which made up their primary sample the combined network number of prosocial acts per half-hour was higher than the number of aggressive acts (a mean of 10.7/half-hour versus 6.10/half-hour). They also found that one quarter of the sample contained no examples of aggression. The one shortcominq noted in Saturday morning television was the lack of examples of preventinq or eliminatinq violence. 20

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In their 1979 study, Friedrich-Cofer, Huston-stein, Kipnis, Susman, and Clewett, also found that reinforcement of the model provided by prosocial programming-can increase its effect. Their study of twoto five-year-old children in a Head start program done over an eight-week exposure period found increased levels of prosocial behavior in children who were provided reinforcement through associated play materials and a greater increase when special teacher training was added to the reinforcement. Some findings in the 1982 NIMH report also indicated that viewing television need not necessarily affect behavior in an adverse way. singer (1982) reported finding that when children's viewing was monitored and mediated by an adult, it helped to improve the children's imagination. Corder-Bolz (1982) called for the teaching of television literacy and parental participation in children's viewing in order for children to better discriminate between reality and fiction and to improve the learning process when educational programs were viewed. Rushton (1982) reviewed numerous experiments that were designed to determine if television could have beneficial as well as detrimental effects on the audience. He defines prosocial material as "that which specifies things that are socially desirable and in some 21

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way benefit another person or society at larqe" (p. 249). He describes four types of prosocial influences. First are those which promote altruism, that is, qenerosity, helpinq behavior, and cooperatinq. The second is friendly behavior; the third is self-control, which is resisting temptation and delaying qratificationr and the fourth is copinq with fears. Rushton's review of both laboratory and naturalistic research desiqns indicated that viewing prosocial material correlated with an improvement in prosocial behavior in the short-term and on delayed measures. However, Rushton (1982) drew a conclusion that supported not only the arqument for beneficial effects from television viewing, but also supported the widely held contention that viewinq television has detrimental effects on behavior. Specifically, "Television is much more than mere entertainment; it is also a major source of observational learninq experiences, a setter of norms. It determines what people judqe to be appropriate behavior in a variety of situations. Indeed it might be that television has become one of the most important aqencies for socialization that our society possesses" (p. 255). Finally, Forqe and Phemister (1987) found that preschool children demonstrated more behaviors of 22

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sharing, cooperation, delay of gratification, and positive social contacts after exposure to prosocial programming. The direction of their research was to determine whether or not the mode of presentation, animated versus non-animated, made a difference. The mode of presentation was not found to be significant but prosocial versus neutral program topics did seem to affect the subjects behavior. This would lead one to believe that the effects of television viewing would be a function of what sorts of messages were being presented. Other Research Given that the studies reviewed in the 1982 NIMH report did not fully establish the causal link between viewing violence on television and aggressive behavior, it could be suggested that establishing a causal link between viewing prosocial television and increases in prosocial behavior would be equally difficult. Although the social learning theory work of Bandura and others provides an explanation of how observed effects could have occurred, other works published since 1980 provide additional information about how and why television might influence audience members. Corder-Bolz (1980) also found that intervention by an adult mediator can affect the learning process and the 23

PAGE 31

formation of attitudes about sex roles and violence. The findings were that primary social agents, those that can demand compliance with their expectations such as parents, teachers, neighbors, the clergy, institutions, organizations, and peer groups, have the most effect on attitudes and beliefs. If secondary social agents, those that cannot demand compliance, such as mass media, present information contradictory to that presented by the primary social aqents, it is likely to be discredited. Huesmann, Eron, Klein, Brice, and Fischer (1983) approach the question from the standpoint of cognition. They suqgest intervention in the form of teachinq that television violence is an unrealistic portrayal, that aqqressive behaviors depicted on television are not as acceptable in the real world as they are in televised stories, and that one should not behave in the same ways as the aggressive characters on television. This intervention would reduce the effect of violent portrayals by altering the cognitive process. They also posit that children who are normally less aggressive will perceive television violence as less realistic and will be less likely to model it. However, a didactic course of treatments designed to produce attitude changes produced no significant results in their second and 24

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fourth grade student subjects. The researchers considered it possible that their didactic treatment was not directed at exactly the right variables and this may have affected the outcome. A second treatment that involved having the subjects produce their own arguments about why television violence is not realistic and should not be modeled did produce some measurable results. However, their primary finding was that identification with a character was the best predictor of whether or not a child was peer nominated as an aggressive individual. They also noted that the subjects who initially had the lowest level of identification showed the most effect. Of special interest is the fact that the amount of television viewed was found to have no significant effect on the other findings. Berkowitz (1984) discussed some factors that extend the "person part" of the social learning interaction beyond age and development. He posited that reaction to a message was dependent on interpretation by the individual as well as the ideas previously held and the thoughts activated by the message. He called this a "priming" effect. That is, a media event leads the viewer to thoughts about similar things without conscious effort. This_effect could be caused by both pro-and antisocial situations depicted in media. However, 25

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Berkowitz states that a semantically related concept, such as a previously acquired set of words or images, must be available to the individual for the effect to occur and that the effect of "priming" decreases over time. Berkowitz also found that viewing fictional material and focusing on the aesthetic aspects of a program reduces the impact of aggression facilitating ideas. Tamborini, Zillmann, and Bryant (1985) expanded this concept with their finding that short-term "priming" occurs after a single exposure to a program but longterm effects result from "repriming" which occurs with multiple exposures. Christenson (1986) determined that the perception of moral lessons improves with age. In a study of kindergartners, first, third, fourth, and sixth graders, the highest level moral lesson as determined by an adult panel was more likely to be perceived by older age groups. The kindergarten and first grade group had no members perceive the highest level lesson while the sixth grade group had 39 percent of its members perceive the lesson. Tan (1986) describes some possible models that can be used to track how children might acquire the cultural values that determine their individual attitudes toward the viewing of violent television programming. The major 26

