Working with semi-free-ranging rhesus monkeys on the island of Cayo Santiago in Puerto Rico, Flombaum and Santos set up a food competition game: Lone monkeys were approached by two human "competitors." Each competitor had a grape affixed to a platform by his feet. In each experiment, one of the competitors could see the monkey in front of them, but the other could not. For example, in Experiment 1, one of the competitors stood with his back to the monkey subject, while the other stood facing the subject. Monkeys in this experiment spontaneously chose to approach and steal a grape from only the competitor with his back toward the monkey. In five more experiments, the monkeys revealed similar preferences for an experimenter who could not see them, rather than one who could. Most notably, they reliably stole food from a competitor with only his eyes averted, rather than one facing perfectly forward, as well as an experimenter with a piece of cardboard over his eyes rather than one with cardboard over his mouth. Together, these results reveal not only that rhesus monkeys prefer to steal food from a competitor who cannot see them, but also that they know exactly how blocking or averting one's eyes can render one unable to see. Thus, even without any training, these monkeys were able to accurately consider the visual perspective of others when deciding from whom to steal.
In previous studies, rhesus monkeys (and other primates) were thought to do no more than merely follow the gaze of others. Primates have typically failed in other, noncompetitive experiments that require surmising what other individuals know or see from where they are looking. In one famous case, for example, rhesus monkeys were unable to find food under a hidden location when the human experimenter wh