Measuring the evolution of facial 'expression' using multi-species FACS.

• Facial Action Coding Systems (FACS) are useful tools for facial behaviour measurement. • FACS is objective, anatomically based and systematic. • FACS have been created to facilitate cross-species comparison. • Homology is indicated by stereotypy, physical and anatomical similarity, and presence across multiple species. • FACS can help determine homology of facial behaviour across species. Darwin observed that form, and in his view, meaning, of facial behaviour (observable changes in the appearance of the face, often termed facial 'expression') is similar between a wide range of species and concluded that this must be due to a shared ancestral origin. Yet, as with all social behaviours, exactly how to define similarity and determine homology is debated. Facial behaviour is linked to specific facial muscle movements, so one important factor in determining homology is the anatomical basis of facial behaviours that appear similar in both appearance and social function. The Facial Action Coding System (FACS) was developed for the scientific measurement of human facial behaviour and is based on individual facial muscle movements (Ekman and Friesen, 1978). FACS has since been modified for use with various non-human primate species (chimpanzees, macaques, hylobatids, orangutans) and domestic species (dogs, cats, horses). These FACS can be used to trace continuity of form in facial behaviour across species and build a better understanding of the evolution of facial communication in mammals. [ABSTRACT FROM AUTHOR]