A series of hide-and-seek tests show that bonobos rely on memory to keep track of several familiar people at once, even when they’re out of sight. The results reveal mental map-making skills once assumed to be uniquely human.
The study, led by Johns Hopkins University’s Social and Cognitive Origins Group, documents these abilities in a bonobo named Kanzi. The findings speak to how animals manage complex social worlds and where humans and apes overlap.
The myth of human-only minds
Study senior author Chris Krupenye is an assistant professor at Johns Hopkins who studies how animals think.
“People think social intelligence is a thing that makes humans unique,” said Krupenye. “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 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,” he said. “They must share with us at least the foundations of our rich social intelligence.”
Humans routinely keep mental tabs on others. If a partner leaves the room, their location stays active in the mind. For bonobos and chimpanzees, who often move through dense forests where companions flicker in and out of view, such memory would be useful – perhaps essential.
Testing bonobo memory skills
Kanzi faced a simple setup with demanding implications. Two caregivers he knew well hid behind two of three identical barriers.
An experimenter then showed Kanzi a photo of one caregiver and asked him to point to where that person was. Trials were repeated in varied orders to prevent rote guessing.
“Kanzi very quickly understood the task and performed well,” said lead author Luz Carvajal, a Ph.D. student in Krupenye’s lab who studies apes’ knowledge of their social relationships.
The team then removed visual cues altogether. Caregivers hid where Kanzi could not see them choose a barrier. From behind the screens they each called out, “Hi Kanzi,” letting him hear who was where.
Only then did the experimenter show a photo of one caregiver and ask Kanzi to indicate their location.
“Here he also performed above chance, and especially well with one of his two caregivers,” Carvajal said. “He does have the capacity to use voice as a marker for identity. This face matches this voice.”
Faces and voices connected
While the bonobo made mistakes – as humans do in memory tasks – the pattern was clear. Kanzi kept separate, accurate records of multiple individuals at once, and he could connect a face with a voice to a place.
“Across these studies, the results suggest that Kanzi has a memory of these individuals that brings together their vocal and visual identities – who they are and what they sound like, and where they are in space,” Krupenye said.
“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,” he said. “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.”
That kind of “bound” memory – linking identity across senses and locations – helps explain how bonobos manage fission-fusion societies, where subgroups split and reunite through the day.
It suggests a shared foundation for the social cognition humans rely on to plan, coordinate, and reconnect.
Bonobo memory, human insight
Field studies have long hinted that great apes mentally track groupmates and recognize familiar voices and faces even after years apart.
Chimpanzees, for instance, can pick out known humans wearing masks. But controlled tests of tracking multiple individuals at once were missing. Kanzi’s performance fills that gap and pushes the conversation forward.
The results also broaden what “voice recognition” means in apes. Bonobos had not been tested on matching voices to specific individuals hidden from view. Here, voice alone guided correct choices, at rates above chance, and in some cases with striking accuracy.
Deep origins of social life
The team now wants to expand the study with new questions: How many individuals can apes track at once? Does performance hold up over minutes, hours, or days? What happens when the voices are recorded, filtered, or partially masked?
The answers could reveal the limits of these mental maps and the cues apes, including bonobos, rely on.
“These animals are rich and complex,” Krupenye said. “Even if we just want to understand ourselves better, there’s an urgency to this work – and to saving this endangered species.”
Human and ape social minds
The findings also blur the line between human and ape social minds. Apes are not planning cities or managing calendars, but they can bind identities across senses, track several individuals at once, and call up that knowledge on demand – much like we do.
For species that must coordinate movement, negotiate alliances, and reunite after separation, those abilities are not luxuries. They are survival tools.
Kanzi’s success suggests the roots of our social lives run deeper than our species – and they’re still visible, if we know how to play the right game of hide-and-seek.
The study is published in the journal Proceedings of the Royal Society B.
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