Inferring the intentions and emotions of others from facial expressions is an integral part of social interaction in anthropoid primates (monkeys, apes and humans). However, the adaptive factors responsible for the elaboration of facial expression are unknown. This is due to a lack of quantitative comparative data, which is necessary for the identification of shared adaptive trends.
A comparative variable of direct relevance to facial expression is facial mobility, or the number of different facial movements an individual is capable of performing. The goal of this study is to develop a new observational method for quantifying facial mobility in nonhuman anthropoids based on the Facial Action Coding System (FACS). FACS is the main method used in psychology today to describe the constituent elements of human facial displays with regard to their muscular basis. The method differentiates "action units" that correspond to the activity of one or more muscles of facial expression. Standardized criteria are used to score the presence of action units from video recordings of facial movements. Since anthropoid facial muscles derive from the same developmental precursors and humans exhibit the greatest differentiation of this muscle group, FACS represents the entire range of possible movements for anthropoids. Thus, facial mobility may be quantified for an individual as the number of different action units exhibited. FACS will be used to estimate facial mobility for up to 75 individuals representing 15 species of anthropoids from zoos in North America. Focal animal samples will be attained using a digital video camcorder. Individuals will be recorded for 10 minutes per sample and action units will be scored from slow-motion and stop-frame playback. Focal sampling will continue for each individual until the number of previously unobserved action units reaches an asymptote of zero. The resulting dataset will facilitate the testing of adaptive hypotheses for the evolution of facial expression, opening new avenues of research into anthropoid behavior and communication.
The broader impacts of this study include the creation of an online database of video and still-frame examples of anthropoid facial behaviors. This resource will help educate the public about the links between human and nonhuman primate emotional expression, firmly placing this important aspect of human social behavior within the context of evolution.
