1Community and Family Medicine, Duke University School of Medicine, 2Evolutionary Anthropology, Duke University, 3Organismal Biology and Anatomy, University of Chicago, 4Anatomy and Neurobiology, Northeast Ohio Medical University
Friday 9:30-9:45, Ballroom C
The jaw-closing muscles are responsible for generating the forces and movements associated with chewing, biting and display behaviors. Addressing size-related trends in the force and excursion capabilities of these muscles is fundamental for testing various scaling hypotheses and for understanding the functional and evolutionary significance of departures from these theoretical predictions.
We measured fiber lengths (Lf) and pinnation angles and computed the physiologic cross-sectional areas (PCSAs) of the masseter and temporalis muscles in adult males and females for 42 anthropoid species (n=19 platyrrhines; n=16 Old World monkeys [OWMs]; n=7 hominoids). We used Lf to estimate maximum muscle excursion/contraction velocity and PCSA to estimate maximum muscle force-generating potential. We employed reduced major axis regression to evaluate scaling relationships of Lf and PCSA relative to jaw length and condyle-M1 distance (load-arm estimates for anterior tooth biting and postcanine mastication, respectively). Correlations with load-arm estimates range between 0.61-0.97 and are all significant (p<0.05). Relative to both load-arm estimates, masseter and temporalis Lfs scale with negative allometry in platyrrhines but positive allometry in OWMs and hominoids. PCSAs scale with negative allometry in platyrrhines and with increasingly positive allometry from OWMs to hominoids. Thus, we observe clade-specific allometric differences underlying potential shifts in the excursion- and force-generating capabilities of the masseter and temporalis across anthropoid clades. Superimposed on these size-correlated trends among clades are differences in jaw-muscle stretch and force estimates that correlate with variation in feeding behavior and diet in platyrrhines and with variation in gape associated with display behaviors in some OWMs.
Funded by the National Science Foundation (BCS 0452160, BCS 0962677, BCS 075261), the National Skeletal Muscle Research Center (NIH R24 HD050837-01) and Duke University Undergraduate Research Support grants.