Powerful or non-powerful? Revisiting hallucal grasping as a key evolutionary innovation in small arboreal mammals

Powerful hallucal grasping is considered a key primate innovation, yet many arboreal mammals show convergent pedal grasping. We hypothesized that hallucal grasping represents a functional continuum beyond the dichotomy of powerful versus non-powerful grasping. Using diceCT to analyze intrinsic hallucal muscle architecture and automated quantification of the entocuneiform-first metatarsal joint (EFMJ) range of motion, we modeled muscle moment arms in 13 small arboreal mammals (primates, scandentians, rodents, and marsupials). All taxa feature a saddle-shaped EFMJ but differ in kinematical emphasis: primates and marsupials favor flexion-extension through facet torsion, enlarged peroneal processes or tilted articulation surfaces, whereas scandentians and rodents prioritize abduction-adduction. Strepsirrhine primates and marsupials possess enlarged flexors and strong adductors enabling sustained torque, while scandentians and rodents are characterized by low anatomical cross-sectional area muscles. Moment-arm analyses reveal torque optimization at different poses, highlighting a spectrum of morphofunctional grasping strategies rather than a single evolutionary pathway.