GibbonFACS: a muscle-based facial movement coding system for hylobatids

The evolution and function of human and nonhuman primate facial expression can be better understood through multispecies comparative analyses. Anatomically based coding systems (Facial Action Coding Systems [FACS]) are developed to enable such analyses because they are standardized and systematic and aid identification of homologous expressions underpinned by similar muscle contractions. To date, FACS has been developed for humans, and subsequently modified for chimpanzees and rhesus macaques. Through detailed examination of facial anatomy and spontaneous movement, here we present a FACS for hylobatids (gibbons and siamangs): apes living in small groups and often thought not to display extensive facial movement. We identified and described 18 independent facial movements (Action Units and Action Descriptors) in the hylobatid face, and compared to humans, chimpanzees, and rhesus macaques. The findings show that the facial movement capacity of hylobatids bears great similarity to species living in much larger groups. The findings could be explained by phylogenetic inertia (especially because pair-bonding is likely to have emerged from social living) and/or the social demands of pair-bonding.