Material property variation of mandibular symphyseal bone in colobine monkeys.

The anterior mandibular corpus of anthropoid primates is routinely subjected to masticatory loads that result in relatively high local levels of stress and strain. While structural morphological responses to these loads have been extensively explored, relatively little is known about material property variation in mandibular bone of nonhuman primates. Consequently, the role of regional and local variation in bone stiffness in conditioning stress and strain gradients is poorly understood. We sampled elastic modulus variation in the bone of the anterior mandibular corpus in two species (N = 3 each) of sympatric colobine monkeys, Procolobus badius and Colobus polykomos. These monkeys were chosen for comparison owing to their distinctive dietary regimens, as P. badius rarely includes hard objects in its diet while C. polykomos habitually processes obdurate items during feeding. Elastic modulus is determined through bone hardness data obtained via microindentation, which enables the description of stiffness variation on sub-millimeter scales. Labial bone stiffness exceeds that of lingual bone in the sample overall. Female mandibular bone is generally stiffer than that found in males, and overall Procolobus mandibular bone is stiffer than that in Colobus. These results, interpreted collectively, suggest that the material response to elevated masticatory stress is increased compliance of the affected bone.