Do chimps possess a true theory of mind? (Sia Kambou/AFP)

You might not know it, but you're a seasoned mind reader. Picture this: Your partner frequently loses the car keys in the couch cushions, and each morning conducts a harried search for them before work. Whenever you see him or her march into the living room with that telltale look of frustration, you know they're about to make a beeline for that blasted couch. You know what's going on in their head, based on their actions and the context of previous behaviors.

This ability to imagine what might be going on in someone else's head is known as the theory of mind. Whether nonhuman animals can possess it is a matter of some debate. But according to a new study in Science, the nonhuman members of the great ape family may indeed have the same mental capacity.

“Decades of research with our closest relatives — chimpanzees, bonobos, gorillas and orangutans — have revealed that great apes do possess many aspects of theory of mind,” study author Christopher Krupenye of the Max Planck Institute (who conducted the research while at Duke University) wrote for the Conversation. “For one, they can identify the goals and intentions behind others’ actions. They’re also able to recognize which features of the environment others can see or know about."

One recent study even found signs of theory of mind in ravens, suggesting aspects of it could be more common than previously assumed. But humans were still thought to have a better handle on the whole business.

“Where apes have consistently failed, though, is on tasks designed to assess their understanding of others’ false beliefs. They don’t seem to know when someone has an idea about the world that conflicts with reality,” Krupenye added.

Imagine you'd just pulled your partner's keys out of the couch yourself and put them on a bookshelf for safe keeping. You know they didn't watch you do so, so you assume they'll start for the couch as usual unless you clue them in. You understand that they have a perception of reality that you know to be false.

That's the tricky aspect of mind reading that Krupenye and his colleagues wanted to study. To do so, they rounded up 40 bonobos, chimpanzees and orangutans and took them to the movies.

The films showed interactions between people and “apes.” (The latter actually were humans — who are technically apes — dressed up as nonhuman apes.) The people searched for objects on-screen with various interventions from the ape characters. Sometimes an ape would hide an object while the person watched, then hide it again while the individual was off-screen.

By tracking the eyes of the great-ape moviegoers, the researchers determined that their audience expected the humans on-screen to look for objects in the last place the humans had seen them. In other words, they seemed to not only understand that the seekers were looking for an object but also that they were operating under false pretenses.

In this experiment, ape subjects watch as a person dressed in an ape costume hides an object from a human. The red dots show where the subjects are looking, showing that they understand the human's false belief about the location of the object. (Christopher Krupenye, Fumihiro Kano, MPI-EVA, Kumamoto Sanctuary)

“They can anticipate that an individual will search for an object where they last saw it, even though the apes know that it’s no longer there,” Krupenye told New Scientist. “That is a really important human skill that has never been shown before in apes.”

“It’s a very surprising and novel finding,” Victoria Southgate, a developmental psychologist at the University of London, told Science Magazine. Southgate wasn't involved in the new study, but she helped develop the eye-tracking technique — which she had previously used to test human infants. “It’s an almost exact replication of the study we did, and the apes appear to pass. It suggests that the capacity to track others’ perspectives and beliefs is not unique to humans,” she said.

In a commentary on the study published in Science, Emory University's Frans de Waal called the research, which he did not help to conduct, "a genuine breakthrough.”

“The results contain a lesson for those who jump on negative outcomes regarding animal mental capacities as proof of human distinctiveness,” de Waal wrote. “As the old mantra goes, absence of evidence is not evidence of absence. We should always keep an open mind about the capacities of nonhuman species.”

Some researchers are still skeptical, arguing that the team could be anthropomorphizing a behavior that doesn't quite reach human levels of empathy. Maybe apes know that individuals will look for an object in the right place they saw it but don't possess a conscious understanding of the fact that those individuals hold a mistaken belief.

But Krupenye and his colleagues are optimistic — and excited about the evolutionary implications for our own species.

“If apes do in fact possess this aspect of theory of mind, the implication is that most likely it was present in the last evolutionary ancestor that human beings shared with the other apes,” he wrote in the Conversation. "By that metric, this core human skill — recognizing others’ false beliefs — would have evolved at least 13 to 18 million years before our own species Homo sapiens hit the scene.”

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