Abstract

Primates communicate in several ways by using multi-modal signals. Recently, researchers
have started to approach primate facial expression in an anatomical, systematic and standardized
way, to try to understand the complexity of their visual communication. In this study,
the widely used tool for analysing human facial expressions, FACS (Facial Action Coding System),
was adapted for orangutans ( Pongo sp.) following the same muscle-based methodology.
Orangutans are an intriguing ape species with conspicuous sexual dimorphism and developmental
changes in facial morphology, possibly attributed to visual communicative signals. The
facial muscular plan of orangutans was considered by reviewing dissections from the available
literature. Individual facial movements were classified according to the visual appearance
changes and the corresponding underlying musculature. A diverse repertoire of detailed facial movements was identified by analysing 64 h of spontaneous orangutan behaviour on video.
Seventeen Action Units (independent muscular movements and the respective changes in appearance)
and seven Action Descriptors (by non-mimetic muscles) were identified and described
in detail in the orangutan face. There is great similarity in the facial musculature of
primates, which demonstrates biological continuity across the order Primates. However, when
comparing orangutans’ Action Units with thse of other primates (namely humans, chimpanzees,
gibbons and rhesus macaques), we found some interesting differences implying that there
is a specialization in visual signals for each species. OrangFACS will facilitate future studies on
orangutan social communication as well as systematic studies on facial expressions between
primate species, thus providing a valuable insight into the evolution and function of communication
and emotion.