Facial expressions are routinely used in social interactions among anthropoids (monkeys, apes, and humans). These displays provide clues to underlying intentions and emotions that help individuals predict the immediate future behavior of other group members. Facial signal complexity and intensity are both a function of the mobility of the muscles of facial expression. Decades of qualitative observations suggest that there is significant interspecific variation in facial mobility, however the evolutionary factors responsible for the variation are unknown. This is largely due to the lack of quantitative comparative data necessary for examining patterns of correlated evolution in a phylogenetic context.

The purpose of my dissertation is to develop an observational method for quantifying facial mobility in nonhuman anthropoids using the Facial Action Coding System (FACS). FACS is the main method used in psychology to study human facial movement. The method differentiates a series of action units (AUs) that correspond to visible changes in the face resulting from individual muscle contractions. Since anthropoid facial muscles are homologous and humans exhibit the greatest differentiation of the muscles of facial expression, FACS represents the entire range of possible movements for nonhuman anthropoids. The facial action repertoire size of a species is an estimate of facial mobility suitable for comparative analysis. These data will help bring facial mobility into the realm of comparative primatology. This research is support by the National Science Foundation and Sigma Xi.