These bonobos know what you don’t know

People are constantly looking at the behavior of others and coming up with ideas about what might be going on in their heads. Now, a new study of bonobos adds to evidence that they might do the same thing.

Specifically, some bonobos were more likely to point to the location of a treat when they knew that a human companion was not aware of where it had been hidden, according to a study which appears in the Proceedings of the National Academy of Sciences.

The findings add to a long-running debate about whether humans have a unique ability to imagine and understand the mental states of others.

Some researchers say this kind of “theory of mind” may be practiced more widely in the animal kingdom, and potentially watching it in action was quite the experience.

“It’s quite surreal. I mean, I’ve worked with primates for quite some years now and you never get used to it,” says Luke Townrow, a PhD student at Johns Hopkins University. “We found evidence that they are tailoring their communication based on what I know.”

Hmmm, where is the grape?

To see what bonobos might know about what humans around them know, Townrow worked with Chris Krupenye of Johns Hopkins University to devise a simple experiment.

“It’s always a challenge for us, that animals don’t speak, so we can’t just ask them what they’re thinking. We have to come up with creative, experimental designs that allow them to express their knowledge,” says Krupenye.

The study involved three male bonobos, all living at an education nonprofit called the Ape Initiative. During each experimental trial, Townrow sat across from one bonobo, who was in an enclosure, but looking through an opening covered by mesh.

The bonobo watched as a helper placed a treat, such as a grape, under one of three blue cups that were lined up in a row.

“We established a co-operative context to this task because if I knew whether the treat or the food item was hidden, I would reveal it and then the bonobo would be able to receive that as a reward,” he explains.

Sometimes, Townrow could see what was happening when the treat got placed under a cup. Other times, his view was obscured by a barrier, so he didn’t know where it was.

No matter what he had or had not seen, Townrow would briefly scan the cups, saying, “Hmmm…where’s the grape?” and then wait for ten seconds.

It turned out that when Townrow had enjoyed an unobscured view of the treat being hidden, the bonobos usually sat still and waited.

But when his eyes had been behind the barrier, which blocked his view and made it so he couldn’t see which cup had been picked as the hiding place, the bonobos tended to point their fingers through the mesh and tap towards the right cup.

“There’s definitely times where you can see that they’re very frantically trying to get my attention and just pointing, pointing, pointing, because they really want me to act, but they have to wait the whole 10 seconds because it’s a controlled setting,” says Townrow.

In the real world

The results didn’t necessarily surprise Krupenye, since he’s worked with apes for over a decade and had a strong suspicion that they understand when another is ignorant of certain information. He says it was just “very exciting” to find a way to test for that so that the bonobos “could really express that understanding.” 

While it would be nice to see this study conducted with more individuals, it’s a “valuable contribution” to the “theory of mind” debate, says Catherine Crockford of the Institute of Cognitive Sciences, CNRS, in Lyon, France.

“At least two of three bonobos communicated more to an ignorant than knowledgeable onlooker who was in a position to help gain access to food, once the bonobo made the necessary information available,” she says.

This suggests that the bonobo could hold two ideas in its mind simultaneously: that the bonobo knew the location of the food, and that the human might have different information about the location that needed updating by the bonobo.

Previously, Crockford and some colleagues found that wild chimps are more likely to emit alarm calls when seeing a risk like a snake if those nearby had not already indicated that they had seen the danger.

Taken together with this new lab experiment, she says, “these studies in alarm and food contexts demonstrate that the capacity is not confined to a specific, narrow context.”