Monkey See, Monkey Think: Grape thefts instigate debate on primate’s mind

Rhesus monkeys may not regard the eyes as windows to the soul, but these animals do treat a competitor’s averted eyes as a license to steal his or her food, a new study suggests. Using the direction of others’ gazes to determine what they can or can’t see is a basic component of social reasoning in monkeys that, until now, has eluded researchers, contend Yale University researchers Jonathan I. Flombaum and Laurie R. Santos.

WATCH IT. In a simple form of mind reading, rhesus monkeys, such as this one, sometimes appear to account for what others can or can’t see. Santos

These results suggest that rhesus monkeys “consider others’ visual perspectives,” says Flombaum. “Without that ability, you can’t reason in more-complex ways about what others know.”

Monkeys’ assumptions about what other animals perceive, at least when they’re competing for food, represent what was perhaps an early evolutionary step toward people’s capacity to reason about what others think and want, the researchers propose in the March 8 Current Biology.

However, there’s less to the new study than meets the eye, argues a critic of much of the research into ape and monkey minds. Psychologist Daniel J. Povinelli of the University of Louisiana at Lafayette argues that Flombaum and Santos show only that monkeys avoid a competitor’s gaze, not that they make assumptions about what others see.

This debate hinges on six new experiments, each conducted with 16 to 20 monkeys. The animals belong to groups that roam the Puerto Rican island of Cayo Santiago.

In each trial, two researchers approached a lone monkey. Each person stood by a small platform on which a grape rested. From one trial to the next, the experimenters stood in different positions—turned toward, away from, or sideways to the grape. In some trials, one researcher turned his head to the side or covered his chest, eyes, or mouth with a board or piece of foam. In each trial, a grape was visible to one experimenter but not the other.

The monkeys almost always took the grape only from the person who couldn’t detect their pilfering.

Situations in which a monkey is competing for food, in this case with the experimenter, elicit a monkey’s capacity for surmising what others can see, in Flombaum’s view. An earlier study by other researchers had suggested that monkeys don’t pay attention to where others are looking, but that test employed a non-competitive setup in which a researcher hid food and then stared at the hiding spot.

Povinelli argues that the Cayo Santiago monkeys acted so as to avoid the experimenter’s direct gaze, which the animals interpreted as a threatening signal. “That doesn’t have anything to do with understanding others’ perceptions,” he says.

Such a threat played no role in one of the experiments, in which one experimenter faced the grapes and the other one faced the opposite direction, Flombaum responds. Neither experimenter faced the monkey. Still, the monkeys filched grapes from the experimenter who looked away from the food.

In further experiments, the Yale scientists plan to examine rhesus reasoning in more detail. Monkeys will watch a grape roll down a ramp to either an open platform or one with walls that block the experimenter’s, but not the monkey’s, view of the food. “We want to see if monkeys take only the hidden grapes,” Flombaum says.

Bruce Bower has written about the behavioral sciences for Science News since 1984. He writes about psychology, anthropology, archaeology and mental health issues.