Christine E. Wall

Finding Our Way through Phenotypes

Imagine if we could compute across phenotype data as easily as genomic data; this article calls for efforts to realize this vision and discusses the potential benefits.

Symphyseal fusion and jaw-adductor muscle force: an EMG study.

The purpose of this study is to test various hypotheses about balancing-side jaw muscle recruitment patterns during mastication, with a major focus on testing the hypothesis that symphyseal fusion in anthropoids is due mainly to vertically- and/or transversely-directed jaw muscle forces. Furthermore, as the balancing-side deep masseter has been shown to play an important role in wishboning of the macaque mandibular symphysis, we test the hypothesis that primates possessing a highly mobile mandibular symphysis do not exhibit the balancing-side deep masseter firing pattern that causes wishboning of the anthropoid mandible. Finally, we also test the hypothesis that balancing-side muscle recruitment patterns are importantly related to allometric constraints associated with the evolution of increasing body size. Electromyographic (EMG) activity of the left and right superficial and deep masseters were recorded and analyzed in baboons, macaques, owl monkeys, and thick-tailed galagos. The masseter was chosen for analysis because in the frontal projection its superficial portion exerts force primarily in the vertical (dorsoventral) direction, whereas its deep portion has a relatively larger component of force in the transverse direction. The symphyseal fusion-muscle recruitment hypothesis predicts that unlike anthropoids, galagos develop bite force with relatively little contribution from their balancing-side jaw muscles. Thus, compared to galagos, anthropoids recruit a larger percentage of force from their balancing-side muscles. If true, this means that during forceful mastication, galagos should have working-side/balancing-side (W/B) EMG ratios that are relatively large, whereas anthropoids should have W/B ratios that are relatively small. The EMG data indicate that galagos do indeed have the largest average W/B ratios for both the superficial and deep masseters (2.2 and 4.4, respectively). Among the anthropoids, the average W/B ratios for the superficial and deep masseters are 1.9 and 1.0 for baboons, 1.4 and 1.0 for macaques, and both values are 1.4 for owl monkeys. Of these ratios, however, the only significant difference between thick-tailed galagos and anthropoids are those associated with the deep masseter. Furthermore, the analysis of masseter firing patterns indicates that whereas baboons, macaques and owl monkeys exhibit the deep masseter firing pattern associated with wishboning of the macaque mandibular symphysis, galagos do not exhibit this firing pattern. The allometric constraint-muscle recruitment hypothesis predicts that larger primates must recruit relatively larger amounts of balancing-side muscle force so as to develop equivalent amounts of bite force. Operationally this means that during forceful mastication, the W/B EMG ratios for the superficial and deep masseters should be negatively correlated with body size. Our analysis clearly refutes this hypothesis. As already noted, the average W/B ratios for both the superficial and deep masseter are largest in thick-tailed galagos, and not, as predicted by the allometric constraint hypothesis, in owl monkeys, an anthropoid whose body size is smaller than that of thick-tailed galagos. Our analysis also indicates that owl monkeys have W/B ratios that are small and more similar to those of the much larger-sized baboons and macaques. Thus, both the analysis of the W/B EMG ratios and the muscle firing pattern data support the hypothesis that symphyseal fusion and transversely-directed muscle force in anthropoids are functionally linked. This in turn supports the hypothesis that the evolution of symphyseal fusion in anthropoids is an adaptation to strengthen the symphysis so as to counter increased wishboning stress during forceful unilateral mastication. (ABSTRACT TRUNCATED)

