Evolution of anthropoid jaw loading and kinematic patterns

Sagittal suture strain in capuchin monkeys (Sapajus and Cebus) during feeding

OBJECTIVES

Morphological variation in cranial sutures is used to infer aspects of primate feeding behavior, including diet, but strain regimes across sutures are not well documented. Our aim is to test hypotheses about sagittal suture morphology, strain regime, feeding behavior, and muscle activity relationships in robust Sapajus and gracile Cebus capuchin primates.

MATERIALS AND METHODS

Morphometrics of sinuosity in three regions of the sagittal suture were compared among museum specimens of Sapajus and Cebus, as well as in robust and gracile lab specimens. In vivo strains and bilateral electromyographic (EMG) activity were recorded from these regions in the temporalis muscles of capuchin primates while they fed on mechanically-varying foods.

RESULTS

Sapajus and the anterior suture region exhibited greater sinuosity than Cebus and posterior regions. In vivo data reveal minor differences in strain regime between robust and gracile phenotypes but show higher strain magnitudes in the middle suture region and higher tensile strains anteriorly. After gage location, feeding behavior has the most consistent and strongest impact on strain regime in the sagittal suture. Strain in the anterior suture has a high tension to compression ratio compared to the posterior region, especially during forceful biting in the robust Sapajus-like individual.

DISCUSSION

Sagittal suture complexity in robust capuchins likely reflects feeding behaviors associated with mechanically challenging foods. Sutural strain regimes in other anthropoid primates may also be affected by activity in feeding muscles.

Biomechanics of the mandible of Macaca mulatta during the power stroke of mastication: Loading, deformation, and strain regimes and the impact of food type

Mandible morphology has yet to yield definitive information on primate diet, probably because of poor understanding of mandibular loading and strain regimes, and overreliance on simple beam models of mandibular mechanics. We used a finite element model of a macaque mandible to test hypotheses about mandibular loading and strain regimes and relate variation in muscle activity during chewing on different foods to variation in strain regimes. The balancing-side corpus is loaded primarily by sagittal shear forces and sagittal bending moments. On the working side, sagittal bending moments, anteroposterior twisting moments, and lateral transverse bending moments all reach similar maxima below the bite point; sagittal shear is the dominant loading regime behind the bite point; and the corpus is twisted such that the mandibular base is inverted. In the symphyseal region, the predominant loading regimes are lateral transverse bending and negative twisting about a mediolateral axis. Compared with grape and dried fruit chewing, nut chewing is associated with larger sagittal and transverse bending moments acting on balancing- and working-side mandibles, larger sagittal shear on the working side, and larger twisting moments about vertical and transverse axes in the symphyseal region. Nut chewing is also associated with higher minimum principal strain magnitudes in the balancing-side posterior ramus; higher sagittal shear strain magnitudes in the working-side buccal alveolar process and the balancing-side oblique line, recessus mandibulae, and endocondylar ridge; and higher transverse shear strains in the symphyseal region, the balancing-side medial prominence, and the balancing-side endocondylar ridge. The largest food-related differences in maximum principal and transverse shear strain magnitudes are in the transverse tori and in the balancing-side medial prominence, extramolar sulcus, oblique line, and endocondylar ridge. Food effects on the strain regime are most salient in areas not traditionally investigated, suggesting that studies seeking dietary effects on mandible morphology might be looking in the wrong places.

Masticatory Loading and Ossification of the Mandibular Symphysis during Anthropoid Origins

An ossified or 'fused' mandibular symphysis characterizes the origins of the Anthropoidea, a primate suborder that includes humans. Longstanding debate about the adaptive significance of variation in this jaw joint centers on whether a bony symphysis is stronger than an unfused one spanned by cartilage and ligaments. To provide essential information regarding mechanical performance, intact adult symphyses from representative primates and scandentians were loaded ex vivo to simulate stresses during biting and chewing - dorsoventral (DV) shear and lateral transverse bending ('wishboning'). The anthropoid symphysis requires significantly more force to induce structural failure vs. strepsirrhines and scandentians with unfused joints. In wishboning, symphyseal breakage always occurs at the midline in taxa with unfused conditions, further indicating that an ossified symphysis is stronger than an unfused joint. Greater non-midline fractures among anthropoids suggest that fusion imposes unique constraints on masticatory function elsewhere along the mandible, a phenomenon likely to characterize the evolution of fusion and jaw form throughout Mammalia.

Mandibular symphyseal fusion in fossil primates: Insights from correlated patterns of jaw shape and masticatory function in living primates

OBJECTIVES

Variation in primate masticatory form and function has been extensively researched through both morphological and experimental studies. As a result, symphyseal fusion in different primate clades has been linked to either the recruitment of vertically directed balancing-side muscle force, the timing and recruitment of transversely directed forces, or both. This study investigates the relationship between jaw muscle activity patterns and morphology in extant primates to make inferences about masticatory function in extinct primates, with implications for understanding the evolution of symphyseal fusion.

MATERIALS AND METHODS

Three-dimensional mandibular landmark data were collected for 31 extant primates and nine fossil anthropoids and subfossil lemur species. Published electromyography (EMG) data were available for nine of the extant primate species. Partial least squares analysis and phylogenetic partial least squares analysis were used to identify relationships between EMG and jaw shape data and evaluate variation in jaw morphology.

