Multiscale adaptations underlie bite force variation in rodents

Area of Science:

• Evolutionary biology
• Biomechanics
• Comparative anatomy

Background:

• Bite force is a crucial indicator of organismal performance.
• Masticatory myosin (MHC-M) expression is linked to high bite force.
• The integration of muscle adaptations across different biological scales remains poorly understood.

Purpose of the Study:

• To investigate how jaw muscle adaptations for force production are integrated across multiple organizational scales in rodents.
• To analyze myosin isoform composition, physiological cross-sectional area, and dynamic moment arms in relation to bite force.

Main Methods:

• Analysis of myosin isoform composition and physiological cross-sectional area of jaw muscles.
• Ex vivo measurement of dynamic moment arms using X-ray Reconstruction Of Moving Morphology (XROMM).
• Comparative analysis across six rodent species with varying feeding ecologies.

Main Results:

• Hard-biting rodents (e.g., grey squirrels) exhibit adaptations at all scales favoring force production.
• Other species display a mosaic of adaptations, achieving similar bite forces through diverse muscle phenotypes.
• Guinea pigs show adaptations consistent with reduced incisor bite force.

Conclusions:

• Craniofacial adaptations for bite force involve coordinated changes across multiple biological scales.
• Selection for traits like MHC-M expression is integrated across these scales.
• Understanding these multiscale adaptations provides insights into evolutionary pressures on feeding mechanics.