Primate Faces and Facial Expressions - )

Social Research, Spring, 2000 by Signe Preuschoft

SIMILARITIES between humans and non-human primates have intrigued people since ancient times. In fact, morphological, behavioral, and genetic evidence confirms that we are indeed close relatives. The morphological evidence includes similarities in the arrangement and position of both the solid and the soft body parts--bones and tissue. The behavioral evidence includes similarities in innate action patterns such as body movements and communication signals--as well as highly flexible behavioral tactics and clever problem-solving strategies.

This spontaneously felt familiarity inspires the imagination of people, and in the countries where primates originate, folklore often murmurs of mystical bonds, a sage decision for non-involvement, breaking of taboos, but also of sudden rescue, provided by the not-quite humans who are: often endowed with super-human strength and perceptibility. At the same time it is impossible to overlook the differences between humans and other primates, which, in combination with the familiarity inspired by likeness, leads to an uncomfortable feeling of grotesqueness and distortion. Human attitudes towards non-human primates wax and wane with cultural and economic conditions, just as does our tolerance for our fellow humans fluctuates, inside and outside our own cultural groups.

At the chimpanzee enclosure in a zoo one often encounters hooting crowds that comment with fascination on how the ape picks her nose (in public), how the males go about shamelessly exhibiting their desire to impress and intimidate. We observe with dismay the sexual swellings of the female chimpanzees and shamefully drag our children away from the place of unconcealed sexual activity. Yet we cheer the exhibition of maternal love, and our children are breathlessly fascinated by the antics of the romping youngsters, and all along we notice their hunched postures, quadrupedal knuckle-walk, untidy, rough fur, dirtiness and lack of manners. But then one of the adult chimpanzees comes over to the glass, and with gracious dignity sits down to inspect the visitors. The visitors gaze back into an alien face in which the teeth are too big, the prognate jaws only slightly reminiscent of a snout, the nostrils are open and the nose is flat. The ape's steady, self-confident look seems to ask, "Why do you think we are different? Look at you, who are you?"

It is the art of the primate ethologist to penetrate through the grotesqueness and locate the fellow being, to strip off the disguise of superficial difference while at the same time recognizing the true, profound and unbridgeable otherness in respect and appreciation.

As a professional observer I have often felt that the face, the eyes, of primates are truly the mirror of their "souls." Different species of primates vary enormously with respect to how much eye contact they tolerate. With habituated primates, however, there invariably comes a point where, looking into their faces, we believe we see them thinking. Primates betray their internal states on their faces to trigger certain responses in the receiver. To read these messages the human observer needs to get accustomed to the codes: What is the "vocabulary" of non-human primate facial expressions? What are their meanings? Primates also display their internal states on their faces to affect the observer or receiver of their expressions in a certain way. To read these messages well the human observer first of all needs to get accustomed to the "screen" on which this drama is displayed: the primate face itself.

To this end, I will introduce some representative primate faces, and briefly discuss the evolutionary dynamics that have shaped them. I will then turn to facial expressions and explain how humans can learn to understand the meaning of the facial behavior of other primates. Are facial displays expressions of emotion, or do they constitute an appeal to the receiver? After this I will discuss the facial displays of the Old World monkeys and apes in relation to those of humans. Using positive emotions as an example, I will illustrate how insights into the evolutionary dynamics of non-human primate facial displays can challenge and improve our understanding of Homo sapiens itself.

I. Primate Faces

The face is comprised of three components: the bone structure of the skull, the facial musculature, and the skin.

The human face has hairy eyebrows arching over the eyes, and a bald, high forehead. The eyes have visible, white eyeballs set inside lidded sockets. The upper eyelid is longer than the lower one and, when blinking, it is the upper eyelid that closes down to meet the lower one. The lids are rimmed with elongated, usually dark hair, the eyelashes. In the center of the eyeball is the iris, which may vary in color. A round pupil dots the middle of the iris. Between the eyes is the bony bridge of the nose. The nose itself often protrudes forward and ends in a cartiligeous nose-tip, and downward facing nostrils. Human mouths are outlined with lips, which contrast with the paler surrounding skin. Humans have cheeks and a chin, and bald ears which are positioned on the sides of the head, below the temples. Humans do not move their ears, but otherwise their faces are moveable and facial movements are accentuated by the hairy brows and eyelashes, as well as by lips in contrasting colors. In humans, there is a sexual dimorphism with regard to facial hairiness. Female faces are hairless. Males grow hair around the mouth and on the cheeks and throat.