RESULTS

Primates with partial and complete symphyseal fusion exhibit shape-function patterns associated with the wishboning motor pattern and loading regime, in contrast to shape-function patterns of primates with unfused jaws. All fossil primates examined (except Apidium) exhibit jaw morphologies suggestive of the wishboning motor pattern demonstrated in living anthropoids and indriids.

DISCUSSION

Partial fusion in Catopithecus, similar to indriids and some subfossil lemurs, may be sufficient to resist, or transfer, some amounts of transversely directed balancing-side muscle force at the symphysis, representing a transition to greater reliance on transverse jaw movement during mastication. Furthermore, possible functional convergences in physiological patterns during chewing (i.e., Archaeolemur) are identified.

Evaluating the triplet hypothesis during rhythmic mastication in primates

Mammalian mastication involves precise jaw movements including transverse movement of the mandible during the power stroke. Jaw elevation and transverse movement are driven by asymmetrical jaw elevator muscle activity, which is thought to include a phylogenetically primitive and conserved triplet motor pattern consisting of: triplet I (balancing side: superficial masseter and medial pterygoid; working side: posterior temporalis), which reaches onset, peak and offset first; and triplet II (working side: superficial masseter and medial pterygoid; balancing side: posterior temporalis), which is active second. Although the presence of a triplet motor pattern has been confirmed in several primate species, the prevalence of this motor pattern - i.e. the proportion of masticatory cycles that display it - has not been evaluated in primates. The present study quantifies the presence and prevalence of the triplet motor pattern in five different primate species, Eulemur fulvus, Propithecus verreauxi, Papio anubis, Macacafuscata and Pan troglodytes, using mean onset, peak and offset time relative to working superficial masseter. In all five of the species studied, the mean triplet motor pattern was observed at peak muscle activation, and in four out of the five species the triplet motor pattern occurred more frequently than expected at random at peak muscle activation and offset. Non-triplet motor patterns were observed in varying proportions at different time points in the masticatory cycle, suggesting that the presence or absence of the triplet motor pattern is not a binomial trait. Instead, the primate masticatory motor pattern is malleable within individual cycles, within individual animals and therefore within species.

Animal Models for Dysphagia Studies: What Have We Learnt So Far

Research using animal models has contributed significantly to realizing the goal of understanding dysfunction and improving the care of patients who suffer from dysphagia. But why should other researchers and the clinicians who see patients day in and day out care about this work? Results from studies of animal models have the potential to change and grow how we think about dysphagia research and practice in general, well beyond applying specific results to human studies. Animal research provides two key contributions to our understanding of dysphagia. The first is a more complete characterization of the physiology of both normal and pathological swallow than is possible in human subjects. The second is suggesting of specific, physiological, targets for development and testing of treatment interventions to improve dysphagia outcomes.

The dilemma of choosing a reference character for measuring sexual size dimorphism, sexual body component dimorphism, and character scaling: cryptic dimorphism and allometry in the scorpion Hadrurus arizonensis

Sexual differences in morphology, ranging from subtle to extravagant, occur commonly in many animal species. These differences can encompass overall body size (sexual size dimorphism, SSD) or the size and/or shape of specific body parts (sexual body component dimorphism, SBCD). Interacting forces of natural and sexual selection shape much of the expression of dimorphism we see, though non-adaptive processes may be involved. Differential scaling of individual features can result when selection favors either exaggerated (positive allometry) or reduced (negative allometry) size during growth. Studies of sexual dimorphism and character scaling rely on multivariate models that ideally use an unbiased reference character as an overall measure of body size. We explored several candidate reference characters in a cryptically dimorphic taxon, Hadrurus arizonensis. In this scorpion, essentially every body component among the 16 we examined could be interpreted as dimorphic, but identification of SSD and SBCD depended on which character was used as the reference (prosoma length, prosoma area, total length, principal component 1, or metasoma segment 1 width). Of these characters, discriminant function analysis suggested that metasoma segment 1 width was the most appropriate. The pattern of dimorphism in H. arizonensis mirrored that seen in other more obviously dimorphic scorpions, with static allometry trending towards isometry in most characters. Our findings are consistent with the conclusions of others that fecundity selection likely favors a larger prosoma in female scorpions, whereas sexual selection may favor other body parts being larger in males, especially the metasoma, pectines, and possibly the chela. For this scorpion and probably most other organisms, the choice of reference character profoundly affects interpretations of SSD, SBCD, and allometry. Thus, researchers need to broaden their consideration of an appropriate reference and exercise caution in interpreting findings. We highly recommend use of discriminant function analysis to identify the least-biased reference character.

What does feeding system morphology tell us about feeding?

Feeding is the set of behaviors whereby organisms acquire and process the energy required for survival and reproduction. Thus, feeding system morphology is presumably subject to selection to maintain or improve feeding performance. Relationships among feeding system morphology, feeding behavior, and diet not only explain the morphological diversity of extant primates, but can also be used to reconstruct feeding behavior and diet in fossil taxa. Dental morphology has long been known to reflect aspects of feeding behavior and diet but strong relationships of craniomandibular morphology to feeding behavior and diet have yet to be defined.

