Separate but equal? Relating morphological divergence to functional performance in the evolution of primate chewing

A long‐standing hypothesis in studies of primate mastication argues that musculoskeletal form has changed markedly during primate evolution while masticatory behaviors, such as the jaw‐muscle activation sequence during chewing, has remained relatively similar across primates. If morphology has evolved within this conserved behavioral framework, then we predict that jaw‐muscle activity patterns exhibit limited variation across primates and show little association with jaw‐muscle and skull form among species. To examine these relationships, we compared jaw‐muscle electromyography (EMG) data to musculoskeletal measures of the masticatory apparatus across several primates including strepsirrhines, monkeys and humans and one non‐primate treeshrew. Contrary to the prediction of little association between jaw‐muscle activity and musculoskeletal form, we find strong associations between the timing and magnitude of jaw‐muscle EMGs and masticatory apparatus shapes. These associations fit in vivo observations and theoretical predictions linking patterns of jaw loading to morphologies providing increased load resistance. We hypothesize that several functional modules exist in the primate masticatory apparatus incorporating both neuromuscular activation and musculoskeletal form. Jaw‐muscle activity patterns appear to be an integrated and evolving part of the primate masticatory apparatus. Supported by NSF.