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 o hid the food preferentially looked at the hidden location. These results suggest that competition-like situations may bring out the primates' abilities more than experiments that don't involve competition.
These latest results, however, suggest that rhesus monkeys can do much more than just follow the gaze of others; they can also deduce what others see and know, based only on their perception of where others are looking. These data potentially push back the time during which our own abilities to "read the minds of others" must have evolved. Moreover, they suggest strongly a reason why these abilities may have evolved in the first place, namely for competitive interactions with others. Finally, these results lay the groundwork for investigating the neural basis for this kind of social reasoning in a readily available laboratory animal ?an urgent endeavor for developing a better neural understanding of diseases such as autism, in which this kind of social reasoning appears impaired.
Jonathan I. Flombaum and Laurie R. Santos: "Rhesus Monkeys Attribute Perceptions to Others"
The other members of the research team include Jonathan I. Flombaum and Laurie R. Santos from Yale University. This research was supported by a National Science Foundation graduate research fellowship to J.I.F. and the Yale University Moore Fund grant to L.R.S. The Cayo Santiago Field Station was supported by the National Institutes of Health (National Center for Research Resources grant CM-5-P40RR003640-13 award to the Caribbean Primate Research Center) and the University of Puerto Rico Medical Sciences Campus.
Publishing in Current Biology, Volume 15, Number 5, March 8, 2005, pages 447?52. http://www.current-biology.com