Primate Faces and Facial Expressions - )

Social Research, Spring, 2000 by Signe Preuschoft

Humans express happiness in laughter, or its presumed diminutive, the smile (e.g. Ekman et al., 1972). Laughter is associated with an unmistakable vocalization, smiling is silent. In the laughter face the jaws are opened widely, in the smile not at all, or only moderately. Otherwise, they are said to look alike: the mouth-corners are retracted sideward and upward, the teeth are usually bared, the eyes squinted and crow's feet wrinkles appear at the outer corner of the eyes.

The comparative evidence, however, strongly suggests that human laughter and smiling are rooted in two independent facial expressions (the play face and the silent bared-teeth display) that have converged only later (van Hooff, 1972; Preuschoft, 1995). Furthermore, the comparison with non-human primates suggests that the human smile and the human fear expression are rooted in the same display (the silent bared-teeth display), and that their different appearance in humans may be a later differentiation. This is an example of how the categories of emotion that correspond to human facial expressions may cross-cut the categories found in other primates. Comparative evidence, however, reveals the evolutionary dynamics that underly the rise and fall of facial displays in the course of phylogeny.

Evolutionary Careers of Facial Displays

When cornered or caught by surprise, many animals, including reptiles, birds and mammals, emit loud, harsh vocalizations. These vocalizations (hissing, mobbing) are uttered with an open mouth (or beak) and consist of explosive vocal, sometimes repetitive, exhalations. These vocalizations occur in situations of self-defense or aggression. Although this is has not been unambiguously established yet, it seems likely that such vocalizations have evolved into screaming in primates. Like other mammals, primates open their jaws widely and expose their teeth while screaming. Some have speculated that this movement is a preparation for biting. They don't want to bite on their lips! In the course of evolution, this preparatory baring of the teeth seems to have acquired signal value in itself. As a result, we find the silent baring of the teeth as a full-fledged ritualized facial display in most species of Old World monkeys and apes (see Figure 5). It has also been documented for several species of New World monkeys, which allows us to date its origins back to Eocene times (over 35 million years ago), before the split between the strepsirhine and the haplorhine lineages (Preuschoft and van Hooff, 1997).

[Figure 5 ILLUSTRATION OMITTED]

The social function of this silent baring of the teeth is in many species a sign of submissiveness and appeasement (see above for Barbary macaques and rhesus monkeys), and it has therefore often been called a "fear grin." It is easy to see a continuous line from self-defense (scream) to fearful appeasement (silent teeth-baring). However, in other, often closely related species the silent bared-teeth display is the expression of a desire to interact socio-positively, and accompanies "polite" initiation of peaceful contact as well as affectionate hugging and cuddling--the very contexts, in which the human smile is observed (Preuschoft, 1995).