The Evolution of Theory of Mind Abilities: How Rhesus Monkeys Reason About The Minds of Others

One of the hallmarks of human psychology is our species' ability to reason about the minds of other individuals, a capacity commonly referred to as a theory of mind (ToM). Although developmental studies have revealed much about ontogeny of our ToM abilities, to date, there is still much controversy concerning the evolution of these capacities and their emergence in non-human primates. Do other primates reason about the minds of other individuals? We present the results of several studies using non-verbal theory of mind (ToM) tasks to explore what rhesus monkeys (Macaca mulatta) know about the minds of human competitors in the absence of training. To date, studies of ToM in this species suggest that although rhesus monkeys follow the gaze of human experimenters, they seem to know little about the significance of eyes. Here, however, we present rhesus monkeys with a different kind of non-verbal ToM task, one that involves reasoning about mental states in a competitive rather than cooperative situation. These studies build on work by Hare, Call, and Tomasello using similar competitive tasks with chimpanzees. In our studies, monkeys received a single trial in which they could attempt to obtain one of two grapes, each of which were guarded by a human experimenter.  The experimenters differed only in the way they oriented towards the contested grapes.   Monkeys preferentially approached an experimenter who was facing away from the grape, whose head and eyes were facing away from the grape, and whose eyes alone were facing away from the grape. These results suggest that, like chimpanzees, rhesus monkeys prefer competing with humans who are unable to see contested foods.  We then tested whether or not subjects understood that barriers could also influence competitors’ visual access.  Monkeys selectively approached an experimenter whose line of sight was blocked by a barrier over an experimenter holding a barrier that did not block visual access.  In the final studies, we investigated whether monkeys reason about the knowledge states of human experimenters.  Taken together, these results from competitive tasks suggest that rhesus monkeys understand more about the minds of others than previously thought.   Specifically, these results suggest that at least one monkey species understands what competitors do and do not know.