What separates humans from “the beasts” of nature? One of the few standout features of human intelligence is often argued to be our ability to deeply understand the desires, knowledge, motives, and intents of others. Known as “theory of mind”, this ability allows us to understand and anticipate the thoughts of others, even when they are different or opposed to one’s own. 

Many have pushed the idea that humans are the only creatures to possess this complex ability, however, a new study on our closest evolutionary cousins is now shaking this longheld assumption.

Reporting their findings in the Proceedings of the National Academy of Sciences, an international team of researchers from Kyoto University in Japan, the Max Planck Institute for Evolutionary Anthropology in Germany, and the University of St Andrews in Scotland has demonstrated that other members of the great ape family – including chimpanzees, bonobos, and orangutans – appear to possess “theory of mind.” 

While this is not the first study to argue this, the new research builds on the team's previous work to provide some of the most convincing arguments yet.

They reached this conclusion through a series of experiments on 47 captive great apes – 29 chimpanzees, 14 bonobos, and four orangutans – at sanctuaries in Japan and Germany.

In the first test, the ape observed a film that showed a human actor being tricked into thinking an object was hidden in a box. After the human watched the object be put in the box, they went behind a screen, after which the object was moved. So, the ape knew the object was not in the box, but the human still thought it was there. Using eye-tracking technology to record their gaze, it appeared that the ape anticipated the human’s false belief that the object was still in the box (even though they were aware it was not).

To dig deeper into whether this displayed true “theory of mind”, they set up another scenario that was a little bit more confusing. The apes were randomly split into two groups and familiarized with one of two types of screen: a barrier made of an opaque material and a “trick” barrier (one which appears to be opaque from a distance but becomes transparent up close). 

The apes watched a new film with the same premise as the first, but with the two new screens at play. As expected, the apes who were used to the opaque barrier anticipated that the human would search for the object under the box where it was last seen. However, the other apes were able to anticipate that the human would go to neither location, as the object had been removed entirely, and both parties were aware of this.

This sharp ability to predict the actions of someone else – even taking into account their different perspectives, knowledge, and intent – shows a remarkable level of intelligence and, according to the team, provides evidence that apes possess “theory of mind.”

“We are excited to find that great apes actually passed this difficult test," study author Fumihiro Kano, of Kyoto University's Kumamoto Sanctuary and Primate Research Institute, said in a statement. "The results suggest that we share this ability with our evolutionary cousins. We plan to continue refining our methods to test further non-mentalistic alternatives to the theory of mind in nonhuman animals."