An anatomical and evolutionary approach to facial expression across species
Facial expressions are ubiquitous among social primates, including humans, often appearing physically and functionally similar across species. Comparisons between species can help us understand evolutionary function. The common assumption that these behaviours are ‘expressions of emotion’, however, can bias our interpretation of meaning and lead us overlook adaptive value. The development of species-specific anatomically based coding systems (Facial Action Coding Systems: FACS) has greatly facilitated within-species and cross-species comparisons, side-stepping the focus on emotion and instead allowing detailed analysis of form and function of signals. FACS can also be used to quantify expressivity within a species and compare between species to explore potential socio-ecological drivers. Overall, our initial studies using such tools are helping to build an evolutionary framework for a better understanding of facial expression.