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structure is the coorientation model, that is, communication requires the participation of at least two persons. According to Tan, three criteria must be met for coorientation to exist. The first is congruency, which is similarity between one person's cognitions and the perceptions of another person's cognitions. The second is agreement, which is the extent to which two people have the same salience evaluations. The third is accuracy, which is the extent to which the estimate of another's cognitions matches what the other actually thinks (Tan, 1986). Tan (1986) places coorientation within families into two patterns and describes four variations of family types based on whether one or the other or both patterns exist. The first pattern is socioorientation in which parents stress the child's relationships with others with the desired outcome of conflict avoidance. The second pattern is concept orientation in which parents stress seeking new ideas, looking at all sides of issues and forming one's own opinions. The first family type is a laissez faire attitude in which neither socioorientation nor concept orientation are stressed and the children are "undirected." The second family type is the protective family in which socioorientation is stressed, social harmony is valued 27

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and the children are "obedient." The third family type is pluralistic in which concept orientation is stressed. In this family type there is no constraint in interpersonal relationships and open discussion is encouraged. The fourth family type is consensual in nature with both orientations being stressed and discussion but not debate is encouraged (Tan, 1986). Some behavioral patterns can be associated with family types. In pluralistic families the children are generally more competent, active in public affairs, more receptive to contradictory ideas and less persuasible. Junior and senior high school students from pluralistic type families also use media more extensively to learn about public affairs and are more positive about the political system. Same age students from protective families generally rank lower in use of media for gaining information, are less politically active, and less positive about the political system (Tan, 1986). In addition to discussing what contributions are made by immediate family members to the acquisition and development of attitudes, Tan (1986) describes some possible effects of violent television and some possible prosocial effects of television. The effects of violent television are: learning new acts, disinhibition and facilitation of aggression, reinforcement of attitudes 28

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held prior to observation, vicarious reinforcement of behavior patterns, and postobservation reinforcement of modeling behavior observed on television. The prosocial effects are divided into cognitive effects and behavioral effects. The cognitive effects are learning useful information and development of cognitive skills, that is, perceptual discrimination, reasoning, and problem solving. The behavioral effects are performance of socially desirable acts, these are: helping others, altruism, controlling aggressive impulses, delaying gratification, persistence, explaining feelings, resisting temptation, adhering to rules; and expressing sympathy to others. Tan states that most of the research done in the area of prosocial effects has been done with preschool children but the results have been positive. In an investigation involving preschoolers and first and second graders, Nikken and Peeters (1988) found that by age seven children realize that what is on television is not necessarily real. Liebert and Sprafkin (1988) found that by age eight, a majority of children understand the purpose of commercial messages and that among fourth grade children, 65 percent of the central content of a program should be remembered. As a means of reducing possible detrimental effects, they suggest that teaching younger children that most programs do not 29

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reflect reality could reduce aggressive reactions. Liebert and Sprafkin also found that adult co-viewers providing commentary on programs improves children's comprehension of the content. These findings indicate that perhaps the best way to limit possible undesirable effects of television on children is to teach them how to discriminate truth from fiction and provide commentary to improve their comprehension. Walker and Morley (1988) found that a liking for violence and a generally aggressive attitude was related to watching violent television and that different types of violence are perceived in differing ways. They also found that adolescents with aggressive behavioral intentions found violent television to be more aesthetically acceptable. These findings could be explained by the proposition that the intended or unintended intervention in the early learning processes by primary social agents helped form the beliefs and attitudes which made the watching of violent television programs more desirable. 30

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CHAPTER 3 RATIONALE Review of research on the effects of mass.media reveals that the topic can cause the researcher a great many problems. Finding causal links between viewing any genre of television and subsequent behavior can become a circular process that the researcher may not see. Are aggressive tendencies acquired from primary social agents with adolescents consequently watching violent programming or, does watching violent programming lead to aggressive tendencies in behavior? Cleary, the processes that occur are difficult to isolate and study and the artifacts that result from the research programs are difficult to sort out. If children are being observed outside the home and are being exposed to researcher selected programming, is such a study externally valid? The bulk of the studies relating to media effects deal with the possible adverse effects. Exploring the possibility that prosocial messages exist in saturday morning children's programs has not received a great deal of attention. Measuring the existence and recognition of these messages in the programs that the children chose to view would begin to balance previous research.

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If social learninq theory is valid and children learn from watchinq others' behavior either in person, or throuqh some mediated means, it can be posited that the existence of prosocial messaqes in Saturday morninq television could provide the prosocial models for children to emulate. Furthermore, if it can be demonstrated that some recognition of the message occurs in younq viewers, then from a social learning theory perspective it would follow that prosocial behavioral changes may be occurring. Existinq prosocial research can be described as hiqhly restricted in that it occurs in laboratory settinqs or is done with groups of subjects in other than an at home setting (e.q., pre-school, lower grade elementary school, or head start). Another problem is that the researcher typically selects the material to be watched rather than the subjects watchinq what they normally select (Bryan and Walbek, 1970a, 1970b;. Stein, Friedrich, & Vondracek, 1972; Friedrich-& Stein, 1975; Gorn, Goldberq, & Kanungo, 1976; Friedrich-Cofer, Huston Stein, Kipnis, Susman, & Clewett, 1979; Sinqer & Singer, 1976). The aqes of the subjects could also be questioned in that normally all are six years old or younger and consequently not of an aqe where they could be expected to have a full comprehension of reality (Christenson, 32

