Apes are capable of understanding what others are thinking – an ability once thought to be unique to humans. In a set of experiments, scientists have shown chimpanzees, orangutans and bonobos can all recognise a human's beliefs, despite knowing better themselves.

"Humans operate with a 'theory of mind' with which they are able to understand that others' actions are driven not by reality but by beliefs about reality, even when those beliefs are false," researchers wrote in the journal Science. "Although great apes share with humans many social-cognitive skills, they have repeatedly failed experimental tests of such false-belief understanding."

To find out if apes can understand others' beliefs, the team led by Christopher Krupenye of Duke University had the apes take part in a visual experiment, where they watched videos on a monitor while their gaze was being tracked.

First, the apes observed a human watching an object being hidden. The object was then moved – either in the presence or absence of the human – by a man dressed in a King Kong suit. When the absent person returned to search for it, scientists monitored where the ape observers looked.

orangutan theory of mind
Orangutans shown to possess 'theory of mind' - the ability to recognise someone elses' thoughts. Reuters

They found 17 of the 22 apes monitored anticipated the human would return to the place they had seen the object hidden to find it – understanding the human had not seen King Kong remove the object like they had. In another experiment, King Kong hid in one of two hay bales while being watched by the man. When the man left, King Kong ran away. When the man returned to look for King Kong, the apes looked at the hay bale where he had been hiding before running away.

Both the experiments show the apes could understand the human's understanding was different to their own and that they could predict their behaviour based on this. Study co-author Michael Tomasello, from the Max Planck Institute for Evolutionary Anthropology, explained. "It means understanding that there exists a mental world distinct from the physical world."

Krupenye added: "This is the first time that any nonhuman animals have passed a version of the false belief test. This cognitive ability is at the heart of so many human social skills. If future experiments confirm these findings, they could lead scientists to rethink how deeply apes understand each other."

Concluding, they wrote: "That great apes operate, at least on an implicit level, with an understanding of false beliefs suggests that this essential theory of mind skill is likely at least as old as humans' last common ancestor with the other apes."