New Developments in Understanding Emotional Facial Signals in Chimpanzees

Abstract

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.

Fig. 1

Prototypical chimpanzee facial expressions and their probability of correct category assignment as identified through discriminant functions analyses and using ChimpFACS coding. (Photographs courtesy L.A. Parr and the Living Links Center, Emory University.)

Fig. 2

An illustration of the matching-to-sample (MTS) facial expression discrimination task. The subject is first presented with a sample stimulus, a facial expression (in this case, a bared-teeth display) and a cross-shaped cursor on the computer screen over a black background (Fig 2a). The sample stimulus is the image to match, and the subject first must orient toward it by touching it with the joystick-controlled cursor. After this, the sample clears the screen and the subject is presented with two alternative stimuli (Fig. 2b)—one matches the sample by showing the same category of expression made by a different individual (left), while the other shows a different expression (relaxed-lip face, right side). The correct choice is to select the stimulus that looks most like the sample.

Fig. 3

Proposed facial expression homologues in chimpanzees and humans. From left to right, the chimpanzee expressions show the bared-teeth display, pant-hoot, play face, scream face, and bulging-lip face. Corresponding human expressions are shown in the top row, along with the action units (AUs) shared by the expressions in both species. (Human photos from Ekman, Friesen, & Hager, 2002; chimpanzee photos courtesy L.A. Parr and the Living Links Center, Emory University.)