Primate Faces and Facial Expressions - )

Social Research, Spring, 2000 by Signe Preuschoft

Monkeys as well as apes can be trained to make same-different judgements, the facial expressions of conspecifics displayed on a computer screen, in an experiment that involves moving a joy-stick-operated cursor.

It has also been shown that chimpanzees can learn to categorize two photos as the "same" if they display the same facial expression, even if these photos show the expression on the faces of two different individuals. It also seems as though chimpanzees process facial expressions as compound displays, and not on the basis of individual facial features: discriminating between two different expressions with a common feature, e.g., an open mouth, did not seem to be more difficult for them than discriminating between two optically very dissimilar expressions (Parr et al., in press).

The pioneering, cross-cultural work on human facial expressions was done by presenting standardized photos of human facial expressions to people and having them assign emotional words or emotional stories to them (Ekman, 1971). A similarly language-based task is to ask people to perform facial expressions of certain emotions upon request (Ekman et al., 1972). However, non-verbal experiments have also been conducted by closely watching the human subjects' facial behavior while exposing them to emotion-eliciting situations, of which the experimenter (also a human!) assumed to know the quality, e.g., embarrassing or amusing, (Goldenthal et al., 1981; Fridlund, 1991), while closely watching the subjects' facial behavior. However, ethological work on the spontaneous use of facial expressions and their functions in the context of natural social interactions is still badly needed--especially in view of the strict ethical limitations of inducing strong emotions in human subjects.

Repertoires

Detailed repertoires of primate displays can be found in van Hooff (1962, 1967, 1973), Chevalier-Skolnikoff (1973), Redican (1975), and Preuschoft and van Hooff (1995, 1997). I will here pursue a different, Homo-centric, approach by comparing the facial expressions of our own species with those of apes and monkeys. It is important to note that in the order of primates there is no progression from few to many facial expressions. Some monkey species have much richer repertoires of facial displays than gibbons, for instance, who belong to the apes. Two lifestyle factors predict the extent to which primates emphasize facial communication: visibility and social complexity (Preuschoft and Preuschoft, 1994; Preuschoft and van Hooff, 1997).

The literature on human facial expressions distinguishes between at least five major categories of facial displays, which are closely linked to what human psychologists call "basic emotions." The categories of human basic emotions--anger, disgust, sadness, fear, and happiness (cf. Ekman, 1998)--seem sometimes to run obliquely to the categories of social functions we can discern in non-human animals. In seeking corresponding non-human primate displays, I therefore give priority to the morphological description of the display (cf. Darwin 1872; Ekman, 1971), and add contextually related, but differently looking displays afterwards.