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19867 Nikken & Peeters, 1988). The present investigation proposes a more naturalistic method that would not select the programs viewed and use measures other than behavioral observation in a laboratory setting to determine what messages are recognized by the children. In order to determine whether or not Saturday morning programming contains prosocial messages, and whether or not those messages are being recognized, two things should be accomplished. First, the existence of prosocial messages needs to be reliably determined based on at least face valid criteria. This requires the use of trained adult rating panels. Second, the recognition of the messages should be measured using an instrument based on the same criteria. In order to avoid the artifacts that might occur in a laboratory setting, the recognition measurement has to be indirect. This requirement somewhat restricts the amount of information that can be gathered in a small scale study. Another consideration that arises from conducting research on a small scale is the limited number of adult participants. In order to fully examine whether or not prosocial messages exist in Saturday morning programming, several panels should be used and interrater reliability be established not only within each panel but across panels. 33

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Within the limitations of scales that exist, the methodology should yield results that are indicative of the efficacy of the training procedures used and the reliability and validity of the survey instruments. A shortcoming of controlled studies that do not attempt to determine whether or not recognition of a particular type of message has occurred in the subjects from a communication standpoint is that the subjective processes within the receivers is not examined. That is, the simple observation of behavior before and after a controlled exposure to some media event has value but fails to take into account what the subjects of the study think about the experience. By developing methods that allow the subjects to express what message is being recognized, it may be possible to describe more completely the that occur during individually selected media consumption. Therefore, the purpose of the present study was to develop and use-methods for examining the possibility that not all media effects are adverse. The first step was to develop a methodology for determining whether or not prosocial messages exist in Saturday morning children's programs. The second step was to develop an instrument which would indirectly measure children's recognition of prosocial messages in those programs. The 34

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third step was the execution of the methodology on a trial basis. Lastly the results of the trial were analyzed to determine whether or not the methodology was viable and what improvements need to be made. 35

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CHAPTER 4 METHODS Procedures The research methodology built on three elements. These were the Saturday morning television programs, a trained adult rating panel, and fourth grade school children. The only experimenter manipulation of any sort was the training of adult rating panel to rate selected programs on prosocial message content. In order to assess the presence of prosocial messages, a sample of Saturday morning programs was drawn for a trained panel of adult observers to evaluate. The programs used were selected based on Nielsen ratings. The programs that held the top five positions for Saturday morning ratings among six to eleven year old children during the November 1989 ratinq period were selected to maximize the likelihood of the survey respondents having viewed them. Since there was a tie for fifth place in the ratings, a disinterested party blindly drew the name of the program to be used. The five programs selected for the study and their rank according to the ratings were: (1) "Slimer and the Real

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Ghostbusters11 (2) "Beetlejuice" (3) "Alvin and the Chipmunks11 (4) "Denver, the Last Dinosaur11 and (5) "Garfield and Friends." The actual episode of each program to be recorded and shown to the adult panel was selected by numbering the Saturdays during the period 30 December 1989 through 27 January 1990 and rolling a die. A panel consisting of four female upper division and graduate students was recruited at the University of Colorado at Colorado Springs. All of the panel members were communication students but only one undergraduate had some background in media effects research. The panel had two training sessions during which interrater reliability was established using examples of programs not included in the study. The definitions of prosocial behaviors given by Liebert and Sprafkin (1988) were used in training the raters. They are: Altruism -consists of sharing, helping and cooperation involving humans or animals. Control of aggressive impulses -involves nonaggressive acts or statements that serve to eliminate or preventaggression by self or others toward humans or animals. Delay of gratification/task persistence -consists of the related acts of delay of gratification and task persistence, expressed either nonverbally or verbally. Explaining feelings .of self or others -consists of statements to another person(s) explaining the feelings, thinking, or action of self or others with the intent of effective positive outcome,including increasing the understanding 37

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of others, resolving strife, smoothing out difficulties, or reassuring someone. Reparation for bad behavior -refers to behavior that is clearly intended as reparation for an act seen as a wrongdoing committed by the person himself/herself. Resistance to temptation -refers to withstanding the temptation to engage in behaviors generally prohibited by society (e.g., stealing), which may be prohibited in the program explicitly or implicitly. sympathy -is a verbal or behavioral expression of concern for others and their problems. (Liebert & Sprafkin, 1988, Table 10.3, p. 230) These definitions provided the criteria to determine the existence of prosocial messages and were explained to the panel during the first training session. An example of a program not broadcast on Saturday mornings was used to illustrate behaviors that fit each of the definitions. Raters were simply asked to count the occurrence of each form of prosocial behavior during a practice rating done on a different program to establish a preliminary level of interrater reliability. The second training session consisted of further discussion and group analysis of a program to establish shared definitions for the panel members. Again an example of a program not broadcast on Saturday mornings was rated by the panel and interrater reliability was checked. The final session for the panel consisted of rating the selected episodes with the commercials being skipped by means of fast forwarding the 38

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tape. The programs rated aired as nominally three and one half hours of programming. A sample of 371 fourth graders from a local school district was surveyed to ascertain whether or not prosocial messages were being recognized by viewers of the programs. There were 96 valid returns for a 25.9% return rate. The responses came from 38 males and 57 females and one response with the gender left blank. Demographics for the schools was not collected at the time the surveys were distributed. When a comparison of the respondents gender distribution to the gender distribution of the school populations seemed to be necessary, the school district was queried for the information. As the term had been over for several weeks, the data files had been purged and the information was not available in a usable form. One survey was returned without being completed and one respondent indicated that neither of the programs had been watched. Neither of these was counted as a valid response. The distribution of the number of responses for each program was not related to the Nielsen ratings. "Alvin and the Chipmunks" which was rated number three had 25 responses, 11Slimer and the Real Ghostbusters" rated number one had 22 responses, 11Beetlejuice" and "Garfield and Friends rated number two and five respectively had 39