Primate craniofacial function and biology

I. Historical perspective on Experimental Research in Biological Anthropology 1. Experimental Comparative Anatomy in Physical Anthropology: The Contributions of Dr. William Hylander to Studies of Skull Form and Function. Daniel Schmitt, Christine E. Wall, and Pierre Lemelin II. In Vivo Research Into Masticatory Function 2. A nonprimate model for the fused symphysis: in vivo studies in the pig. Susan W. Herring, Katherine L. Rafferty, Zi Jun Liu, and Zongyang Sun 3. Symphyseal fusion in selenodont artiodactyls: new insights from in vivo and comparative data. Susan H. Williams, Christine E. Wall, Christopher J. Vinyard, William L. Hylander 4. Does the primate face torque? Callum F. Ross 5. Motor control of masticatory movements in the Southern hairy-nosed wombat (Lasiorhinus latifrons). Alfred W. Crompton, Daniel E. Lieberman, Tomasz Owerkowicz, Russell V. Baudinette and Jayne Skinner 6. Specialization of the Superficial Anterior Temporalis in Baboons for Mastication of Hard Foods. Christine E. Wall, Christopher J. Vinyard, Susan H. Williams, Kirk R. Johnson, William L. Hylander III. Modeling Masticatory Apparatus Function 7. Effects of Dental Alveoli on the Biomechanical Behavior of the Mandibular Corpus. David J. Daegling, Jennifer L. Hotzman and Andrew J. Rapoff 8. Surface strain on bone and sutures in a monkey facial skeleton: an in vitro approach and its relevance to Finite Element Analysis. Qian Wang, Paul C. Dechow, Barth W. Wright, Callum F. Ross, David S. Strait, Brian G. Richmond, Mark A. Spencer and Craig D. Byron. 9. Craniofacial Strain Patterns During Premolar Loading: Implications for Human Evolution. David S. Strait, Barth Wright, Brian G. Richmond, Callum F. Ross, Paul C. Dechow, Mark A. Spencer, Qian Wang. IV. Jaw-Muscle Architecture 10. Scaling of reduced physiologic cross-sectional area in primate muscles of mastication. Fred Anapol, Nazima Shahnoor and Callum F. Ross 11. Scaling of the Chewing Muscles in Prosimians. Jonathan M.G. Perry and Christine E. Wall 12. The relationship between jaw-muscle architecture and feeding behavior in primates: Tree-gouging and nongouging gummivorous callitrichids as a natural experiment. Andrea B. Taylor and Christopher J. Vinyard V. Bone and Dental Morphology 13. Relationship between three dimensional microstructure and elastic properties of cortical bone in the human mandible and femur. Paul C. Dechow, Dong Hwa Chung, and Mitra Bolouri 14. Adaptive plasticity in the mammalian masticatory complex: You are what, and how, you eat. Matthew J. Ravosa, Ravinder Kunwar, Elisabeth K. Nicholson, Emily B. Klopp, Jessie Pinchoff, Stuart R. Stock, M. Sharon Stack, and Mark W. Hamrick 15. Mandibular corpus form and its functional significance: Evidence from marsupials. Aaron S. Hogue 16. Putting shape to work: Making functional interpretations of masticatory apparatus shapes in primates. Christopher J. Vinyard 17. Food physical properties and their relationship to morphology: The curious case of kily. Nayuta Yamashita 18. Convergence and frontation in Fayum anthropoid orbits. Elwyn L. Simons 19. What else is the tall mandibular ramus of the robust australopiths good for? Yoel Rak and William L. Hylander 20. Framing the question: Diet and evolution in early Homo. Susan C. Anton

The Evolutionary Morphology of Tree Gouging in Marmosets

The marmosets, Callithrix spp. and Cebuella pygmaea, are unique among anthropoids in their habitual biting of trees with their anterior teeth to elicit exudate flow. This tree-gouging behavior is thought to offer certain ecological benefits to marmosets, such as routine access to an under-exploited resource, as well as have specific influences on their behavioral ecology.

The Jaw Adductors of Strepsirrhines in Relation to Body Size, Diet, and Ingested Food Size

Body size and food properties account for much of the variation in the hard tissue morphology of the masticatory system whereas their influence on the soft tissue anatomy remains relatively understudied. Data on jaw adductor fiber architecture and experimentally determined ingested food size in a broad sample of 24 species of extant strepsirrhines allows us to evaluate several hypotheses about the influence of body size and diet on the masticatory muscles. Jaw adductor mass scales isometrically with body mass (β = 0.99, r = 0.95), skull size (β = 1.04, r = 0.97), and jaw length cubed (β = 1.02, r = 0.95). Fiber length also scales isometrically with body mass (β = 0.28, r = 0.85), skull size (β = 0.33, r = 0.84), and jaw length cubed (β = 0.29, r = 0.88). Physiological cross‐sectional area (PCSA) scales with isometry or slight positive allometry with body mass (β = 0.76, r = 0.92), skull size (β = 0.78, r = 0.94), and jaw length cubed (β = 0.78, r = 0.91). Whereas PCSA is isometric to body size estimates in frugivores, it is positively allometric in folivores. Independent of body size, fiber length is correlated with maximum ingested food size, suggesting that ingestive gape is related to fiber excursion. Comparisons of temporalis, masseter, and medial pterygoid PCSA in strepsirrhines of different diets suggest that there may be functional partitioning between these muscle groups. Anat Rec, 2011.

Genomic signatures of diet-related shifts during human origins.

There are numerous anthropological analyses concerning the importance of diet during human evolution. Diet is thought to have had a profound influence on the human phenotype, and dietary differences have been hypothesized to contribute to the dramatic morphological changes seen in modern humans as compared with non-human primates. Here, we attempt to integrate the results of new genomic studies within this well-developed anthropological context. We then review the current evidence for adaptation related to diet, both at the level of sequence changes and gene expression. Finally, we propose some ways in which new technologies can help identify specific genomic adaptations that have resulted in metabolic and morphological differences between humans and non-human primates.

