APES understand what others see and believe, a skill previously believed to be unique to humans, according to new research from the University of St Andrews.

An international team of researchers, including Dr Christopher Krupenye and Dr Josep Call from St Andrews and colleagues from the Kyoto University in Japan, studied apes to see if they, like humans, possess a theory of mind – the ability to attribute mental states, like desires and beliefs, to oneself and others.

Theory of mind is critical to human social life as human beings consider others’ goals, knowledge and beliefs when we co-operate or compete, communicate or teach. Now there is evidence that apes can also project their inner experiences to understand others’ states of mind. The study, published yesterday in PNAS, challenges a long-standing hypothesis that apes can only respond to cues of others’ behaviour rather than truly understand what others can see and believe.

Krupenye, of the School of Psychology and Neuroscience at St Andrews, said: “This is an important advance in our effort to determine how richly our closest relatives can understand others’ perspectives.”

The researchers showed chimpanzees, bonobos and orangutans videos of a human being dressed as a gorilla hiding objects behind an unusual barrier from a human actor. They tracked the animals eye movements to see if they understood how others see the world when the viewpoint differs from their own.

The key finding of the study was that how the apes experienced the barrier themselves is how they expected the actor to experience it. If they experienced it as opaque, they expected the actor to behave as though he hadn’t seen through it either and if they experienced the barrier as translucent they anticipated that the actor would also be able to see through it.

The study showed apes can successfully predict that an actor will search for an object where he has last seen it (and believes it to be), even if the apes know it is no longer there (it was removed while the actor was behind the barrier).

Call said: “This study suggests that apes consult their own past perceptual experience to understand the actor’s perspective and predict his subsequent actions.”