Facial expression production and perception in non-human primates

Facial expressions are ubiquitous among primates and many clear similarities across species suggest a shared evolutionary history for these behaviours. Investigations of non-human primate facial expressions are therefore vital to enable us to understand the form and function of human facial behaviour. Methodological developments over the past two decades enable fine-grained quantitative comparisons across species and have demonstrated that similar processes underpin facial expressions in non-human and human primates. However, there is a tendency to focus on facial expression as a repertoire of discrete, prototypical expressions that does not always reflect naturalistic behaviour. In this Review, we review the production and perception of non-human primate facial expressions, focusing on methodological approaches and how to address the challenges of studying dynamic facial expressions. Moving forwards, methods that enable the study of dynamic and variable communicative exchange within social interaction might provide a better understanding of how facial expressions function and what (if anything) about facial expression is unique to humans.