Adductor Muscle Function and Fiber Type in Macaca and Papio

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Understanding the ways in which primates use their jaws during chewing is of great importance to physical anthropology because chewing has a major influence on the form of the skull. Knowledge of the influences on skull form helps anthropologists interpret the evolutionary history and behavior of extinct primates and understand the relationship between anatomy and function in living primates. The goal of this study is to determine whether, for the chewing muscles, variation in fiber type is linked to muscle activity during chewing. This research will (1) document the proportion and cross-sectional area of the Type I (slow) and Type II (fast) fibers in the chewing muscles of adult male and female macaques and baboons using histochemistry and electrophoresis, (2) determine whether sexual dimorphism in fiber type is present, and (3) determine how the proportions of fiber types are correlated with electromyographic activity during chewing in these animals. The hypothesis that the proportion of fast fibers is directly related to the rapid production of large bite forces during chewing will be tested. This research is important because little is known about the relation between muscle fiber type and activity during chewing, and it will provide new information about structure-function links in the skull. Furthermore, the data will resolve the controversy over whether there is sexual dimorphism in the fiber type of the chewing muscles. If sexual dimorphism is present, the data will provide a baseline for understanding the functional implications of sexual dimorphism.