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

Abstract

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

Development and application of CatFACS: Are human cat adopters influenced by cat facial expressions?

“…This system has been widely used for humans (Cohn et al, 2007) and has been successfully adapted and applied with other species (e.g. Waller et al, 2012;Caeiro et al, 2013;. FACS bases its coding of facial movements on the muscular activity instead of the traditional emotion labelling system, by assigning independent codes (Action Units) to each facial muscle contraction (Ekman et al, 2002;Cohn et al, 2007).…”

Facial expression: An under-utilised tool for the assessment of welfare in mammals

“…Eyebrow raising through activation of the medial portion of the frontalis muscle is associated with the negative states of surprise and fear in humans (Waller et al, 2008b). Primates, horses, and dogs also have the capacity for a similar expression Parr et al, 2010;Caeiro et al, 2012;Gleerup et al, 2015a). There is some evidence that brow raising is activated by pain states in horses (Gleerup et al, 2015a), although this action is caused by activation of the levator anguli occuli medialis muscle in this species (Wathan et al, 2015).…”

Social variables exert selective pressures in the evolution and form of primate mimetic musculature

“…Although social relationships may be more fluid than in Old World monkeys, social interactions in apes are typified by complex facial display repertoires (e.g. Ekman & Friesen, 1978;Goodall, 1986;Ekman et al 2002;Vick et al 2007;Waller et al 2012;Caeiro et al 2013).…”

“…), orangutans (OrangFACS: Caeiro et al. ), domestic dogs (DogFACS: Waller et al. ), domestic cats (CatFACS: Caiero et al.…”

“…Despite the large cheek flanges that some mature males form and the relatively low frequencies of social interactions with multiple individuals, orangutans have been documented to produce about the same number of facial displays as chimpanzees, but fewer facial displays than humans (Caeiro et al. ).…”