Developing a musculoskeletal model of the primate skull: predicting muscle activations, bite force, and joint reaction forces using multibody dynamics analysis and advanced optimisation methods

An accurate, dynamic, functional model of the skull that can be used to predict muscle forces, bite forces, and joint reaction forces would have many uses across a broad range of disciplines. One major issue however with musculoskeletal analyses is that of muscle activation pattern indeterminacy. A very large number of possible muscle force combinations will satisfy a particular functional task. This makes predicting physiological muscle recruitment patterns difficult. Here we describe in detail the process of development of a complex multibody computer model of a primate skull (Macaca fascicularis), that aims to predict muscle recruitment patterns during biting. Using optimisation criteria based on minimisation of muscle stress we predict working to balancing side muscle force ratios, peak bite forces, and joint reaction forces during unilateral biting. Validation of such models is problematic; however we have shown comparable working to balancing muscle activity and TMJ reaction ratios during biting to those observed in vivo and that peak predicted bite forces compare well to published experimental data. To our knowledge the complexity of the musculoskeletal model is greater than any previously reported for a primate. This complexity, when compared to more simple representations provides more nuanced insights into the functioning of masticatory muscles. Thus, we have shown muscle activity to vary throughout individual muscle groups, which enables them to function optimally during specific masticatory tasks. This model will be utilised in future studies into the functioning of the masticatory apparatus.

The mechanical significance of morphological variation in the macaque mandibular symphysis during mastication

Catarrhine symphyseal morphology displays considerable variation. Although this has been related to dentition, phylogeny, sexual dimorphism, and facial orientation, most emphasis has been given to the functional significance of the symphysis to mechanical loading during mastication. The current state of knowledge regarding the mechanical significance of the symphysis is based on a combination of in vivo experimental and comparative studies on Macaca fascicularis. These approaches have provided considerable insight into the stereotypical patterns of loading in the symphyseal region during chewing and hypotheses related to the associated symphyseal morphologies. Finite element analysis (FEA) was used to assess how in silico manipulation translates into the mechanical loading hypotheses previously proposed experimentally. In particular, this study tests the form-function relationship of the symphysis of an adult M. fascicularis mandible during lateral transverse bending and dorsoventral shear of the mandibular symphysis, and a series of modified hypothetical morphologies including absence/presence of tori and variation in the inclination and depth of the symphysis. FEA results of this study support previous findings that stresses associated with lateral transverse bending and dorsoventral shear of the mandibular symphysis can be minimized via an increased labio-lingual thickness in the superior transverse torus, an oblique symphyseal inclination, and/or an increased symphyseal depth. The finding that reduction of strains related to lateral transverse bending and dorsoventral shear can be achieved through a number of different morphologies contributes to our understanding of the influence of morphological and/or developmental constraints, such as dental development, on symphyseal form.

A preliminary analysis of correlations between chewing motor patterns and mandibular morphology across mammals

The establishment of a publicly-accessible repository of physiological data on feeding in mammals, the Feeding Experiments End-user Database (FEED), along with improvements in reconstruction of mammalian phylogeny, significantly improves our ability to address long-standing questions about the evolution of mammalian feeding. In this study, we use comparative phylogenetic methods to examine correlations between jaw robusticity and both the relative recruitment and the relative time of peak activity for the superficial masseter, deep masseter, and temporalis muscles across 19 mammalian species from six orders. We find little evidence for a relationship between jaw robusticity and electromyographic (EMG) activity for either the superficial masseter or temporalis muscles across mammals. We hypothesize that future analyses may identify significant associations between these physiological and morphological variables within subgroups of mammals that share similar diets, feeding behaviors, and/or phylogenetic histories. Alternatively, the relative peak recruitment and timing of the balancing-side (i.e., non-chewing-side) deep masseter muscle (BDM) is significantly negatively correlated with the relative area of the mandibular symphysis across our mammalian sample. This relationship exists despite BDM activity being associated with different loading regimes in the symphyses of primates compared to ungulates, suggesting a basic association between magnitude of symphyseal loads and symphyseal area among these mammals. Because our sample primarily represents mammals that use significant transverse movements during chewing, future research should address whether the correlations between BDM activity and symphyseal morphology characterize all mammals or should be restricted to this "transverse chewing" group. Finally, the significant correlations observed in this study suggest that physiological parameters are an integrated and evolving component of feeding across mammals.

The influence of experimental manipulations on chewing speed during in vivo laboratory research in tufted capuchins (Cebus apella)

Even though in vivo studies of mastication in living primates are often used to test functional and adaptive hypotheses explaining primate masticatory behavior, we currently have little data addressing how experimental procedures performed in the laboratory influence mastication. The obvious logistical issue in assessing how animal manipulation impacts feeding physiology reflects the difficulty in quantifying mechanical parameters without handling the animal. In this study, we measured chewing cycle duration as a mechanical variable that can be collected remotely to: 1) assess how experimental manipulations affect chewing speed in Cebus apella, 2) compare captive chewing cycle durations to that of wild conspecifics, and 3) document sources of variation (beyond experimental manipulation) impacting captive chewing cycle durations. We find that experimental manipulations do increase chewing cycle durations in C. apella by as much as 152 milliseconds (ms) on average. These slower chewing speeds are mainly an effect of anesthesia (and/or restraint), rather than electrode implantation or more invasive surgical procedures. Comparison of captive and wild C. apella suggest there is no novel effect of captivity on chewing speed, although this cannot unequivocally demonstrate that masticatory mechanics are similar in captive and wild individuals. Furthermore, we document significant differences in cycle durations due to inter-individual variation and food type, although duration did not always significantly correlate with mechanical properties of foods. We advocate that the significant reduction in chewing speed be considered as an appropriate qualification when applying the results of laboratory-based feeding studies to adaptive explanations of primate feeding behaviors.

