Relationship of bone utilization and biomechanical competence in hominoid mandibles

This investigation explores regional variation in bone mass in the mandibles of large-bodied hominoids with respect to the masticatory biomechanical environment. Cortical area, subperiosteal area, mandibular length, maximum and minimum area moments of inertia are sampled at 7 sections along the mandibular corpus in 20 specimens each of Homo sapiens, Pan troglodytes, Pongo pygmaeus and Gorilla gorilla. The null hypothesis is that bone is utilized similarly among species, between sexes and among corpus locations in terms of economy of bone deployment (relative to subperiosteal area) and efficiency in producing structural stiffness (relative to cross-sectional moments of inertia). The alternative hypothesis is that dietary toughness and the scaling of muscular force recruitment produces an unfavourable stress environment in the mandible such that larger species (Gorilla and Pongo) use relatively more cortical bone than Pan and Homo. Three-way factorial analysis of variance (with species, sex and location as main effects) indicates significant interaction of species and location for all indices of bone economy and efficiency. Sex is significant as a main effect or interacting with location in all indices of cortical area. While allometric effects are not readily discernible in these data, the null hypothesis of a common pattern of bone utilization is decisively rejected. Human mandibles use relatively more cortical bone than those of great apes, particularly in anterior regions of the corpus. Among the apes, orangutans use very little cortical bone to achieve mechanical stiffness.