The Perception of Facial Expressions By Chimpanzees, Pan Troglodytes

The production of facial expressions is important for social communication, and has been described for many primate species. The perception of facial expressions, however, has received considerably less attention. This study reports the results of two experiments that investigated the ability of chimpanzees to discriminate five of their species-typical facial expressions: bared-teeth display, hoot face, relaxed-lip face, relaxed open mouth face, and scream face. Unfamiliar con-specifics' facial expressions were presented as black and white, digitized images using a computerized joystick-testing apparatus and matching-to-sample paradigm. Subjects were first presented with a sample facial expression, and were then required to choose between two comparison, one of which showed the same expression as the sample. The first task demonstrated that all expressions except the relaxed-lip face were discriminated from a neutral face significantly above chance, several on the first day. The second task showed impaired discrimination when the sample and nonmatching comparison expressions shared similar features, supporting the hypothesis that facial expressions were categorized using distinctive features. The overall correlation between the number of shared features and performance was only weakly negative, suggesting that additional discrimination strategies may be involved.