Bite force-gape curves and passive tension costs in Macaca mulatta

Passive forces generated by the jaw adductor muscles and their connective tissues are thought to play a protective role in the feeding system by limiting gape to avoid hyperextension and minimize distractive forces at the temporomandibular joint. However, passive muscle forces have only been measured in individual jaw adductors of two non-primate mammals, and it is unknown how these forces translate to bite force at the occlusal surface and affect gape behaviors. We measured in vivo passive bite forces in eight adult Macaca mulatta at anterior (I1) and posterior (M1) bite points across linear gapes ranging from 15 to 50 mm. Active bite force data were collected at the anterior bite point from two of these macaques (one male, one female) using a custom-built bite force transducer across linear gapes ranging from 10 to 60 mm. We demonstrate that M. mulatta passive bite forces increase with gape and vary by bite point, with forces larger at M1 compared with I1 for both linear and angular gapes. Our experimental data and Hill-type muscle models of both active and passive forces suggest that passive bite forces are absolutely and relatively small at the occlusal surface in macaques and play a minimal role in constraining gape. These are the first empirical data on bite force passive tension in primates, and the first data to suggest that the macaque jaw adductor muscles exhibit unusually high compliance, potentially relating to selection for large gape behaviors.