OrangFACS: A Muscle-Based Facial Movement Coding System for Orangutans (Pongo spp.)

Comparing homologous expressions between species can shed light on the phylogenetic and functional changes that have taken place during evolution. To assess homology across species we must approach primate facial expressions in an anatomical, systematic, and standardized way. The Facial Action Coding System (FACS), a widely used muscle-based tool for analyzing human facial expressions, has recently been adapted for chimpanzees (Pan troglodytes: ChimpFACS), rhesus macaques (Macaca mulatta: MaqFACS), and gibbons (GibbonFACS). Here, we present OrangFACS, a FACS adapted for orangutans (Pongo spp.). Orangutans are the most arboreal and the least social great ape, so their visual communication has been assumed to be less important than vocal communication and is little studied. We scrutinized the facial anatomy of orangutans and coded videos of spontaneous orangutan behavior to identify independent movements: Action Units (AUs) and Action Descriptors (ADs). We then compared these facial movements with movements of homologous muscles in humans, chimpanzees, macaques, and gibbons. We also noted differences related to sexual dimorphism and developmental stages in orangutan facial morphology. Our results show 17 AUs and 7 ADs in orangutans, indicating an overall facial mobility similar to that found in chimpanzees, macaques, and gibbons but smaller than that found in humans. This facial movement capacity in orangutans may be the result of several, nonmutually exclusive explanations, including the need for facial communication in specialized contexts, phylogenetic inertia, and allometric effects.