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18 responses each, and "Denver, the Last Dinosaur" rated number four had 13 responses. Fourth graders were determined to be capable of reading the simple questions used on the survey instrument. Further, only three simple answer choices of yes, no, and don't know were provided (See Appendix B). Two program surveys were given to each student in order to increase the probability of getting one valid program evaluation from each respondent. For those indicating the same frequency of viewing for both programs, a coin flip was used to determine which one would be used in the data analysis. For differing frequencies of viewing, the program watched most was used. Instruments The panel used a tally sheet with each of the individual prosocial behaviors listed and briefly defined. While viewing the program episodes selected, they noted the occurrence of each behavior (See Appendix A). The survey forms for the fourth graders were tailored to the programs. That is, each form had the name of the program at the top and the questions pertained to that programs title character (See Appendix B). Prior to distribution, the questionnaires were paired equally 40

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and shuffled so that the programs any particular-child was asked about occurred randomly. The two questionnaires were stapled to a parental permission form prior to being distributed. The sets of questionnaires were broken down prior to being delivered to the participating schools and were picked up after being returned to the schools by those who chose to participate. The seven questions about the title characters paralleled the Liebert and Sprafkin prosocial behaviors listed on the trained rating panel's sheets and the respondents were asked whether or not the main character of the program would choose to behave in that manner. For example, on the altruism behavior the respondents were asked to reply yes, no, or dont know to the statement, (Character Name) likes to share and help others. Additionally, the respondents were asked if they would like to have the title character as a friend, how often they viewed the program in question, and their gender and age (See Appendix B). Data Analysis The rating panel tally sheets were compiled to determine a rate per program of each prosocial behavior and a total rate. These results were put into a common 41

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measurement on a per hour basis for rankinq the proqrams accordinq to the rate of occurrence of prosocial behaviors. Interrater reliability was established by intercorrelatinq the four raters evaluations of each program. Specifically, each rater's count of each type of prosocial behavior were intercorrelated with the other raters' counts of each type of prosocial behavior. This yielded six correlations per program which were converted to z scores and averaged to arrive at interrater reliability for each program and across all programs. The data from the children's survey instruments were used to calculate a total positive rating score. This score was the total of positive answers given by the respondents for each program. A prosocial ratio score was also determined for each program. This was done by dividing the number of positive answers by the total of the positive and neqative answers. This ratio was used to determine the prosocial message recognition rank of the programs. A one between analysis of variance was done to determine if siqnificant relationships existed amonq the program rankinqs as determined by the adult panel and the survey respondents' evaluations. The dependent variable in this procedure was the student derived positiveness ratio for each program. The independent variable was the 42

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prosocial rankinq of the proqram as determined by the adult panel. Thus, if the children were recoqnizinq prosocial messaqes in the same way as the adults, the orderinq of the children's positiveness ratio means should be the same as the prosocial orderinq of the adults. Analyses of variance were also done usinq the respondent's qender and the answer to the question about whether or not they would like to have the main character of the proqram as a friend as independent variables. The positive answer totals for the various behaviors were used as dependent variables in order to determine whether there was more recoqnition of any of the behaviors based on qender or the desirability of the character as a friend. In order to explore the relationship between whether or not the various characters would be desirable as friends and the number of positive opinions assigned to the programs by the children, the correlation between these items was checked. The number of positive opinions was counted and used as a score in this case and the friend dimension was left on its one to three scale. Finally, a rank order correlation was done between the prosocial rankings assigned by the adult panel and children's prosocial rankinqs of the programs. A 43

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positive correlation would indicate similarities in adult-child recognition of prosocial messages while a negative correlation would indicate dissimilarity. 44

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CHAPTER 5 RESULTS The first item that needed to be established was the interrater reliability of the adult panel. The reliable recognition of the occurrence of prosocial behaviors in the programs was the basis for determining whether or not the methodology used in this project was viable. The statistical operation used for establishing reliability in this case was the average intercorrelation of the scores given by the four panel members. The resultant average intercorrelations among raters are listed in Table 5.1. The overall good intercorrelation among adult raters indicates that the training sessions established a shared set of definitions. Table 5.1 Interrater Reliability Program Title Garfield and Friends Slimer and the Real Ghostbusters Denver, the Last Dinosaur Alvin and the Chipmunks Beetlejuice Across all programs Correlation 0.935 0.915 0.910 0.890 0.775 0.895

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The question of the occurrence of prosocial behaviors in saturday morning children's programs that were evaluated was the next consideration. The rate of occurrence differed widely across the set of programs but all were determined to portray at least some prosocial behaviors. The adult panel means for the number of prosocial occurrences on a per hour basis is given in Table 5.2. Table 5.2 Adult Rating Panel Observation of Prosocial Behaviors (Panel mean of occurrences per hour) Rank Program Title Score l. Alvin and the Chipmunks 48.5/hour 2. Beetlejuice 48.0/hour 3. Garfield and Friends 26.75/hour 4. Slimer and the Real Ghostbusters 23.75/hour 5. Denver, the Last Dinosaur 22.0/hour In order to establish a basis for comparing what the opinions of the fourth graders about the probability of the main characters of the programs behaving in a prosocial manner with the results of the. adult panel's evaluation, an index of-some sort was necessary. The children's total positive answers about the main characters for each program were divided by the total of 46

