HOMOLOGIZING PRIMATE FACIAL DISPLAYS - A CRITICAL-REVIEW OF METHODS

The applicability of previously established criteria for identifying h omologous behavioural traits to the ritualized facial displays of prim ates is discussed and illustrated with several examples. (1) Homologiz ing of facial displays is based on (a) their stereotypy, (b) their nat ure as compounds of individual facial actions, and - to a lesser exten t - (c) the homology of the muscle substrate by which the displays are accomplished. (2) Sequences of gradually changing displays ('Merkmals reihen') can be constructed by considering related extant species, dif ferent ontogenetic stages, and variation and blends of the underlying motivations. (3) The pattern of distribution of the display in the pri mate phylogenetic tree reflects the direction of changes ('Lesrichtung '), and thus yields insights into the phylogeny of the displays. (4) T he behavioural context of a display reflects its motivational embeddin g and is inseparable from its function. Therefore the behavioural cont ext does not differentiate between similarities due to common descent and similarities due to identical function.