The 81st Annual Meeting of the American Association of Physical Anthropologists (2012)


Scaling of jaw-muscle fiber architecture in platyrrhines: a preliminary assessment

ANDREA B. TAYLOR1,2, TIAN YUAN2, CALLUM F. ROSS3 and CHRISTOPHER J. VINYARD4.

1Community and Family Medicine, Duke University School of Medicine, 2Evolutionary Anthropology, Duke University, 3Organismal Biology and Anatomy, University of Chicago, 4Anatomy and Neurobiology, NEOMED

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Recent comparative investigations of jaw-muscle fiber architecture in callitrichids and cebids have provided important insights into how jaw-muscle architecture relates to masticatory apparatus form and feeding mechanics in platyrrhines. Here we extend these analyses to assess the relationships among jaw-muscle fiber architecture, jaw size, and jaw robusticity across platyrrhines. We computed the normalized fiber lengths and physiologic-cross sectional areas (PCSAs) of the superficial masseter and temporalis muscles in 11 platyrrhine species. Our initial reduced-major axis regression of a combined estimate of masseter and temporalis PCSA suggests positive allometry relative to jaw length (slope = 2.3; p<0.01), but does not rule out isometry. Alternatively, masseter and temporalis fiber lengths may scale with negative allometry (slope=0.9 and 0.7, respectively; p<0.03). In addition to a potential size-related increase in muscle force-producing capacity, hard-object feeders such as Cebus apella exhibit relatively large PCSAs. By contrast, the more folivorous Alouatta seniculus displays a relatively reduced jaw-muscle PCSA, suggesting that large body size, rather than relative increases in jaw-muscle force production, plays an important functional role in howler feeding behaviors. Relative PCSA is significantly correlated (r=0.7, p=0.02) with a relative jaw robusticity index that tracks variation in the bony masticatory apparatus. Thus, hard-object feeders tend to have both relatively robust mandibles and relatively large PCSAs, while more folivorous taxa such as howlers exhibit both relatively reduced jaw robusticity and jaw-muscle PCSA. Based on these preliminary findings, muscular and skeletal morphologies of the masticatory apparatus are co-evolving in platyrrhines and both play integral roles in feeding performance.

This work was funded by grants from the National Science Foundation (BCS 0452160; BCS 0962677; BCS-0552285), the National Skeletal Muscle Research Center (NIH R24 HD050837-1), and the Duke University Undergraduate Research Support Office.

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