Bioanthropologists studying skeletal function typically infer behavior and performance from bony size and shape. This approximation has fostered a basic notion of how primate skeletons relate to their growth and lifestyles. However, estimating performance from skeletal shape becomes difficult when one examines structures that are oddly-shaped, differ in growth patterns, or vary in tissue composition. The primate mandibular symphysis is one such joint that defies simplistic modeling of its performance. Symphyseal anatomy ranges from the primitive mammalian condition of smooth, opposing right and left jaws loosely connected by fibrous tissue, fibrocartilage and some ligaments to a tightly bound joint with greater sutural complexity consisting of interlocking bony rugosities and numerous calcified ligaments to the more unique condition of an ossified joint seen in humans and various other primates. The behavioral and mechanical correlates of this remarkable variation are poorly understood. Indeed, failure to relate symphyseal form to joint integrity undermines our knowledge of the function and evolution of the feeding apparatus and behavioral reconstructions of fossil primates. The first objective is to quantify symphyseal strength during simulated loading regimes in primate and non-primate mammals. Measures of joint performance will be integrated with data on joint proportions, biomineralization and internal anatomy. The second goal is to examine the plasticity and properties of symphyseal tissues. To obtain variation in joint loads, cohorts of genetically similar rabbits will be raised on foods of different consistencies. By quantifying joint integrity within and across species, this experimental study links load-resisting properties and performance to joint shape, addresses major issues involving the symphysis, and serves as a model for future work on joint function. A broader impact of this study is its expansion of an ongoing multidisciplinary partnership between bioanthropologists and engineers. Its educational merit is that students and teachers, especially under-represented groups, will benefit via long-term apprenticeship, interdisciplinary training, and outreach. The benefit to society includes providing key basic data for future evolutionary and clinical projects on joint loading, tissue responses, and performance.
