March 23, 2005 — -- Rhesus monkeys on an island just off the coast of Puerto Rico have figured out a fundamental characteristic of social behavior:
If you're going to steal another guy's grape, make sure he isn't looking.
Psychologists from Yale University have been studying the monkeys on the island of Cayo Santiago for several years now to see if they share a human ability to analyze the thought processes of a human competitor, who also wants the grape, simply by studying the eyes. They have conducted a series of six experiments during which the monkeys could swipe a grape either from someone who was looking the other way, or from someone who was clearly looking right at the coveted grape.
Almost invariably the monkeys stole the grape from the human whose eyes were either blocked or averted.
Laurie R. Santos, an assistant professor of psychology who specializes in behavioral neuroscience, and Jonathan I. Flombaum, a doctoral candidate in cognitive psychology, tested 115 monkeys on the island to see if they assess a competitor's mental state by "attending to the eyes." In an article in the March 8 issue of Current Biology they say they found the first solid evidence that the monkeys do exactly that.
"Until now, we didn't know that rhesus monkeys could do these things, and in fact there are many scientists who said they couldn't," says Flombaum.
He adds that humans nearly always do that, unless they are suffering from some sort of mental impairment, and that's why the monkeys are of such interest. Young children with autism, for example, are unable to "attend to the eyes" of another human, as he puts it, and the rhesus monkey may guide researchers toward a treatment for autism as well as other disabilities.
By studying the cellular structure of the monkey's brain, for example, scientists might now be able to determine exactly how the monkey uses another's eyes to determine the mental state, and thus why a child with autism cannot do the same.
The research was conducted on a gorgeous island where rhesus monkeys roam about freely with only an occasional interaction with humans. The island is a breeding colony for rhesus monkeys, used widely in research projects around the world.
"Some of the baby monkeys are culled out every year and shipped off to various labs," Flombaum says. "But the ones that aren't culled are unharmed for the rest of their lives. They just kind of hang out on the island."
They are fed "monkey chow," which is about as appetizing as it sounds, so when the researchers showed up with fresh grapes, word probably shot out over the monkey grapevine to the 1,000 monkeys that call Cayo Santiago home.
The researchers, consisting of two human "competitors" and two camera operators, looked for monkeys that were somewhat isolated from other monkeys. The rhesus is a very sociable creature, at least among its own kind, so isolation was important to minimize interference from other monkeys.
The monkeys aren't real fond of humans, but they have grown accustomed to having people around, so all it took was one glimpse of a delicious grape to win their attention.
Six experiments were carried out with each monkey. Each human participant had one grape, which was placed on a platform midway between the human and the monkey. Sometimes one human faced away from the grape, and sometimes the eyes of one human were hidden by a sheet of cardboard, and sometimes both humans faced the grapes head on, but one had his or her eyes averted.
The idea was to determine if the monkeys studied the eyes of the human competitors closely enough to know whether the humans were really paying attention to what was going on, or were distracted enough that it might be possible to slip in there and grab that grape.
Each set of experiments involved around 20 monkeys, and all but three or four of them swiped the grape from the person who, based on the eyes, wasn't watching the grape. The researchers say the fact that even the person whose eyes were averted lost the grape is particularly significant because that shows it wasn't human intimidation but human inattention that determined the monkey's behavior. In other words, the monkey knew just what the human was thinking because the eyes were diverted upward and not locked on the grape.
That level of sophistication is probably surprising, even to monkey lovers, but Flombaum says the monkeys probably learned how to analyze a competitor's eyes because it's necessary for survival.
"When you live in a pretty large group of intelligent animals, especially where there's a lot of social behavior, it's a crucial ability," he says.
So is it something they've learned, or has evolution given them an ability that is so much like ours?
"I think it's a little of both," Flombaum says. "Evolution has equipped these animals to be able to quickly figure out how they need to behave, and how to figure out what others are thinking."
And evolution has probably "pre-wired" the monkeys' brains to "tell them that the way you know what others are thinking is to look at their eyes."
Humans do that, even on casual meetings with old friends, he says. A distracted gaze suggests the other person really isn't all that interested. Intense eye contact suggests genuine interest, and an inability to "attend to the eyes of others" may suggest some sort of mental impairment.
As the old saying goes, the eyes are the window into the soul, and as modern science adds, into the brain as well.
But another old saying might have to be revised. Now it will probably have to be something like this:
Hear no evil. Speak no evil. See if anyone's watching the grapes.
Lee Dye's column appears weekly on ABCNEWS.com. A former science writer for the Los Angeles Times, he now lives in Juneau, Alaska.