A series of hide-and-seek experiments with a bonobo named Kanzi shows for the first time that apes can mentally keep track of multiple familiar humans at once, even when they are out of sight.
Kanzi could also recognize caregivers from their voices alone, an ability never before tested on bonobos.
The work, led by Johns Hopkins University's Social and Cognitive Origins Group , answers key questions about how animals manage to keep track of their groupmates and uncovers yet another aspect of human social cognition shared with our closest relatives.
"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," says senior author Chris Krupenye , a Johns Hopkins assistant professor who studies how animals think. "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."
The findings are published today in Proceedings of the Royal Society B.
Humans intuitively track the whereabouts of others, mentally. 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. 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 groupmates they cannot see.
Research has shown that bonobos and chimpanzees recognize the faces and vocalizations of familiar groupmates, even after years apart . Chimpanzees have recognized familiar humans, even when they had masks on. While field studies have hinted that apes might be able to mentally track groupmates, this is the first study 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. An experimenter 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 many times.
"Kanzi very quickly understood the task and performed well," said lead author Luz Carvajal, a PhD student in Krupenye's lab who studies apes' knowledge of their social relationships, adding that the team also wanted to see if Kanzi could identify the caregivers not just by photos of their faces, but by the sound of their voices.
For this 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, however, they each called out to Kanzi, saying "Hi Kanzi," so that he could hear who was behind each barrier. The experimenter would then show Kanzi a photo of one of the caregivers and ask Kanzi to point to where they were.
"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."
While Kanzi did make mistakes across trials, the results demonstrate a fundamental capacity to mentally track, and keep straight, the locations of multiple familiar people at once.
"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," 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. 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."
Next the team hopes 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 the minds of apes during these separations.
"These animals are rich and complex," Krupenye says. "Even if we just want to understand ourselves better there's an urgency to this work and to saving this endangered species."