The facial skeleton in patients with osteoporosis: a field for disease signs and treatment complications

Osteoporosis affects all bones, including those of the facial skeleton. To date the facial bones have not drawn much attention due to the minimal probability of morbid fractures. Hearing and dentition loss due to osteoporosis has been reported. New research findings suggest that radiologic examination of the facial skeleton can be a cost-effective adjunct to complement the early diagnosis and the follow up of osteoporosis patients. Bone-mass preservation treatments have been associated with osteomyelitis of the jawbones, a condition commonly described as osteonecrosis of the jaws (ONJ). The facial skeleton, where alimentary tract mucosa attaches directly to periosteum and teeth which lie in their sockets of alveolar bone, is an area unique for the early detection of osteoporosis but also for the prevention of treatment-associated complications. We review facial bone involvement in patients with osteoporosis and we present data that make the multidisciplinary approach of these patients more appealing for both practitioners and dentists. With regard to ONJ, a tabular summary with currently available evidence is provided to facilitate multidisciplinary practice coordination for the treatment of patients receiving bisphosphonates.

A preliminary analysis of the relationship between jaw-muscle architecture and jaw-muscle electromyography during chewing across primates

The architectural arrangement of the fibers within a muscle has a significant impact on how a muscle functions. Recent work on primate jaw-muscle architecture demonstrates significant associations with dietary variation and feeding behaviors. In this study, the relationship between masseter and temporalis muscle architecture and jaw-muscle activity patterns is explored using Belanger's treeshrews and 11 primate species, including two genera of strepsirrhines (Lemur and Otolemur) and five genera of anthropoids (Aotus, Callithrix, Cebus, Macaca, and Papio). Jaw-muscle weights, fiber lengths, and physiologic cross-sectional areas (PCSA) were quantified for this preliminary analysis or collected from the literature and compared to published electromyographic recordings from these muscles. Results indicate that masseter architecture is unrelated to the superficial masseter working-side/balancing-side (W/B) ratio across primate species. Alternatively, relative temporalis architecture is correlated with temporalis W/B ratios across primates. Specifically, relative temporalis PCSA is inversely related to the W/B ratio for the anterior temporalis, indicating that as animals recruit a larger relative percentage of their balancing-side temporalis, they possess the ability to generate relatively larger amounts of force from these muscles. These findings support three broader conclusions. First, masseter muscle architecture may have experienced divergent evolution across different primate clades related to novel functional roles in different groups. Second, the temporalis may be functionally constrained (relative to the masseter) across primates in its functional role of creating vertical occlusal forces during chewing. Finally, the contrasting results for the masseter and temporalis suggest that the fiber architecture of these muscles has evolved as distinct functional units in primates.

Ecological consequences of scaling of chew cycle duration and daily feeding time in primates

Feeding systems and behaviors must evolve to satisfy the metabolic needs of organisms. This includes modifications to feeding systems as body size and metabolic needs change. Using our own data and data from the literature, we examine how size-related changes in metabolic needs are met by size-related changes in daily feeding time, chew cycle duration, volume of food processed per chew, and daily food volume intake in primates. Increases in chew cycle duration with body mass in haplorhine primates are described by a simple power function (cycle time alpha body mass(0.181)). Daily feeding time increases with body mass when analyzed using raw data from the "tips" of the primate phylogenetic tree, but not when using phylogenetically independent contrasts. Whether or not daily feeding time remains constant or increases with body mass, isometry of ingested bite size and the slow rate of increase in chew cycle time with body size combine to allow daily ingested food volume to scale faster than predicted by metabolic rate. This positive allometry of daily ingested food volume may compensate for negative allometry of nutrient concentration in primate foods. Food material properties such as toughness and hardness have little impact on scaling of chew cycle durations, sequence durations, or numbers of chews in a sequence. Size-related changes in food processing abilities appear to accommodate size-related changes in food material properties, and primates may alter ingested bite sizes in order to minimize the impacts of food material properties on temporal variables such as chew cycle duration and chew sequence duration.

Control of jaw movements in two species of macropodines (Macropus eugenii and Macropus rufus)

The masticatory motor patterns of three tammar wallabies and two red kangaroos were determined by analyzing the pattern of electromyographic (EMG) activity of the jaw adductors and correlating it with lower jaw movements, as recorded by digital video and videoradiography. Transverse jaw movements were limited by the width of the upper incisal arcade. Molars engaged in food breakdown during two distinct occlusal phases characterized by abrupt changes in the direction of working-side hemimandible movement. Separate orthal (Phase I) and transverse (Phase II) trajectories were observed. The working-side lower jaw initially was drawn laterally by the balancing-side medial pterygoid and then orthally by overlapping activity in the balancing- and working-side temporalis and the balancing-side superficial masseter and medial pterygoid. Transverse movement occurred principally via the working-side medial pterygoid and superficial masseter. This pattern contrasted to that of placental herbivores, which are known to break down food when they move the working-side lower jaw transversely along a relatively longer linear path without changing direction during the power stroke. The placental trajectory results from overlapping activity in the working- and balancing-side adductor muscles, suggesting that macropods and placental herbivores have modified the primitive masticatory motor pattern in different ways.

