Functional Analysis of the Skull in South American Capuchin Monkeys

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Using an in vivo analysis of bone strain during vigorous biting and chewing of tough foods from gages affixed to the interorbit, body, and symphysis of the mandible in adult black-capped capuchin monkeys, this research will directly evaluate the functional morphology of the area around the eye orbit and mandibular symphysis in higher primates as it relates to the masticatory complex. Capuchins, unlike many other primates, often use the incisors as well as molars to process tough foods. This study is designed to investigate the relative effects on symphyseal and browridge form of loading regimes during forceful biting with the incisors and during chewing in the same animal. Two hypotheses regarding cranial form and function will be tested: symphyseal fusion in higher primates is linked to stresses experienced during heavy use of the incisors, and browridge formation in neandertals and other hominoids is due to greater incisal loading. If maximal strain levels at the symphysis are significantly higher during powerful incisor biting and clenching, then it will be possible to link vertical incisors and incisal loading to the evolution of symphyseal fusion in higher primates. In assessing the relative influence of incisor loading on browridge formation, mandibular and interorbital strain levels will be compared to test for a cranial strain gradient. For a circumorbital structure to be considered a functional adaptation to dissipate masicatory stress, no difference should be observed between interorbital and mandibular strain levels. Alternatively, the presence of a strain gradient would suggest a secondary effect of a loading regime on browridge formation. In this case, genetic and epigenetic controls of circumorbital form would be viewed as relatively independent of factors affecting jaw form. This would also support the claim that bone along the upper face serves to protect the special sense organs from traumatic forces.