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the positive and negative answers for each program to determine a "positiveness ratio" for each program. The "Don't Know" were considered to be a "no opinion" and omitted from the calculation. The resultant ratios established the ranking given in Table 5.3. Table 5.3 Fourth Grader Ranking of Positiveness Ratios* Rank Program Title 1. Denver, the Last Dinosaur 2. Slimer and the Real Ghostbusters 3. Alvin and the Chipmunks 4. Beetlejuice 5. Garfield and friends Ratio 0.98 0.74 0.42 0.34 0.24 *(Ratio=Positive AnswersjPositive+Negative Answers) In a one between analysis of variance the number of positive answers given for the program rated by each respondent was entered as a separate case. This was used as the dependent variable with the adult ranking of the programs as the independent variable. The results indicated that significant differences, F(4,92) = 20.86 p<.OOl, existed in the number of positive answers given for each program. Specifically the results in Table 5.2 indicate that adults ranked the prosocialness of programs from most to least as: (l) "Alvin and the Chipmunks" 47

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(2) "Beetlejuice" (3) "Garfield and Friends" (4) "Slimer and the Real Ghostbusters" (5) "Denver, the Last Dinosaur." In contrast, the results in Table 5.3 revealed that children ranked the prosocialness of the lead characters in these proqrams from most to least as: (1) "Denver, the Last Dinosaur" (2) "Slimer and the Real Ghostbusters" (3) "Alvin and the Chipmunks" (4) "Beetlejuice" (5) "Garfield and Friends." The Tukey HSD procedure indicated that the differences at the p<.OS level existed between both "Beetlejuice" and "Garfield and Friends" and the other three programs. This procedurewas indicated by the sharp differences in the ordering of the programs according to prosocial content between the adult panel and the children. The results indicate that the children rated the proqrams as being siqnificantly different in content. Rank order correlation between the adult and children's prosocial rankinqs of the five proqrams indicated that the adults and children have different opinions about the amount of prosocial behavior that occurred in the proqrams (Spearman's rho = 0.60, p=O.l64). The correlation did not approach acceptable siqnificance due to a small sample of only five proqrams. Analysis variance usinq qender as the independent variable revealed siqnificant differences in the 48

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children's answers on two of the behavior descriptions. The first was reparation for bad behavior which on the survey constituted apologizing for wrongdoing, F(l,91) = 6.45 p<.Ol3. The second was sympathy which on the survey was helping others to feel better, F(l,91) = 8.89 p<.004. Males rated the characters as exhibiting both of these traits more than did the females. For analysis of variance with the answer to whether or not the respondent would like to have the main character as a friend, there were significant differences in the answers to the same two behavior descriptions. That is, reparation for bad behavior, F(2,9l) = 4.56 p<.Ol3, and sympathy, F(2,9l) = 6.76 p<.002, again were the dimensions that were rated differently. Those who would like the character as a friend rated them as exhibiting both the traits more than did those who did not desire the character as a friend. The correlation between the total of positive opinion answers assigned to the programs by children and their desire to have the character as a friend was contrary to eXpectations. The negative correlation of -0.444 p<.OOl across all programs, between these items indicates that the less perceived prosocial behavior by a character predicts greater desire by children to want the 49

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character as a friend. This is well illustrated by the fact that the program "Garfield and Friends" ranked lowest on the prosocial scale derived from the children's opinions and yet 13 of 18 total respondents indicated they would like Garfield as a friend. 50

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CHAPTER 6 CONCLUSIONS The goal of this research was to develop a reliable and valid methodology in order to examine the existence and recognition of prosocial messages in Saturday morning children's television programs. The bulk of previous investigation of the effects of television viewing on children has been focused on the possible detrimental effects. The research that has explored the possibility of prosocial effects occurring has been done largely in laboratory settings and has used preschool age children as subjects. In order to determine whether or not prosocial messages exist in Saturday morning programs, it was necessary to use adults as raters in order to increase the objectivity of the observations. Fourth grade students; who are of an age to understand that the programs are fiction, were surveyed about. their opinions of how the characters in top rated Saturday morning programs behave in order to establish whether or not the messages were being recognized by the audience members. The investigator did not control the students viewed thereby minimizing experimental effects. Using a national rating service's ratings to determine what

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programs were to be investigated also eliminated researcher bias from that part of the process. In effect two parts of a communication process were investigated. First, the content of a message, that is, the occurrence of prosocial behaviors within the programs, was examined as objectively as possible. Second, the receivers recognition of the content was indirectly measured by soliciting their opinions about how the main characters behave. The question of whether or not the Liebert and Sprafkin typology provided the basis for a reliable method of measuring the prosocial message content of saturday morning children's programs can be at least partially resolved. The high degree of interrater reliability among the members of the adult rating panel indicates that agreement can be reached-about the meaning of a set of criteria for such measurement. Although unknown factors may have contributed to the ease of establishing the common definitions that the panel used in their rating, it is probable that a high degree of interrater reliability could be established with appropriate training of any similar rating panel. The success with establishing a high degree of interrater reliability among the adult panel members implies that the occurrence of prosocial behaviors in the 52

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program episodes that were rated is a valid finding. This finding can be used to support the hypothesis that prosocial models exist for the audience members to emulate if other individual psychographic factors provide the initial influence for such behavior. An important fact to keep in mind is that the presentation of a prosocial model for a child to emulate is not likely in itself to cause an alteration of behavior patterns. The influence of primary social agents would in large part determine what behaviors would be acceptable for emulation and this would provide the criteria that a child used for categorizing the modeled behaviors as to acceptability. The survey instrument used in assessing the recognition of prosocial messages by children yielded somewhat ambiguous results. Although the respondents gave positive answers about at least some of the behaviors in question for all the programs, their ranking of which programs contained the most prosocial content varied widely from the rankings assigned by the adult raters. It can be concluded from this that the subjective recognition of the prosocial messages by the children differs from the trained objective observation of the programs by the adult raters. This may be a function of identity with the characters in question. 53

