Function and Evolution of Jaw-Muscle Fiber Type in Primates

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Studying muscle can help us to understand the diets of early humans and other extinct primates. Jaw-muscle fiber types play an important role in fine-tuning the jaw muscles for specific motor tasks such as chewing and powerful biting. This research will study jaw-muscle fiber types across a range of primate species and use those data to test three hypotheses about the evolution of jaw muscle patterns in primates. Results of this work will provide new insights into the relationship between muscle and diet by i) increasing knowledge about the behavioral and ecological factors that shape feeding-system anatomy; ii) advancing our understanding of how jaw-muscle fiber types and whole muscle anatomy work together to generate the jaw movements and jaw forces necessary during feeding and aggressive biting; and (iii) refining models of chewing biomechanics in living and fossil primates. A central component of this work includes the training and mentoring of undergraduate and graduate students and postdoctoral scholars, including women and underrepresented minorities, in state-of-the-art methods for muscle fiber typing. In addition, results will be incorporated in community outreach programs aimed at exposing primary school girls to science. Lastly, this project will generate a unique collection of digital images and slides of muscle tissue that will be made available to the scientific community for teaching and research.The jaw-closing muscles are the motors of the masticatory system. Collectively, these muscles are responsible for generating the jaw movements and forces associated with feeding behaviors as well as non-feeding oral behaviors such as aggressive biting and wide-mouth opening for canine threat display. While fiber architecture is an important determinant of whole muscle function, its functional and adaptive significance cannot be fully appreciated in the absence of information on fiber type. This lack of data on primate fiber phenotype limits 1) knowledge about variation in jaw-muscle fiber phenotype; 2) the functional significance of this variation for behavior and performance; and 3) the specificity with which we can model and test hypotheses of feeding-system function and performance in both living and fossil species. To provide a more complete framework for linking measures of primate masticatory anatomy and physiology with function and performance, the investigators will collect and analyze novel data on jaw-muscle fiber type composition and distribution in select, closely related species of strepsirrhine and anthropoid primates that diverge in a key feeding or biting behavior. The data collected will be used to: (i) quantify the myosin heavy chain (MHC) fiber type composition and distribution in primate jaw-closing muscles using state-of-the-art immunohistochemistry; (ii) test three key functional and adaptive hypotheses that have been advanced to explain the evolution of fiber phenotype in mammalian jaw muscles: the frequent recruitment hypothesis, the aggressive bite hypothesis, and the high occlusal force hypothesis; and (iii) assess the correspondence among fiber phenotype, fiber architecture and leverage with the goal of refining models of masticatory biomechanics. This project will provide novel data on the contractile proteins found in primate jaw-closing muscles, advance knowledge of the ecological and behavioral factors that drove changes in the primate jaw-muscle fiber phenotype, and yield new insights into how key components of feeding-system morphology such as fiber architecture, fiber phenotype and leverage combine to facilitate or constrain feeding behavior in primates, including species that have been advanced as models for interpreting feeding behavior and diet in early human ancestors.