Facial expressions are a critical mode of non-vocal communication for many mammals, particularly non-human primates. Although chimpanzees ( Pan troglodytes ) have an elaborate repertoire of facial signals, little is known about the facial expression (i.e. mimetic) musculature underlying these movements, especially when compared with some other catarrhines. Here we present a detailed description of the facial muscles of the chimpanzee, framed in comparative and phylogenetic contexts, through the dissection of preserved faces using a novel approach. The arrangement and appearance of muscles were noted and compared with previous studies of chimpanzees and with prosimians, cercopithecoids and humans. The results showed 23 mimetic muscles in P. troglodytes , including a thin sphincter colli muscle, reported previously only in adult prosimians, a bi-layered zygomaticus major muscle and a distinct risorius muscle. The presence of these muscles in such definition supports previous studies that describe an elaborate and highly graded facial communication system in this species that remains qualitatively different from that reported for other non-human primate species. In addition, there are minimal anatomical differences between chimpanzees and humans, contrary to conclusions from previous studies. These results amplify the importance of understanding facial musculature in primate taxa, which may hold great taxonomic value.

A comparative perspective has remained central to the study of human facial expressions since Darwin's [(1872Darwin's [( /1998. The expression of the emotions in man and animals (3rd ed.). New York: Oxford University Press] insightful observations on the presence and significance of cross-species continuities and species-unique phenomena. However, cross-species comparisons are often difficult to draw due to methodological limitations. We report the application of a common methodology, the Facial Action Coding System (FACS) to examine facial movement across two species of hominoids, namely humans and chimpanzees. FACS [Ekman & Friesen (1978). Facial action coding system. CA: Consulting Psychology Press] has been employed to identify the repertoire of human facial movements. We demonstrate that FACS can be applied to other species, but highlight that any modifications must be based on both underlying anatomy and detailed observational analysis of movements. Here we describe the ChimpFACS and use it to compare the repertoire of facial movement in chimpanzees and humans. While the underlying mimetic musculature shows minimal differences, important differences in facial morphology impact upon the identification and detection of related surface appearance changes across these two species.

Faces are one of the most salient classes of stimuli involved in social communication. Three experiments compared face-recognition abilities in chimpanzees (Pan troglodytes) and rhesus monkeys (Macaca mulatta). In the face-matching task, the chimpanzees matched identical photographs of conspecifics' faces on Trial 1, and the rhesus monkeys did the same after 4 generalization trials. In the individual-recognition task, the chimpanzees matched 2 different photographs of the same individual after 2 trials, and the rhesus monkeys generalized in fewer than 6 trials. The feature-masking task showed that the eyes were the most important cue for individual recognition. Thus, chimpanzees and rhesus monkeys are able to use facial cues to discriminate unfamiliar conspecifics. Although the rhesus monkeys required many trials to learn the tasks, this is not evidence that faces are not as important social stimuli for them as for the chimpanzees.

Five chimpanzees were tested on their ability to discriminate faces and automobiles presented in both their upright and inverted orientations. The face stimuli consisted of 30 black and white photographs, 10 each of unfamiliar chimpanzees (Pan troblodytes), brown capuchins (Cebus apella), and humans (Homo sapiens). Ten black and white photographs of automobiles were also used. The stimuli were presented in a sequential matching-to-sample (SMTS) format using a computerized joystick-testing apparatus. Subjects performed better on upright than inverted stimuli in all classes. Performance was significantly better on upright than inverted presentations of chimpanzee and human faces but not on capuchin monkey faces or automobiles. These data support previous studies in humans that suggest the inversion effect occurs for stimuli for which subjects have developed an expertise. Alternative explanations for the inversion effect based on the type of spatial frequency contained in the stimuli are also discussed. These data are the first to provide evidence for the inversion effect using several classes of face stimuli in a great ape species.

Over 125 years ago, Charles Darwin suggested that the only way to fully understand the form and function of human facial expression was to make comparisons to other species. Nevertheless, it has been only recently that facial expressions in humans and related primate species have been compared using systematic, anatomically-based techniques. Through this approach, large scale evolutionary and phylogenetic analyses of facial expressions, including their homology, can now be addressed. Here, the development of a muscular-based system for measuring facial movement in rhesus macaques (Macaca mulatta) is described based on the well-known FACS (Facial Action Coding System) and ChimpFACS. These systems describe facial movement according to the action of the underlying facial musculature, which is highly conserved across primates. The coding systems are standardized, so their use is comparable across laboratories and study populations. In the development of MaqFACS, several species differences in the facial movement repertoire of rhesus macaques were observed in comparison to chimpanzees and humans, particularly with regard to brow movements, puckering of the lips, and ear movements. These differences do not appear to be the result of constraints imposed by morphological differences in the facial structure of these three species. It is more likely that they reflect unique specializations in the communicative repertoire of each species.