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Adult raters would have little or no identification with the characters during a one-time observation of an episode of a program while the regular child viewer would develop identification with the main character over time .. The children's opinion of how a character behaves would evolve from seeing how the character acted in several episodes of a program. If the episode viewed by the adult rating panel happened to be one in which the prosocial content was low, differences would occur in the rankinqs assiqned. Other possible processes which might account for the fact that the children's opinions differed from the adult countinq of behaviors are recall and context. The children's opinions would be based on the recall of final outcomes of stories rather than concentration on detectinq and counting occurrences in a single episode. This would tend to blend the memory of individual antisocial acts that might have occurred into the context . of a prosocial outcome such as ultimately helping another to solve a problem or escape from a perilous situation. The differences among the children's opinions of whether or not the main characters exhibit various prosocial behaviors occurred in two areas. Reparation for bad behavior and sympathy were rated differently according to the gender of the child and whether or not 54

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the character was desirable as a friend. These differences could be attributed to the socialization process. Nine and ten year old fourth graders have probably started to develop gender related social characteristics. Desirable characteristics in a friend could be expected to be different for girls and boys depending on what sort of role modeling had been done by primary social agents. The negative correlation between the overall positive answer score for the programs and the desirability of the characters as friends must also be attributed to identification with the characters. Over a period of time, it is reasonable that children would not repeatedly choose to watch a particular program unless they felt some sort of friendship based on identification with the main character. The method used in this project to determine whether or not prosocial messages are included in the content of Saturday morning children's television seems to be reliable and valid. High interrater reliability was established and the programs that were rated were found to have some prosocial message content. The method used to determine whether or not these messages are received by the audience appears to have worked to a degree but did not yield sufficiently clear results to suggest that 55

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the prosocial messages are being recognized. If the adult rankings are used as a standard of measurement, it could be arqued that the differences between the adult panel and the children indicate that the children do not recognize the prosocial messages in Saturday morning programming. This is most likely a reflection of the different ways in which the measurement was done. The adult rating panel was counting overall occurrences to include the actions of supporting characters in the programs. This would mask somewhat the fact that the main character was not behaving in what would be considered a prosocial manner. The children were restricted to evaluating the main character and this would mask the prosocial actions of supporting characters. In order to resolve this problem of measurement, it would be necessary to word the survey questions to cover the entire cast of characters in a program or to have the adult rating panel count only prosocial behaviors exhibited by the main character. However, the fact that the children's opinions of the prosocial content of the various programs were significantly different indicates that their responses were nonrandom. This finding suggests some sort of systematic recognition of a perceived prosocial pattern of behavior on the part of the characters by the 56

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children. The finding of an inverse correspondence between the adults' and children's prosocial rankings muddies the finding that the children recognized some prosocial messages and requires further investigation with more comprehensive research tools. The questions which arose during the analysis of the results of the survey administered to the children concern the validity of the method. The first problem area which must be noted is the fact that the surveys were accompanied by a parental consent form and were completed by the children at home. This may have led to consulting with the parents about the meanings of the questions and what the "appropriate" answer might be. This would introduce some parental bias into the answers given and would not be detectable in the course of analyzing the results. In order to avoid this, it would be better to obtain parental permission separately from the administration of the survey instrument. The necessarily simple wording of the questions_on the survey was another problem area. Asking the children their opinion of overall behavior patterns makes it easy for them to answer but does not elicit a truly thoughtful response. An instrument consisting of various scenarios with the character name inserted would provide a method for more precisely measuring children's opinions of a 57

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character's behavior patterns. Additionally, it is difficult to determine whether or not the behavioral descriptions used carry the same connotation in the children's minds as in the mind of an adult. Resolving this problem would require interviewing a large sample of children in order to determine what best describes to them the various behavior patterns being investigated. once the most common connotations used by children in a particular age group were determined, the questions used in soliciting their opinions could be worded in a way that would make them a more valid measurement. Another extension of the opinion measurement would be to recruit the parents of the children to also view the programs and evaluate the main characters on the Liebert and Sprafkin typology. This would allow for cross referencing the opinions of the parents and their children to determine what similarities exist between the opinions of primary social agents and the children using a common basis of measurement. The results -obtained would provide indications of how much influence is exerted by primary social agents on the attitudes of children. This two pronged approach could also provide indications as to how much the training of a rating panel contributes to the reliability of its measurements. Comparing the results obtained from two different adult 58

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groups with those of a group of children would yield more comprehensive findings on which to base conclusions and determine what the next level of refinement should be in order to more firmly establish the existence and recognition of prosocial messages in Saturday morning programming. The results of this research indicate that the prosocial messages do exist in Saturday morning children's television programs and that it can be reliably measured. There were also indications that children recognize these messages. The reliability and validity of the method used to measure the children's recognition of prosocial messages needs to be greatly improved over what was used in this project. This improvement would require more resources to be available to conduct the survey of children's opinions. The possible influence of parental bias needs to be eliminated by administering the survey outside the home. Using a national rating service s ratings to select subject programs and not controlling the children's viewing should be retained in order to prevent researcher bias from influencing the results. It is evident that examining the possible effects of television viewing on children in a naturalistic manner is. achievable. Taking such research outside the 59

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laboratory and controlled field experiment setting -provides a more realistic picture of what children are recognizing as the message of what they view. The communication concept of the meaning of a message residing in the receiver can be applied only by determining what meaning is given to the message content of television programs by the receiver, in this case the children who are the intended audience of those who produce what is broadcast on Saturday mornings. 60

