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.