The discrimination of faces and their emotional content by chimpanzees (Pan troglodytes).

The ability to recognize and discriminate conspecific faces and facial expressions has played a critical role in the evolution of social communication. Darwin was one of the first to speculate that human and nonhuman primate facial expressions share similar mechanisms for production and functions in expressing emotion. Since his seminal publication, numerous studies have attempted to unravel the meaning of animal signals, with the most success coming from the field of vocal communication, where researchers have identified the referential and emotional nature of specific vocalizations. Studies specifically addressing nonverbal facial displays, however, have faced numerous methodological challenges, including how to objectively describe facial movements and how to study the perception and production of these signals within a social context. In this paper, I will review my studies on chimpanzee face recognition, their ability to categorize facial expressions, and the extent to which chimpanzee facial expressions may convey information about emotion. Finally, recent studies from my lab have begun to address the role of auditory and visual cues in facial expression categorization. Chimpanzees were given the task of matching expressions according to which sensory modality was more salient, the visual or auditory component. For some expressions the visual modality was preferred, while for others the auditory modality was preferred. These data suggest that different social and ecological pressures may shift attention towards one sensory modality over another, such as during long-distance communication or emotional conflict.