Masseter electromyography during chewing in ring-tailed lemurs (Lemur catta)

We examined masseter recruitment and firing patterns during chewing in four adult ring-tailed lemurs (Lemur catta), using electromyography (EMG). During chewing of tougher foods, the working-side superficial masseter tends to show, on average, 1.7 times more scaled EMG activity than the balancing-side superficial masseter. The working-side deep masseter exhibits, on average, 2.4 times the scaled EMG activity of the balancing-side deep masseter. The relatively larger activity in the working-side muscles suggests that ring-tailed lemurs recruit relatively less force from their balancing-side muscles during chewing. The superficial masseter working-to-balancing-side (W/B) ratio for lemurs overlaps with W/B ratios from anthropoid primates. In contrast, the lemur W/B ratio for the deep masseter is more similar to that of greater galagos, while both are significantly larger than W/B ratios of anthropoids. Because ring-tailed lemurs have unfused and hence presumably weaker symphyses, these data are consistent with the symphyseal fusion-muscle recruitment hypothesis stating that symphyseal fusion in anthropoids provides increased strength for resisting forces created by the balancing-side jaw muscles during chewing. Among the masseter muscles of ring-tailed lemurs, the working-side deep masseter peaks first on average, followed in succession by the balancing-side deep masseter, balancing-side superficial masseter, and finally the working-side superficial masseter. Ring-tailed lemurs are similar to greater galagos in that their balancing-side deep masseter peaks well before their working-side superficial masseter. We see the opposite pattern in anthropoids, where the balancing-side deep masseter peaks, on average, after the working-side superficial masseter. This late activity of the balancing-side deep masseter in anthropoids is linked to lateral-transverse bending, or wishboning, of their mandibular symphyses. Subsequently, the stresses incurred during wishboning are hypothesized to be a proximate reason for strengthening, and hence fusion, of the anthropoid symphysis. Thus, the absence of this muscle-firing pattern in ring-tailed lemurs with their weaker, unfused symphyses provides further correlational support for the symphyseal fusion late-acting balancing-side deep masseter hypothesis linking wishboning and symphyseal strengthening in anthropoids. The early peak activity of the working-side deep masseter in ring-tailed lemurs is unlike galagos and most similar to the pattern seen in macaques and baboons. We hypothesize that this early activity of the working-side deep masseter moves the lower jaw both laterally toward the working side and vertically upward, to position it for the upcoming power stroke. From an evolutionary perspective, the differences in peak firing times for the working-side deep masseter between ring-tailed lemurs and greater galagos indicate that deep masseter firing patterns are not conserved among strepsirrhines.

Mechanical properties of foods used in experimental studies of primate masticatory function

In vivo studies of jaw-muscle behavior have been integral factors in the development of our current understanding of the primate masticatory apparatus. However, even though it has been shown that food textures and mechanical properties influence jaw-muscle activity during mastication, very little effort has been made to quantify the relationship between the elicited masticatory responses of the subject and the mechanical properties of the foods that are eaten. Recent work on human mastication highlights the importance of two mechanical properties-toughness and elastic modulus (i.e., stiffness)-for food breakdown during mastication. Here we provide data on the toughness and elastic modulus of the majority of foods used in experimental studies of the nonhuman primate masticatory apparatus. Food toughness ranges from approximately 56.97 Jm(-2) (apple pulp) to 4355.45 Jm(-2) (prune pit). The elastic modulus of the experimental foods ranges from 0.07 MPa for gummy bears to 346 MPa for popcorn kernels. These data can help researchers studying primate mastication select among several potential foods with broadly similar mechanical properties. Moreover, they provide a framework for understanding how jaw-muscle activity varies with food mechanical properties in these studies.

A comparative analysis of temporomandibular joint morphology in the African apes

A number of researchers have suggested a functional relationship between dietary variation and temporomandibular joint (TMJ) morphology, yet few studies have evaluated TMJ form in the African apes. In this study, I compare TMJ morphology in adults and during ontogeny in Gorilla (G.g. beringei, G.g. graueri, and G.g. gorilla) and Pan (P. paniscus, P. troglodytes troglodytes, P.t. schweinfurthii, and P.t. verus). I test two hypotheses: first, compared to all other African apes, G.g. beringei exhibits TMJ morphologies that would be predicted for a primate that consumes a diet comprised primarily of moderately to very tough, leafy vegetation; and second, all gorillas exhibit the same predicted morphologies compared to Pan. Compared to all adult African apes, G.g. beringei has higher rami and condyles positioned further above the occlusal plane of the mandible, relative to jaw length. Thus, mountain gorillas have the potential to generate relatively more muscle force, more evenly distribute occlusal forces along the postcanine teeth, and generate relatively greater jaw adductor moment. G.g. beringei also exhibits relatively wider mandibular condyles, suggesting these folivorous apes are able to resist relatively greater compressive loads along the lateral and/or medial aspect of the condyle. All gorillas likewise exhibit these same shape differences compared to Pan. These morphological responses are the predicted consequences of intensification of folivory and, as such, provide support for functional hypotheses linking these TMJ morphologies to degree of folivory. The African apes to not, however, demonstrate a systematic pattern of divergence in relative condylar area as a function of intensification of folivory. The ontogenetic trajectories for gorillas are significantly elevated above those of Pan, and to a lesser but still significant degree, mountain gorillas similarly deviate from lowland gorillas (G.g. gorilla and G.g. graueri). Thus, adult shape differences in ramal and condylar heights do not result from the simple extrapolation of common growth allometries relative to jaw length. As such, they are suggestive of an adaptive shift towards a tougher, more folivorous diet. However, the allometric patterning for condylar area and condylar width does not systematically conform to predictions based on dietary specialization. Thus, while differences in condylar shapes may confer functional advantages both during growth and as adults, there is no evidence to suggest selection for altered condylar proportions, independent of the effects of changes in jaw size.

