Exploring Femoral Diaphyseal Shape Variation in Wild and Captive Chimpanzees by Means of Morphometric Mapping: A Test of Wolff's Law

Abstract

Long bone shafts (diaphyses) serve as load-bearing structures during locomotion, implying a close relationship between diaphyseal form and its locomotor function. Diaphyseal form-function relationships, however, are complex, as they are mediated by various factors such as developmental programs, evolutionary adaptation, and functional adaptation through bone remodeling during an individual's lifetime. The effects of the latter process ("Wolff's Law") are best assessed by comparing diaphyseal morphologies of conspecific individuals under different locomotor regimes. Here we use morphometric mapping (MM) to analyze the morphology of entire femoral diaphyses in an ontogenetic series of wild and captive common chimpanzees (Pan troglodytes troglodytes). MM reveals patterns of variation of diaphyseal structural and functional properties, which cannot be recognized with conventional cross-sectional analysis and/or geometric morphometric methods. Our data show that diaphyseal shape, cortical bone distribution and inferred cross-sectional biomechanical properties vary both along ontogenetic trajectories and independent of ontogeny. Mean ontogenetic trajectories of wild and captive chimpanzees, however, were found to be statistically identical. This indicates that the basic developmental program of the diaphysis is not altered by different loading conditions. Significant differences in diaphyseal shape between groups could only be identified in the distal diaphysis, where wild chimpanzees exhibit higher mediolateral relative to anteroposterior cortical bone thickness. Overall, thus, the hypothesis that Wolff's Law predominantly governs long bone diaphyseal morphology is rejected.