Sociality, ecology, and relative brain size in lemurs

The social brain hypothesis proposes that haplorhine primates have evolved relatively large brains for their body size primarily as an adaptation for living in complex social groups. Studies that support this hypothesis have shown a strong relationship between relative brain size and group size in these taxa. Recent reports suggest that this pattern is unique to haplorhine primates; many nonprimate taxa do not show a relationship between group size and relative brain size. Rather, pairbonded social monogamy appears to be a better predictor of a large relative brain size in many nonprimate taxa. It has been suggested that haplorhine primates may have expanded the pairbonded relationship beyond simple dyads towards the evolution of complex social groups. We examined the relationship between group size, pairbonding, and relative brain size in a sample of 19 lemurs; strepsirrhine primates that last share a common ancestor with monkeys and apes approximately 75 Ma. First, we evaluated the social brain hypothesis, which predicts that species with larger social groups will have relatively larger brains. Secondly, we tested the pairbonded hypothesis, which predicts that species with a pairbonded social organization will have relatively larger brains than non-pairbonded species. We found no relationship between group size or pairbonding and relative brain size in lemurs. We conducted two further analyses to test for possible relationships between two nonsocial variables, activity pattern and diet, and relative brain size. Both diet and activity pattern are significantly associated with relative brain size in our sample. Specifically, frugivorous species have relatively larger brains than folivorous species, and cathemeral species have relatively larger brains than diurnal, but not nocturnal species. These findings highlight meaningful differences between Malagasy strepsirrhines and haplorhines, and between Malagasy strepsirrhines and nonprimate taxa, regarding the social and ecological factors associated with increases in relative brain size. The results suggest that factors such as foraging complexity and flexibility of activity patterns may have driven selection for increases in brain size in lemurs.

A model of temporomandibular joint function in anthropoid primates based on condylar movements during mastication

The hypothesis that the shape of the bony temporomandibular joint (TMJ) is functionally related to sagittal sliding of the condyle during mastication is tested, and a model of the relation of sagittal sliding to mandibular size, TMJ shape, and diet is developed. Sagittal sliding is defined as fore-aft motion of the condyle during mandibular translation and/or angular rotation. Ascending ramus height is used as a structural correlate of the distance between the condyle and the mandibular axis of rotation (CR). Cineradiographic data on sagittal sliding and gape during mastication in Ateles spp., Macaca fascicularis, Papio anubis, and Pan troglodytes in conjunction with comparative data on mandibular size and TMJ shape are used to evaluate the hypothesis. The results show that 1) linear and angular gape are highly positively correlated with sagittal sliding, 2) pure mandibular translation is rare during mastication, 3) the CR is rarely if ever located at the condyle during mastication, 4) angular gape should be standardized in interindividual comparisons of sagittal sliding, and 5) the height of the ascending ramus (and by inference the CR-to-condyle distance) is highly positively correlated with absolute sagittal sliding. Sagittal sliding relative to the length of the articular eminence was the variable used to explore the relation between TMJ shape and sliding. This variable standardized absolute sagittal sliding relative to joint size. The relative depth and orientation of the articular eminence were not correlated with relative sagittal sliding. The anteroposterior curvature of the condyle was highly negatively correlated with relative sagittal sliding. Flat condyles are associated with large amounts of relative sagittal sliding. A flat condyle increases joint contact area, which reduces joint stress. A flat condyle also increases joint congruence, and this may facilitate the combined sliding and rolling motion of the condyle when the sliding motion is relatively large. The shape of the entoglenoid process was also positively correlated with relative sagittal sliding. A relatively large entoglenoid process may help to guide sagittal sliding and prevent excessive mediolateral sliding of the condyle. The functional model makes a number of predictions about the correlations between food consistency and food object size, mandibular size, TMJ shape, and sagittal sliding of the condyle during mastication and incision.

A Biomechanical Analysis of Skull Form in Gum-Harvesting Galagids

Among primates, some highly gummivorous species habitually gouge trees to elicit exudate flow whereas others scrape the hardened gums from trees. These foraging behaviors are thought to require high external forces at the anterior dentition. In this study, we test whether skull form in gouging and scraping galagids corresponds to this suggested need to produce these higher external forces and to resist increased internal loads in the jaws. We find few consistent morphological patterns linking skull form and the generation of high forces during gouging. However, there is some tendency for gougers and scrapers to show increased load resistance capabilities in their mandibles. Future research on the mechanical properties of trees exploited by these species and on jaw function during gouging and scraping will improve our understanding of the mechanical demands of gum feeding on the galagid skull form.

Jaw-Muscle Function and the Origin of Primates

Anthropologists studying primate chewing have focused on the origins and evolution of the masticatory apparatus of anthropoids and humans. We know far less about the functional morphology and evolution of the masticatory apparatus in the earliest euprimates (e.g., Jablonski, 1986). A more complete understanding of masticatory apparatus function in the earliest primates would greatly benefit studies of chewing behavior in both strepsirrhines and haplorhines. We begin addressing this shortcoming in this chapter by asking, “To what extent do treeshrews share similar jaw-muscle activity patterns during chewing with living primates?” We use the small, nonprimate mammal, Belanger’s treeshrew (Tupaia belangeri), as an extant model of jaw-muscle activity during chewing, or mastication, in early euprimates. By comparing