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

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


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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