Jaw-muscle electromyography during chewing in Belanger's treeshrews (Tupaia belangeri)

We examined masseter and temporalis recruitment and firing patterns during chewing in five male Belanger's treeshrews (Tupaia belangeri), using electromyography (EMG). During chewing, the working-side masseters tend to show almost three times more scaled EMG activity than the balancing-side masseters. Similarly, the working-side temporalis muscles have more than twice the scaled EMG activity of the balancing-side temporalis. The relatively higher activity in the working-side muscles suggests that treeshrews recruit less force from their balancing-side muscles during chewing. Most of the jaw-closing muscles in treeshrews can be sorted into an early-firing or late-firing group, based on occurrence of peak activity during the chewing cycle. Specifically, the first group of jaw-closing muscles to reach peak activity consists of the working-side anterior and posterior temporalis and the balancing-side superficial masseter. The balancing-side anterior and posterior temporalis and the working-side superficial masseter peak later in the power stroke. The working-side deep masseter peaks, on average, slightly before the working-side superficial masseter. The balancing-side deep masseter typically peaks early, at about the same time as the balancing-side superficial masseter. Thus, treeshrews are unlike nonhuman anthropoids that peak their working-side deep masseters early and their balancing-side deep masseters late in the power stroke. Because in anthropoids the late firing of the balancing-side deep masseter contributes to wishboning of the symphysis, the treeshrew EMG data suggest that treeshrews do not routinely wishbone their symphyses during chewing. Based on the treeshrew EMG data, we speculate that during chewing, primitive euprimates 1) recruited more force from the working-side jaw-closing muscles as compared to the balancing-side muscles, 2) fired an early group of jaw-closing muscles followed by a second group of muscles that peaked later in the power stroke, 3) did not fire their working-side deep masseter significantly earlier than their working-side superficial masseter, and 4) did not routinely fire their balancing-side deep masseter after the working-side superficial masseter.

THE EVOLUTION OF STATIC ALLOMETRY IN SEXUALLY SELECTED TRAITS

Although it has been the subject of verbal theory since Darwin, the evolution of morphological trait allometries remains poorly understood, especially in the context of sexual selection. Here we present an allocation trade-off model that predicts the optimal pattern of allometry under different selective regimes. We derive a general solution that has a simple and intuitive interpretation and use it to investigate several examples of fitness functions. Verbal arguments have suggested cost or benefit scenarios under which sexual selection on signal or weapon traits may favor larger individuals with disproportionately larger traits (i.e., positive allometry). However, our results suggest that this is necessarily true only under a precisely specified set of conditions: positive allometry will evolve when the marginal fitness gains from an increase in relative trait size are greater for large individuals than for small ones. Thus, the optimal allometric pattern depends on the precise nature of net selection, and simple examples readily yield isometry, positive or negative allometry, or polymorphisms corresponding to sigmoidal scaling. The variety of allometric patterns predicted by our model is consistent with the diversity of patterns observed in empirical studies on the allometries of sexually selected traits. More generally, our findings highlight the difficulty of inferring complex underlying processes from simple emergent patterns.

Comparative functional analysis of skull morphology of tree-gouging primates

Many primates habitually feed on tree exudates such as gums and saps. Among these exudate feeders, Cebuella pygmaea, Callithrix spp., Phaner furcifer, and most likely Euoticus elegantulus elicit exudate flow by biting into trees with their anterior dentition. We define this behavior as gouging. Beyond the recent publication by Dumont ([1997] Am J Phys Anthropol 102:187-202), there have been few attempts to address whether any aspect of skull form in gouging primates relates to this specialized feeding behavior. However, many researchers have proposed that tree gouging results in larger bite force, larger internal skull loads, and larger jaw gapes in comparison to other chewing and biting behaviors. If true, then we might expect primate gougers to exhibit skull modifications that provide increased abilities to produce bite forces at the incisors, withstand loads in the skull, and/or generate large gapes for gouging. We develop 13 morphological predictions based on the expectation that gouging involves relatively large jaw forces and/or jaw gapes. We compare skull shapes for P. furcifer to five cheirogaleid taxa, E. elegantulus to six galagid species, and C. jacchus to two tamarin species, so as to assess whether gouging primates exhibit these predicted morphological shapes. Our results show little morphological evidence for increased force-production or load-resistance abilities in the skulls of these gouging primates. Conversely, these gougers tend to have skull shapes that are advantageous for creating large gapes. For example, all three gouging species have significantly lower condylar heights relative to the toothrow at a given mandibular length in comparison with closely related, nongouging taxa. Lowering the height of the condyle relative to the mandibular toothrow should reduce the stretching of the masseters and medial pterygoids during jaw opening, as well as position the mandibular incisors more anteriorly at wide jaw gapes. In other words, the lower incisors will follow a more vertical trajectory during both jaw opening and closing. We predict, based on these findings, that tree-gouging primates do not generate unusually large forces, but that they do use relatively large gapes during gouging. Of course, in vivo data on jaw forces and jaw gapes are required to reliably assess skull functions during gouging.

