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.

She 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 visiting Kendall two to three times a week for about eight months, she left for a summer 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 other pictures.