Chimpanzees recognise pals not seen for decades, scientists find

Researchers have found that chimpanzees can recognise friends and relatives they have not seen for more than 20 years.

A chimpanzee in Kibale Forest National Park, Uganda. Picture: Getty Images.

When a photograph of an old friend flashes up, it can prompt a flood of memories when you recognise a face from decades past. Chimpanzees and other great apes feel the same about their long-lost pals, scientists have found.

Researchers have uncovered evidence of the longest “social memories” ever observed outside of human beings, finding that chimps and bonobos can recognise friends or relatives they have not seen for more than 20 years.

In one case, a bonobo called Louise had not seen her sister, Loretta, or her nephew, Erin, for more than 26 years, but a study found that she recognised them when shown photographs of them.

The scientists, from the University of California, Berkeley, and Johns Hopkins University in Baltimore, used infrared eye-tracking cameras to record where 26 chimpanzees and bonobos were gazing when shown two images side by side of other bonobos or chimpanzees.

They chose animals, including from Edinburgh Zoo in Scotland, who had been separated from friends and family, in some cases to prevent inbreeding within a zoo. In other cases, the animals had died.

One picture depicted a stranger they had never seen before and the other showed an animal that they had lived with as a group-mate for at least a year in the past and had not seen for at least nine months, using a photograph taken at around the time they last saw each other.

The apes’ eyes were drawn to and lingered for much longer on the photos showing their former acquaintances, demonstrating greater recognition, the scientists said.

While elephants are usually held up as the animals with the longest memories, a previous study found that dolphins performed best, finding that some can recognise vocalisations from other dolphins for up to 20 years.

“That, up until this point, had been the longest long-term social memory ever found in a non-human animal,” said Laura Simone Lewis, a fellow at Berkeley’s psychology department. “What we’re showing here is that chimps and bonobos may be able to remember that long, or longer.

“It was a really simple test: do they look longer at their previous group-mate, or are they looking longer at the stranger? And we found that, yes, they are looking significantly longer at the pictures of their previous group-mates.”

The animals did not display any signs of distress when seeing images of old friends, but at times they were “seemingly mesmerised” by seeing a familiar face, prompting them to stop drinking the juice they had been given as a reward for taking part in the study, which was published in the journal Proceedings of the National Academy of Sciences.

“Chimpanzees and bonobos recognise individuals even though they haven’t seen them for multiple decades,” said Christopher Krupenye, an assistant professor in the department of psychological and brain sciences at Johns Hopkins University. He said they linger longer when looking at individuals with whom they had had a close or positive relationship, compared with those who had been rivals.

“There’s this small but significant pattern of greater attention toward individuals with whom they had more positive relationships,” he said. “It suggests that this is more than just familiarity, that they’re keeping track of aspects of the quality of these social relationships.”

Lewis said: “We tend to think about great apes as quite different from ourselves, but we have really seen these animals as possessing cognitive mechanisms that are very similar to our own, including memory. And I think that is what’s so exciting about this study.”

Studies suggest that, in humans, our memories of long-lost friends start to fade after 15 years, but can last as long as 48 years after separation.

The researchers suggested that long-term “social” memories must date back at least as far as the last common ancestor of humans and great apes, between six and nine million years ago.

“This work clearly shows how fundamental and long-lasting these relationships are,” Krupenye said. “Disruption to those relationships is likely very damaging.”

The Times