Chimps and bonobos remember friendly faces even after decades apart, study finds

When Laura Simone Lewis visited Kendall the chimpanzee for the first time in six months, she wasn't sure how he would react.

The researcher had been working with the chimp at the North Carolina Zoo in 2015 as part of her undergraduate research on primate cognition, and they'd developed a special bond.

But, after regularly visiting Kendall two to three times a week, she left for a stint to study baboons in South Africa.

"I was nervous that the chimps might not remember me when I returned, but sure enough, when I returned, Kendall came right up to me," Lewis, a comparative psychologist at the University of California, Berkeley, told As It Happens host Nil Köksal.

"It seemed clear that he remembered and recognized me. He was calm and interested in me, acting just like, you know, I had never left."

Her experience isn't unusual. Lewis says plenty of people who work with primates tell similar stories. But now, she and her colleagues say they have the data that backs up what they have long suspected: chimpanzees and bonobos remember each other's faces even after spending years — or even decades — apart.

The findings were published Monday in the journal Psychological and Cognitive Sciences.

The study's authors looked at chimps and bonobos at zoos in Scotland, Japan and Belgium. The two ape species are humanity's closest living relatives. We share common ancestry, and about 99 per cent of our DNA, with them.

Participation in the study was voluntary. The apes were given access to a study room, which they could visit at their leisure. Inside was a juice dispenser facing a screen.

While they sipped their juice, the 26 primates who came in were shown images of other chimps and bonobos — some they'd never seen before, and some they formerly lived with who had either died or left the group. The scientists used eye-tracking cameras to study their reactions.

"The question was very simple. We were asking, would the apes recognize their previous group mates and would they look longer at the images of their previous group mates? And that's exactly what [we] found," Lewis said.

"Both chimpanzees and bonobos looked significantly longer at the images of their previous group mates as compared to the images of strangers."

One bonobo, Louise, honed in on images of her sister, Loretta, and nephew, Aaron — neither of whom she had seen in 26 years.

In fact, on average, the study's participants spent more time looking the apes they had close bonds with — family members, or unrelated individuals they'd had positive social relationships with — than they did looking at the strangers.

"We characterize these positive social relationships as relationships where individuals spend a lot of time in proximity together and spend a lot of time grooming each other," Lewis said.

"We don't want to anthropomorphize these species, but these relationships are very positive, are vital to their survival and to their well-being. And so we really are starting to characterize these positive social relationships as somewhat similar to friendship in humans."

Impacts of family separation

Ammie Kalan, a University of Victoria primatologist who studies chimpanzees, called the study "elegant and simple in its design and, therefore, really convincing because of the clear results.

"I always tell my students how easy it is to identify individual chimpanzees because their faces, gestures and voices are all so unique. It's great to have scientific evidence to support the fact that among chimpanzees, individuals are uniquely memorable to one another, even across decades," Kalan, who was not involved in the research, told CBC in an email.

"This finding is important more broadly for great ape welfare, particularly with regards to the potential effects of separating families and social groups and conversely, uniting them with these individuals later in life."

Study co-author Christopher Krupenye, who studies animal cognition at Johns Hopkins University, says apes are often torn apart from family and group mates through poaching and deforestation.

"This work clearly shows how fundamental and long lasting these relationships are. Disruption to those relationships is likely very damaging," he said in a press release.

Benefits of long-term memories for social species

Lewis says the findings make sense when you consider how chimps and bonobos live and interact in the wild.

They tend to form large communities with upwards of 150 individuals, she says, while also breaking off into smaller groups to perform tasks like border patrol.

They also interact with members of neighbouring communities, and young females will often leave one group for another to mate.

"These chimpanzees and bonobos, you know, have strong social relationships that can last decades, but they don't see each other all the time. They can be away from individuals in their groups and communities for days or weeks or months at a time," she said.

"They may not see … individuals of neighbouring groups for months or sometimes even years at a time."

It's not unlike human society, she says.

"If we have long-term memory that's shaped by our social relationships, and they do too, that it likely means that our last common ancestor that lived somewhere between five to seven million years ago likely also had long-term memory," she said.

"This may have aided us, especially in our own human evolution, when we started to travel farther distances [and] trade with populations that lived far away from us."