Medial metatarsal rigidity in extant non-human catarrhine primates

Objectives This study quantifies hallucal metatarsal (Mt1) and second metatarsal (Mt2) polar second moment of areas (J) in hominoids and cercopithecoids to address three questions. Which catarrhines have more rigid Mt1s versus Mt2s? Is asymmetry in medial metatarsal rigidity (e.g., Mt1 > Mt2) different among taxa? Does asymmetry in medial metatarsal rigidity correlate with relative intrinsic hallucal muscle mass across species.Materials and Methods J was calculated from CT images in 345 specimens and compared between groups: Pan, Gorilla, Pongo, hylobatids, cercopithecines, and colobines. Differences between the Mt1 and Mt2 within groups were assessed using paired t tests. Log(10)-transformed Mt1:Mt2 J ratios were used to assess asymmetry in rigidity and inter-group comparisons were made. Additionally, correlations were performed between species means of log(10)-transformed Mt1:Mt2 J ratios and species means of relative intrinsic hallucal muscle mass.Results African ape and hylobatid Mt1s are significantly more rigid than their Mt2s. Rigidity in medial metatarsals are more variable in Pongo, but most individuals have stronger Mt2s. Greatest variation is observed in cercopithecoids, with some species having consistently stronger Mt1s or Mt2s, but the majority show no bias for either. Log(10)-transformed Mt1:Mt2 J ratios are significantly correlated with relative intrinsic hallucal muscle mass; taxa with more rigid Mt1s have more developed muscle.Discussion Differences between metatarsals in rigidity do not correlate with previously documented regional differences in peak plantar pressure during weight support. Rather, asymmetry in metatarsal rigidity, specifically Mt1 > Mt2, reflect adaptations of larger intrinsic hallucal musculature capable of producing greater hallucal grasping force.