Multiscale adaptations underlie bite force variation in rodents

Bite force is a key metric of organismal performance, and expression of masticatory myosin (MHC-M) is associated with high bite force. However, skeletal muscles are multiscale structures, and it remains unclear how adaptations for force production are integrated across scales. We analyzed myosin isoform composition and physiological cross-sectional area of the jaw muscles and measured their dynamic moment arms ex vivo using XROMM (X-ray Reconstruction Of Moving Morphology) in six rodent species. We found modifications at all scales in hard biters (grey squirrels) to prioritize force production. Other species - chipmunk, woodchuck, red squirrel and rat - showed a mix of adaptations across scales, with different muscle phenotypes producing equivalent bite force outputs. By contrast, guinea pigs showed modifications at all scales consistent with reduced incisor bite force production. Our results suggest that selection for ecologically relevant traits - including MHC-M expression - occurs at multiple organizational scales within the rodent craniofacial system.