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APPENDIX A -Adult Rating Panel Coding Sheet CODING SHEET NAME OF DATE TAPED---------------------------------As you note the occurrence of one of the listed behaviors, mark its occurrence in the count column for that behavior. MESSAGE TYPE COYNT ALTRUISM -sharinq, helpinq & cooperation involvinq humans or animals. CONTROL OF AGGRESSIVE IMPULSES -nonaqqressive acts or statements thatprevent aqqression. DELAY OF GRATIFICATION/PERSISTENCE -acts of delaying qratification & task persistence. EXPLAINING FEELINGS statements explaininq the feelinqs, thinkinq or action of self or others. REPARATION FOR BAD BEHAVIOR -behavior intended as reparation for wrongdoing by person. RESISTANCE TO TEMPTATION -withstandinq the temptation to engage in prohibited behaviors. SYMPATHY -verbal or behavioral expression of concern for others. 61

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APPENDIX B -Fourth Grade Student Survey Instrument I watch (name of program) ALMOST ALWAYS SOMETIMES ALMOST NEVER HAVEN'T SEEN IT (Character name) likes to share and help others. (Character name) likes to fight. (Character name) keeps working until the job is done before playing. (Character name) helps settle arguments by explaining why YES NO DON'T KNOW YES NO DON'T KNOW YES NO DON'T KNOW they started. YES NO DON'T KNOW (Character name) apologizes when he/she does something wrong. YES NO DON'T KNOW (Character name) takes things that belong to someone else. YES NO DON'T KNOW (Character name) would help someone who was feeling bad to feel better. YES NO DON'T KNOW I would like (Character name) to be my friend. YES NO DON'T KNOW I AM A BOY __ GIRL,_ __ WHO IS _____ YEARS OLD. 62

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BIBLIOGRAPHY Anderson, J. A., & Meyer, T. P. (1988). Mediated communication g social action perspective. Newbury Park, CA: Sage Publications. Bandura, A. (1977). Social learning theory. Englewood Cliffs, NJ: Prentice-Hall, Incorporated. Becker, s. L. (1987). communication (2nd ed.). Glenview, IL: Scott, Foresman and Company. Berkowitz, L. (1984). Some effects of thoughts on antiand prosocial influences of media events: A cognitiveneoassociation analysis. Psychological Bulletin, 95, 410427. Bryan, J. H., & Walbek, N.H. (1970a). Preaching and practicing generosity: Children's actions and reactions. Child Development. 41, 329-353. Bryan, J. H., & Walbek, N.H. (1970b). The impact of words and deeds concerning altruism upon children. Child Development, 41, 747-757. Christenson, P. G. (1986). Children's perceptions of moral themes in television drama. In M. L. McLaughlin (Ed.), Communication Yearbook 9. Beverly Hills, CA: Sage Publications, Incorporated. Collins, w. (1982). Cognitive processing in television viewinq. In o. Pearl, L. Bouthilet, & J. Lazar (Eds.), Television and behavior: Ten years of scientific progress and implications for the eighties (vol. 2) (pp. 9-23). Rockville, MD: National Institute of Mental Health. Comstock, G. (1982). Violence in television content: An overview. In D. Pearl, L. Bouthilet, & J. Lazar (Eds.), Television and behavior: Ten years scientific progress gng implications for the eighties (vol. 2) (pp. 108125). Rockville, MD: National Institute of Mental Health.

PAGE 71

Cook, T. o., Kendzierski, o. A., & Thomas, s. v. (1983). The implicit assumptions of television research: An analysis of the 1982 NIMH report on television and behavior. Public Opinion Quarterly, 47, 161-201. corder-Bolz, c. (1980). Mediation: The role of significant others. Journal of Communication, 30(3), 106118. corder-Bolz, c. (1982). Television literacy and critical viewing skills. In D. Pearl, L. Bouthilet, & J. Lazar (Eds.), Television and behavior: Ten years of scientific progress and implications for the eighties (vol. 2) (pp. 91-103). Rockville, MD: National Institute of Mental Health. Dorr, A. (1982). Television and affective development and functioning. In D. Pearl, L. Bouthilet, & J. Lazar (Eds.), Television and behavior: Ten years of scientific progress and implications for the eighties (vol. 2) (pp. 68-77). Rockville, MD: National Institute of Mental Health. Fishbein, M., & Ajzen, I. (1975). Belief, attitude, intention and behavior: An introduction to theory and research. Reading, PA: Addison-Wesley Publishing Company. Forge, K. L. s., & Phemister, s. (1987). prosocial cartoons on preschool children. Journal, 12(2), 83-88. The effect of Child Study Freedman, J. L. (1984). Effect of television violence on aggressiveness. Psychological Bulletin, 92, 227-246. Freedman, J. L. (1986). Television violence and aggression: A rejoinder. Psychological Bulletin, 100, 372-378.; Friedrich, L. K., & Stein, A. H. (1975). Prosocial television and young children: The effects of verbal labeling and role playing on learning and behavior. Child Development, 46, 27-38. Friedrich-Cofer, L., & Huston, A. c. (1986). Television violence and aggression: The debate continues. Psychological Bulletin, 100, 364-371. 64

