Decoding function in primate facial expressions: re-evaluating broader contexts and terminology

In this paper, we reflected on the role of pioneering studies in shaping research on nonhuman primate facial communication, as well as on how their terminology for describing certain expression types may have imposed limitations over time and, in some cases, misdirected research in the field. We then examined the extent to which more recent nonhuman primate studies have applied novel methods to reveal the complexity inherent in their facial morphology, while studies of their functions have not kept pace. Drawing on the current state of research on nonhuman primate facial communication, we concluded that important opportunities exist to investigate the likely complexity of these functions, representing a fundamental step toward reconstructing the evolution of nonverbal communication in humans and their closest relatives.