Cross-sectional geometry and morphology of the mandibular symphysis in Middle and Late Pleistocene Homo

Studies of the evolutionary emergence of the human "chin" have been investigated from a phylogenetic perspective during the later Pleistocene or from a biomechanical perspective across extant primates. Since it was during the Middle and Late Pleistocene that the distinctive human mentum osseum emerged, the relationship between mentum osseum form and resistance to mechanical stress at the mandibular symphysis was examined for forty-two Middle and Late Pleistocene human mandibles. Mentum osseum variation was scored on a five-point ordinal scale (mentum osseum rank). Resistance to bending was represented by second moments of area calculated from symphyseal cross-sections. Relative strength in bending was represented by second moments of area divided by estimated moment arm or beam length. Vertical bending resistance in the coronal plane was maintained across the range of mentum osseum variation within and between later Pleistocene human groups. In contrast, resistance to lateral transverse bending (wishboning) was significantly negatively correlated with the emergence of a protruding mentum osseum. However, Neandertals and early modern humans were equivalent in their abilities to resist this bending regime, while both groups were less resistant in wishboning than earlier archaic humans. In addition, symphyseal inclination, which decreased throughout the later Pleistocene, was highly correlated with mentum osseum rank. Although the overall pattern of differential stasis and change in vertical bending and wishboning resistance at the symphysis is consistent with aspects of the current biomechanical model of the "chin," the decoupling of bending resistance and mentum osseum form in the Late Pleistocene suggests that the evolutionary emergence of the modern human "chin" was at least partly independent of the biomechanical demands placed on the symphysis.

Masticatory form and function in the African apes

This study examines variability in masticatory morphology as a function of dietary preference among the African apes. The African apes differ in the degree to which they consume leaves and other fibrous vegetation. Gorilla gorilla beringei, the eastern mountain gorilla, consumes the most restricted diet comprised of mechanically resistant foods such as leaves, pith, bark, and bamboo. Gorilla gorilla gorilla, the western lowland gorilla subspecies, consumes leaves and other terrestrial herbaceous vegetation (THV) but also consumes a fair amount of ripe, fleshy fruit. In contrast to gorillas, chimpanzees are frugivores and rely on vegetation primarily as fallback foods. However, there has been a long-standing debate regarding whether Pan paniscus, the pygmy chimpanzee (or bonobo), consumes greater quantities of THV as compared to Pan troglodytes, the common chimpanzee. Because consumption of resistant foods involves more daily chewing cycles and may require larger average bite force, the mechanical demands placed on the masticatory system are expected to be greater in folivores as compared to primates that consume large quantities of fleshy fruit. Therefore, more folivorous taxa are predicted to exhibit features that improve load-resistance capabilities and increase force production. To test this hypothesis, jaw and skull dimensions were compared in ontogenetic series of G. g. beringei, G. g. gorilla, P. t. troglodytes, and P. paniscus. Controlling for the influence of allometry, results show that compared to both chimpanzees and bonobos, gorillas exhibit some features of the jaw complex that are suggestive of improved masticatory efficiency. For example, compared to all other taxa, G. g. beringei has a significantly wider mandibular corpus and symphysis, larger area for the masseter muscle, higher mandibular ramus, and higher mandibular condyle relative to the occlusal plane of the mandible. However, the significantly wider mandibular symphysis may be an architectural response to increasing symphyseal curvature with interspecific increase in size. Moreover, Gorilla and Pan do not vary consistently in all features, and some differences run counter to predictions based on dietary variation. Thus, the morphological responses are not entirely consonant with predictions based on hypothesized loading regimes. Finally, despite morphological differences between bonobos and chimpanzees, there is no systematic pattern of differentiation that can be clearly linked to differences in diet. Results indicate that while some features may be linked to differences in diet among the African apes, diet alone cannot account for the patterns of morphological variation demonstrated in this study. Allometric constraints and dental development also appear to play a role in morphological differentiation among the African apes.