PAGE 72

Friedrich-Cofer, L. K., Huston-stein, A. H., Kipnis, o. M., Susman, E. J., & Clewett, A. s., ( 1979). Environmental enhancement of prosocial television content: Effects on interpersonal behavior, imaginative play, and self-regulation in a natural setting. Developmental Psychology, 15(6), 637-646. Fry, D. L., & Fry, V. H. (1986). the study of mass communication. (Ed.), Communication Sage Publications, Incorporated. A semiotic model for In M. L. McLaughlin Beverly Hills, CA: Gerbner, G. (1971). Violence in television drama: Trends and symbolic functions. In G. A. Comstock, & E. A. Rubinstein (Eds.), Television and social behavior reports and papers volume I: Media content and control. Rockville, MD: National Institute of Mental Health. Gorn, G. J., Goldberg, M. E., & Kanungo, R. N. (1976). The role of educational television in changing intergroup attitudes of children. Child Development, 47, 277-280. Hirsch, P.M. (1981). On not learning from one's own mistakes, a reanalysis of Gerbner at al.'s findings on cultivation analysis part II. Communication Research, 3-37. Huesmann, L. R. (1982). Television violence and aggressive behavior. In D. Pearl, L. Bouthilet, & J. Lazar (Eds.), Television and behavior: Ten years of scientific progress and implications for the eighties (vol. 2) (pp. 126-137). Rockville, MD: National Institute of Mental Health. Huesmann, L. R., Eron, L. D., Klein, R., Brice, P., & Fischer, P. (1983). Mitigating the imitation of aggressive behaviors by changing children's attitudes about media violence. Journal of Personality and Social Psychology, !,!, 899-910 Huesmann, L. R., Lagerspetz, K., & Eron, L. D. (1984). variables in the TV violence-aggression relation: Evidence from two countries. Developmental Psychology, 746-775. Idea of boycotting TV advertisers gaininq. (1989, June. 3). Colorado Springs Gazette Telegraph. pp. El, E6. 65

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Koop takes aim at alcohol. Springs Gazette Telegraph. (1989, June 1). p. A6. Colorado Liebert, R. M., & Sprafkin, J. (1988). The early window: Effects of television on children and youth (3rd ed.). New York: Pergamon Press. Lowery, S., & DeFleur, M. L. (1988). Milestones in mass communication research: Media effects (2nd ed.). New York: Lonqman Inc. Loye, D., Gorney, R., & Steele, G. (1977). Effects of television: An experimental field study. Journal of Communication, 27(3), 206-216. Murray, J. P., Rubinstein, E. A., & Comstock, G. A. (Eds.), (1972). Television and social behavior. Reports and papers vol II: Television and social learning. Rockville, MD: National Institute of Mental Health. National Institute of Mental Health. (1982). Television and behavior: Ten years Qf scientific progress and implications for the eighties. (DHHS Publication No. ADM 82-1195). Washington, DC: u.s. Government Printinq Office. Nikken, P., & Peeters, A. L. (1988). Children's perceptions of television reality. Journal of Broadcasting and Electronic Media. 32, 441-452. Norusis, M. J./SPSS Inc. (1988). SPSS-X(tm) Advanced statistics guide. (2nd ed.). Chicago: SPSS Inc. 'Poulos, R. w., Harvey, s. E., & Liebert, R. M. (1976). Saturday morninq television: A profile of the 1974-75 children's season. Psychological Reports, 39, 1047-1057. Rosenthal, T. L., & Zimmerman, B. J. (1978). Social learning and cognition. New York: Academic Press, Inc. Rushton, J.P. (1979). Effects of prosocial television and film material on the behavior of viewers. In L. Berkowitz (Ed.), Advances !n experimental social psychology. Vol. 12. New York: Academic Press. pp. 321-351. 66

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Rushton, J.P. (1982). Television and prosocial behavior. In D. Pearl, L. Bouthilet, & J. Lazar (Eds.), Television and behavior: Ten years of scientific progress and implications for the eighties (vol. 2) (pp. 248-257). Rockville, MD: National Institute of Mental Health. Schramm, w., Lyle, J., & Parker, E. B. (1961). Television in the lives of children. Stanford, CA: Stanford University Press. Singer, D. G. (1982). Television and the developing Imagination of the child. In D. Pearl, L. Bouthilet, & J. Lazar (Eds.), Television and behavior: Ten years of scientific progress and implications for the eighties (vol 2) (pp. 39-52). Rockville, MD: National Institute of Mental Health. Singer, J. (1980). The power and limitations of television: A cognitive-affective analysis. In P. H. Tannenbaum (Ed.), The entertainment functions of television. (pp. 31-65). Hillsdale, NJ: Lawrence Erlbaum Associates, Inc. Publishers. Singer, J. L. and Singer, D. G. (1976). Can TV stimulate imaginative play? Journal of Communication, 46(3), .74-80. Stein, A. H., Friedrich, L. K., & Vondracek, F. (1972). Television content and young children's behavior. In J. P. Murray, E. A. Rubinstein, & G. A. comstock (Eds.), Television and social behavior: Reports and papers, vol II: Television and social learning. Rockville, MD: National Institute of Mental Health. Stein, G. M., & Bryan, J. H. (1972). The effects of a television model upon rule adoption behavior of children. Child Development, 43, 268-273. Surgeon General's Scientific Advisory Committee on Television and Social Behavior. (1971). Television and growing YR: The impact .of televised violence. Report to the Surgeon General. Washington, DC: United States Public Health Service. 67

PAGE 75

Tamborini, R., Zillmann, D., & Bryant, J. (1985). Fear and Victimization: Exposure to television and perceptions of crime and fear. In R. N. Bostrom, & B. H. westley (Eds.), Communication Beverly Hills, CA: Sage Publications, Incorporated. Tan, A. s. (1986). research (2nd ed.). Company. Mass communication theories and New York: MacMillan Publishing Tannenbaum, P. H. (Ed.), (1980). The entertainment functions of television. Hillsdale, NJ: Lawrence Erlbaum Associates, Inc. Publishers. Walker, K. B., & Morley, D. D. (1988). Attitudes and parental factors as intervening variables in the television violence-aggression relation. Unpublished manuscript, University of Colorado at Colorado Springs, Colorado springs, co. Walker, K. B., & Morley, D. D. (in press). Adolescent perceptions of context in violent media portrayals. Speech Communication Annual. Williams, F. (1986). Reasoning with statistics. (3rd ed.). New York: Holt, Rinehart and Winston. Wood, D. N. (1983). Mass media and the individual. st. Paul, MN: West Publishing Company. 68 . I