Integration of distal tibial shape and internal trabecular bone structure among catarrhine primates

Primates display a wide range of locomotor behaviours, each resulting in distinct mechanical demands on the ankle joint. This study examines how variation in distal tibial shape and internal bone structure captures these locomotor adaptations. We examined the morphology of distal tibiae from eight extant catarrhine genera using canonical holistic morphometric analysis. Relative trabecular bone volume fraction was quantified across homologous three-dimensional volumes, and shape was assessed via a new geometric morphometric-based approach. Our analyses reveal taxonomic and locomotor differences in both distal tibial trabecular distribution and shape. Humans exhibit a squared tibial plafond with trabecular reinforcement at its center, reflecting vertical axial loading. African apes show a trapezoidal plafond with trabecular reinforcement linked to ankle flexion during climbing and quadrupedalism. Orangutans and hylobatids exhibit diverse trabecular distribution and morphology, consistent with multidirectional loading. Cercopithecoids display posterior trabecular reinforcement and narrower joint morphology, consistent with plantarflexed postures during quadrupedalism. Substantial covariation between tibial shape and trabecular distribution was identified across taxa, with functional gradients aligning with ankle loading regimes, showing that analyzing trabecular structure and external shape enhances our understanding of joint mechanics, offering critical insight for interpretation of primate locomotor diversity and evolution.