Apes can mentally track friends even when they’re hidden

By Stephen Beech

Out of sight isn’t out of mind for apes, reveals new research.

Bonobos can mentally track multiple members of their social circle - even when they can't see them, say American scientists.

A series of "hide-and-seek" experiments with a bonobo named Kanzi have shown for the first time that apes can keep track of several familiar humans at once - even when they are hidden from view.

The findings also showed that Kanzi could also recognize caregivers from their voices alone - an ability never before tested on the species of endangered great ape.

The study, led by researchers from Johns Hopkins University in Baltimore, answers key questions about how animals manage to keep track of their pals.

And the team says their findings, published in the journal Proceedings of the Royal Society B, also uncover another aspect of human social cognition shared with our closest relatives.

Study senior author Dr. Chris Krupenye said: “People think social intelligence is a thing that makes humans unique - that because we have to manage so many different relationships, we might have a range of cognitive tools for doing so that will only be found in an ultra-social species like humans.

“But most of us who study apes have a strong intuition that because the social world is so important for them too, they must, like humans, be keeping track of these critical social partners.

"They must share with us at least the foundations of our rich social intelligence.”

He explained that humans intuitively track the whereabouts of others, mentally.

Dr. Krupenye said, "If you’re at home and your partner leaves the room, they don’t leave your mind - you mentally maintain information about their whereabouts, as well as the locations of your other friends and family in the world, near and far."

He says bonobos and chimpanzees in the wild often live in dense forests where their groupmates regularly go out of view, so they too would benefit from the ability to keep mental tabs on mates they cannot see.

Previous research has shown that bonobos and chimps recognise the faces and vocalisations of familiar groupmates, even after years apart.

Chimps have recognized familiar humans, even when they had masks on.

While field studies have suggested that apes might be able to mentally track groupmates, the new study is the first to test in a controlled environment whether any animal can track multiple individuals at once.

During experiments, as Kanzi watched, two caregivers that he knew well would hide behind different barriers, in an array of three, that blocked them from his view.

A researcher would hold up a photo of one caregiver and ask Kanzi to point to where that person was.

The test was repeated and switched up multiple times.

Study lead author Luz Carvajal, a PhD student in Dr. Krupenye’s lab, said: “Kanzi very quickly understood the task and performed well."

The team also wanted to find out if Kanzi could identify the caregivers not just by photos of their faces, but by the sound of their voices.

For the further test, the caregivers again hid behind barriers, but this time Kanzi was not able to see which barrier they hid behind.

Once they were hidden, they called out “Hi Kanzi,” so that he could hear who was behind each barrier.

The researcher would then show Kanzi a photo of one of the caregivers and ask him to point to where they were.

Carvajal said: "Here he also performed above chance and especially well with one of his two caregivers.

“He does have the capacity to use voice as a marker for identity. This face matches this voice.”

While Kanzi did make mistakes across trials, the researchers say the results shiw a "fundamental capacity" to mentally track, and keep straight, the locations of several familiar people at once.

Dr. Krupenye, an Assistant Professor at Johns Hopkins, said: "Across these studies the results suggest that Kanzi has a memory of these individuals that brings together their vocal and visual of identities - who they are and what they sound like, and where they are in space.

“If he hears them, he might imagine what they look like.

"If he sees them, he might bring to mind an idea of what they sound like.

"We think this is one integrated memory. He’s using the same photo prompt to refer to an individual, whether he can see them or not.”

The research team hope to test the boundaries of how many individuals can apes mentally track at once and how long those memories last, to better understand what is happening in their minds during separations.

Dr. Krupenye added, “These animals are rich and complex.

“Even if we just want to understand ourselves better, there’s an urgency to this work and to saving this endangered species.”