Homologizing primate facial displays: a critical review of methods.

The applicability of previously established criteria for identifying homologous behavioural traits to the ritualized facial displays of primates is discussed and illustrated with several examples. (1) Homologizing of facial displays is based on (a) their stereotypy, (b) their nature as compounds of individual facial actions, and--to a lesser extent--(c) the homology of the muscle substrate by which the displays are accomplished. (2) Sequences of gradually changing displays ('Merkmalsreihen') can be constructed by considering related extant species, different ontogenetic stages, and variation and blends of the underlying motivations. (3) The pattern of distribution of the display in the primate phylogenetic tree reflects the direction of changes ('Lesrichtung'), and thus yields insights into the phylogeny of the displays. (4) The behavioural context of a display reflects its motivational embedding and is inseparable from its function. Therefore the behavioural context does not differentiate between similarities due to common descent and similarities due to identical function.