Astragalar morphology of late Eocene anthropoids from the Fayum Depression (Egypt) and the origin of catarrhine primates

The phylogenetic relationships of the late Eocene anthropoids Catopithecus browni and Proteopithecus sylviae are currently a matter of debate, with opinion divided as to whether these taxa are stem or crown anthropoids. The phylogenetic position of Catopithecus is of particular interest, for, unlike the highly generalized genus Proteopithecus, this taxon shares apomorphic dental and postcranial features with more derived undoubted catarrhines that appear in the same region 1-2 Ma later. If these apomorphies are homologous and Catopithecus is a stem catarrhine, the unique combination of plesiomorphic and apomorphic features preserved in this anthropoid would have important implications for our understanding of the crown anthropoid morphotype and the pattern of morphological character transformations that occurred during the early phases of stem catarrhine evolution.Well-preserved astragali referrable to Proteopithecus, Catopithecus, and the undoubted early Oligocene stem catarrhine Aegyptopithecus have provided additional morphological evidence that allows us to further evaluate competing hypotheses of interrelationships among Eocene-Oligocene Afro-Arabian anthropoids. Qualitative observations and multivariate morphometric analyses reveal that the astragalar morphology of Proteopithecus is very similar to that of early Oligocene parapithecids and living and extinct small-bodied platyrrhines, and strengthens the hypothesis that the morphological pattern shared by these taxa is primitive within crown Anthropoidea. In contrast, Catopithecus departs markedly from the predicted crown anthropoid astragalar morphotype and shares a number of apomorphic features (e.g., deep cotylar fossa, laterally projecting fibular facet, trochlear asymmetry, mediolaterally wide astragalar head) with Aegyptopithecus and Miocene-Recent catarrhines. The evidence from the astragalus complements other independent data from the dentition, humerus and femur of Catopithecus that support this taxon's stem catarrhine status, and we continue to maintain that oligopithecines are stem catarrhines that constitute the sister group of a clade containing propliopithecines and Miocene-Recent catarrhines.

Transverse masticatory movements, occlusal orientation, and symphyseal fusion in selenodont artiodactyls

Based on extensive experimental work on primates, two masticatory loading regimes have emerged as the likely determinants of mandibular symphyseal fusion-dorsoventral shear and lateral transverse bending (wishboning) (Ravosa and Hylander, 1994; Hylander et al., 1998, 2000). Recently, however, it has been argued that, rather than functioning to strengthen the symphysis during mastication, fusion serves to stiffen the symphyseal joint so as to facilitate increased transverse jaw movements during occlusion (Lieberman and Crompton, 2000). As part of this transverse stiffness model, it has been suggested that taxa with fused symphyses should also exhibit more horizontally oriented occlusal wear facets. Using a series of univariate and bivariate analyses, we test predictions of these three models in a sample of 44 species of selenodont artiodactyls. Consistent with the wishboning and transverse stiffness models, taxa with fused symphyses (camelids) have more horizontally oriented M(2) and M(2) occlusal wear facets, anteroposteriorly (AP) elongate symphyses, and relatively wider corpora. Contrary to the dorsoventral shear model, camelids do not have relatively deeper corpora (due to greater parasagittal bending). While taxa with ossified symphyses have relatively larger symphysis cross-sectional areas, this appears to be the byproduct of an increase in AP symphysis length due to greater lateral transverse bending of the mandible. Theoretical consideration of the biomechanics of mastication further suggests that strength, not stiffness, is the critical factor in determining symphyseal ossification. Thus, like anthropoid primates, fusion in selenodont artiodactyls appears to function in resisting increased wishboning stresses arising from an emphasis on transverse occlusal/mandibular movements and loads.

Mammalian feeding and primate evolution: An overview

Most of the papers included in this volume are derived from presentations in a symposium on Mammalian Feeding at the 65th Annual Meetings of the American Association of Physical Anthropologists in North Carolina in 1996. The aims of this symposium were to gather together the preeminent researchers on mammalian mastication and document the state of research in that field. The symposium emphasized in vivo studies of mammalian feeding because of a paucity of recent reviews of this field, but included morphometric and modeling papers as well. Subsequently the papers were revised, and were submitted in spring 1998 for publication, pending the outcome of peer review. Copyright 2000 Wiley-Liss, Inc.

Masticatory stress, orbital orientation and the evolution of the primate postorbital bar

A postorbital bar is one of a suite of derived features which distinguishes basal primates from their putative sister taxon, plesiadapiforms. Two hypotheses have been put forward to explain postorbital bar development and variation in circumorbital form: the facial torsion model and visual predation hypothesis. To test the facial torsion model, we employ strain data on circumorbital and mandibular loading patterns in representative primates with a postorbital bar and masticatory apparatus similar to basal primates. To examine the visual predation hypothesis, we employ metric data on orbit orientation in Paleocene and Eocene primates, as well as several clades of visual predators and foragers that vary interspecifically in postorbital bar formation.A comparison of galago circumorbital and mandibular peak strains during powerful mastication demonstrates that circumorbital strains are quite low. This indicates that, as in anthropoids, the strepsirhine circumorbital region is excessively overbuilt for countering routine masticatory loads. The fact that circumorbital peak-strain levels are uniformly low in both primate suborders undermines any model which posits that masticatory stresses are determinants of circumorbital form, function and evolution. This is interpreted to mean that sufficient cortical bone must exist to prevent structural failure due to non-masticatory traumatic forces. Preliminary data also indicate that the difference between circumorbital and mandibular strains is greater in larger taxa.Comparative analyses of several extant analogs suggest that the postorbital bar apparently provides rigidity to the lateral orbital margins to ensure a high level of visual acuity during chewing and biting. The origin of the primate postorbital bar is linked to changes in orbital convergence and frontation at smaller sizes due to nocturnal visual predation and increased encephalization. By incorporating in vivo and fossil data, we reformulate the visual predation hypothesis of primate origins and thus offer new insights into major adaptive transformations in the primate skull.