Bone geometry in cercopithecoid mandibles

Abstract

This study explores the relation between cortical bone geometry in the mandibular corpus and in vivo masticatory stress patterns and dietary specialization in cercopithecoid primates. Cortical bone distribution in the mandibles of three species of Old World monkeys ( Macaca fascicularis, Procolobus badius, Lophocebus albigena) was measured by computed tomography. The arrangement of bone within sections was quantified as (1) the ratio of cortical area to the enclosed (total) subperiosteal area; (2) the ratio of orthogonal second moments of area; and (3) size-adjusted measures of cortical area and regional thickness. Cross-sectional geometry differed among samples, but consistent patterns of cortical thinning and bone area were found within individual sections. This consistency was despite the marked differences in diet and feeding behavior that distinguish the three taxa. Lingually thin and basally thick cortical bone was found in the three monkeys; previously published data suggest that this pattern may be stereotypical among anthropoid primates. It is hypothesized that the interactive effects of shear, bending and torsion produce eccentric loads in corpus sections, which are mirrored by this asymmetrical arrangement of cortical bone. When interpreted against existing data for other primate groups, these results are consistent with the hypothesis that masticatory-loading profiles are broadly similar across anthropoids despite the distinctive occlusions found among the suborder. Understanding of the impact of diet on jaw morphology is, therefore, not improved by considerations of cortical bone distribution, i.e. the inference of diet from jaw form is best predicated on considerations of relative corpus size rather than cross-sectional geometry.