Classifying chimpanzee facial expressions using muscle action.

The Chimpanzee Facial Action Coding System (ChimpFACS) is an objective, standardized observational tool for measuring facial movement in chimpanzees based on the well-known human Facial Action Coding System (FACS; P. Ekman & W. V. Friesen, 1978). This tool enables direct structural comparisons of facial expressions between humans and chimpanzees in terms of their common underlying musculature. Here the authors provide data on the first application of the ChimpFACS to validate existing categories of chimpanzee facial expressions using discriminant functions analyses. The ChimpFACS validated most existing expression categories (6 of 9) and, where the predicted group memberships were poor, the authors discuss potential problems with ChimpFACS and/or existing categorizations. The authors also report the prototypical movement configurations associated with these 6 expression categories. For all expressions, unique combinations of muscle movements were identified, and these are illustrated as peak intensity prototypical expression configurations. Finally, the authors suggest a potential homology between these prototypical chimpanzee expressions and human expressions based on structural similarities. These results contribute to our understanding of the evolution of emotional communication by suggesting several structural homologies between the facial expressions of chimpanzees and humans and facilitating future research.

New Developments in Understanding Emotional Facial Signals in Chimpanzees.

There has been little research over the past few decades focusing on similarities and differences in the form and function of emotional signals in nonhuman primates, or whether these communication systems are homologous with those of humans. This is, in part, due to the fact that detailed and objective measurement tools to answer such questions have not been systematically developed for nonhuman primate research. Despite this, emotion research in humans has benefited for over 30 years from an objective, anatomically based facial-measurement tool: the Facial Action Coding System. In collaboration with other researchers, we have now developed a similar system for chimpanzees (ChimpFACS) and, in the process, have made exciting new discoveries regarding chimpanzees' perception and categorization of emotional facial expressions, similarities in the facial anatomy of chimpanzees and humans, and we have identified homologous facial movements in the two species. Investigating similarities and differences in primate emotional communication systems is essential if we are to understand unique evolutionary specializations among different species.