Jaw-muscle force and excursion scale with negative allometry in platyrrhine primates

Platyrrhines span two orders of magnitude in body size and are characterized by diverse feeding behaviors and diets. While size plays an important role in primate feeding behavior and masticatory apparatus morphology, we know little about size-correlated changes in the force-generating (physiologic cross-sectional area; PCSA) and excursion/stretch (fiber length; L_f ) capabilities of the jaw-closing muscles in platyrrhines.

We examined scaling relationships of the superficial masseter and temporalis muscles in 21 platyrrhine species. Previous work suggests that larger platyrrhines are at a mechanical disadvantage for generating bite forces compared with smaller platyrrhines. We hypothesize that scaling of jaw-muscle fiber architecture counters this size-correlated decrease in mechanical advantage. Thus, we predicted that jaw-muscle PCSAs and muscle weights scale with positive allometry while L_f s scale with negative allometry, relative to load-arm estimates for incisor/molar biting and chewing.

Jaw-muscle PCSAs and L_f s appear to scale with negative allometry relative to load-arm estimates and body size. Negative allometry of jaw-muscle weights partially accounts for the size-correlated decreases in PCSA and L_f . Estimates of bite force also scale with negative allometry.

Large-bodied platyrrhines (e.g., Alouatta) are at a relative disadvantage for generating jaw-muscle and bite force as well as jaw-muscle stretch, compared with smaller species (e.g., Callithrix). The net effect is that larger platyrrhines likely produce relatively smaller maximal bite forces compared with smaller taxa. Relative to small- and intermediate-sized platyrrhines, large-bodied platyrrhines feed on some of the least mechanically challenging foods, consistent with the size-correlated decrease in relative muscle and bite forces across the clade. Am J Phys Anthropol, 2015. © 2015 Wiley Periodicals, Inc. Am J Phys Anthropol 158:242-256, 2015. © 2015 Wiley Periodicals, Inc.