Memo to zoo visitors making faces at the chimps and gorillas on the other side of the glass: they know what you're thinking. Or, more precisely, feeling.

The extent and limits of ape intelligence is a hot area in science, but most of the research has focused on cognition. Now a team of scientists has turned the spotlight on emotions, and how well apes can read the human kind as displayed in our facial expressions. Earlier studies had shown that apes understand people's goals and perceptions. But whether apes understand our emotional expressions was pretty much a mystery, even though there are striking similarities between the facial expressions that we and our more hirsute cousins make, as researchers as far back as Darwin noted. Both human babies and newborn chimps make a pouting face to get mom’s attention, for instance, and bare their teeth in something like a smile in order to make nice—or "achieve social bonding," as primatologists put it.

A cool paper in the September issue of the journal Developmental Science describes studies on 17 chimps, five bonobos, five gorillas and five orangutans from the Wolfgang Köhler Primate Research Center in Leipzig, Germany. In the first test, a researcher sat at a table on one side of a plexiglass panel while an ape sat on the other side. Two opaque boxes rested on the table. The scientist opened one box (making sure the ape could not see inside) and smiled with pleasure. He next opened the other and made a disgusted face. The ape was then allowed to reach through one of the holes in the panel and pick one box. Which would he choose?

In 57 percent of the tests, the ape chose the box that elicited a smile from the scientist rather than an expression of disgust. Good choice. The box that brought the smile contained a grape, and the ape was rewarded for his perspicacity in reading human facial expressions. The other box contained dead cockroaches. The apes' skill at reading an expression of happiness, write David Buttelmann of the Max Planck Institute for Evolutionary Anthropology in Leipzig and his colleagues, indicates that they can read meaning in the emotional expressions on human faces, suggesting that despite 6 million years of separate evolution apes and humans share a common emotional language. (It's always interesting to compare apes to babies: human infants can read facial expressions and act on them at around 14 to 18 months.)

In the next experiments, the set-up with the plexiglass was the same. An ape saw the scientist hold up a grape and a slice of banana, but his view was then blocked as the scientist put one treat under one cup and the other under the other cup. The ape then watched as the scientist looked under each of the two cups in turn, making an expression of happiness at one and of disgust at the other. The scientist next reached under one cup (at this point, the ape's view was again blocked, so he could not see which cup the scientist chose) and ate what was inside. His view restored, the ape saw the scientist munching on something, and then was allowed to choose a cup for himself.

This time the apes tended to choose the cup that had triggered the expression of disgust. Counterintuitive? Not at all. The apes went beyond the simplistic "pick cup that elicited happy face" to make a fairly sophisticated computation. That is, they seemed to reason that the human would eat the food that made him smile, emptying that cup, with the result that only the disgust-inducing cup would still contain a snack.

That was even stronger evidence than in the first test that the apes understood the meaning of the human's facial expression, and were not simplistically equating "disgust" with "stay away from this cup." Their skill at inferring how people will act on their emotions, conclude the scientists, suggests that they "understand facial expressions as expressing internal states that cause certain [human] actions—in this case, eating one food but not another." The apes had to understand that expressions of disgust or happiness reflect an internal state that people will then act on, and infer that the person was more likely to eat the food that made him look happy. Species did not matter—chimps, gorillas, and the others all did about the same—but age did: the older the animal, the better he did. Call it the wisdom of old age.

In my recent story describing the rising criticism of evolutionary psychology and the idea of human universals, I noted that emotions and emotional expression do seem to be universals (unlike, say, rape). If so, that suggests that emotional expressions have deep evolutionary roots, perhaps reaching as far back as the common ancestors of humans and modern apes. That possibility looks even more likely as evidence accumulates that apes can read the emotions on our faces.

© 2009