Apes recognize pictures of friends they haven't seen for more than 26 years

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Louise hadn’t seen her sister, Loretta, or nephew Erin for more than 26 years, not since leaving San Diego for Cincinnati in 1992, but when she saw their photos through a see-through plexiglass panel, she seemed to recognize their faces.

Louise isn’t a human; she’s a 51-year-old bonobo, and she showed a strikingly “robust attentional bias” toward both Loretta and Erin as she sipped a favourite fruit juice from a nozzle.

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Specifically, Louise focused significantly more on Loretta and Erin than on the faces of total strangers.

A new study suggests bonobos and chimpanzees, humans’ closest living relatives, possess, like us, “an enduring memory for previous social partners,” and that those relationships live on in the great apes’ minds for decades.

The work is being described as evidence of the “longest-lasting nonhuman social memory documented to date,” and not only tells us how similar great apes are to humans but also how distinctly social these endangered creatures are, said senior author Christopher Krupenye.

“It uncovers the richness of the social and mental lives of our closest relatives. In some sense, their social relationships clearly continue to exist beyond the present,” said Krupenye, a Johns Hopkins University assistant professor who studies animal cognition.

“They exist in some ways in their minds for many years without continued interaction.”

Long-lasting social memory likely provided key foundations for the evolution of human culture and sociality as they extended across time, space and group boundaries

That raises important questions: are they missing old mates and relatives they’re no longer with? Are they replaying past events? Imagining the good times they’ve had, or the bad?

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The study can’t say, but it raises the possibility that they may have the ability to do so, said lead author Laura Lewis, a biological anthropologist at University of California, Berkeley.

Humans are believed to be special animals for our ability to remember names and faces for decades, but the evolutionary history of that long-lasting social memory remained unclear, Lewis and her co-authors said.

The findings in our closest relatives suggest this kind of rich memory isn’t, in fact, unique to humans and was likely present at least six to nine million years ago in our last common ancestor.

“Long-lasting social memory likely provided key foundations for the evolution of human culture and sociality as they extended across time, space and group boundaries,” the researchers wrote.

Their own experience working with apes was the driving inspiration for the study, published today in the journal Proceedings of the National Academy of Sciences.

Krupenye has travelled to zoos and sanctuaries the world over, spending weeks, months, sometimes years with great apes “trying to uncover the secrets of their minds.”

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Apes housed in zoos see dozens, if not hundreds of unfamiliar people every day, people they mostly ignore. “But I’ve had the experience, many times, of returning even years later and it seems like very clearly they are recognizing me,” Krupenye said.

How so? The apes make facial expressions. They sometimes like it if those familiar humans bob a little bit or make faces, and they’ll make faces back.

“Most people who work closely with great apes have the impression they must clearly remember us, even over years of separation,” Krupenye said.

“We were interested in seeing if we could design an experiment that would allow us to pin that down empirically.”

In 2019, the team studied a total of 26 zoo- or sanctuary-housed chimpanzees and bonobos from facilities in Scotland, Belgium and Japan.

The animals ranged in age from four to 46 years.

The study was voluntary. The apes were “invited” to participate by offering them a low sugar, diluted juice from a plastic nozzle that was strategically mounted to the bottom of a plexiglass panel.

On the other side of the plexiglass was a computer monitor that showed, in three-second spurts, two side-by-side, high-quality close-up photos — one an ape of the same species they’d once known but that had either died or been moved to another group or facility, the other a perfect stranger they’d never met.

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Photos taken from as close as possible to the last time the animals saw former groupmates were used.

Positioned directly below the monitor was an eye-tracker, a harmless infrared camera that measured where the apes looked and for how long, tracking changes in the target of their gaze.

Data from humans suggest it takes about half a second to process a face for recognition.

Given the short trials, the researchers weren’t expecting to find 100 per cent attention toward one image. But the apes, on average, looked for a quarter-second — 11 per cent — longer at images of former groupmates relative to strangers, and slightly longer still at those they’d had more “positive interactions” with.

The apes had last seen old companions at least nine months ago, and, in the most extreme case, bonobo Louise hadn’t seen Loretta or Erin in nearly 27 years at the time of testing.

All three had lived together at the San Diego Zoo until 1992, when Louise was moved to the Cincinnati Zoo and then, in 2014, to the Kumamoto Sanctuary in Japan.

In seven out of eight trials, Louise, born in 1972, showed a pronounced bias to look toward Loretta and Erin, one that was significantly greater than chance.

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The chimps and bonobos at the Kumamoto Sanctuary, more “research-experienced” apes, showed the most robust biases toward former groupmates, the researchers said.

The results suggest that apes likely have memory for non-kin lasting at least 9.5 years, and up to 26 years for social partners — a good proportion of their 40- to 60-year lifespans.

Other species — fur seals, elephants, Japanese macaques, orangutans — have been shown to remember old acquaintances for years, especially through vocalizations or calls. The standouts were dolphins: One study reported bottlenose dolphins could recognize the signature whistles of former tank mates for at least 20 years in what was then described as the longest social memory for a non-human species.

Great apes may have even more enduring memories.

“These are, unfortunately, highly endangered species,” Krupenye said. “It’s critical that we invest in efforts to conserve them in the wild … to protect them for them, and also, selfishly, for us.”

The study was partly funded by the Canadian Institute for Advanced Research, a Canadian-based global research organization.

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