Chimpanzees can still remember faces after a quarter of a century

In 2015, while working as an undergraduate researcher at the North Carolina Zoo, Laura Lewis befriended a male chimpanzee named Kendall. When she visited the chimpanzees, Kendall gently took her hands and inspected her fingernails.

Then she disappeared for the summer to study baboons in Africa. When she returned to North Carolina, she wondered if Kendall would still remember her face. Sure enough, as soon as she stepped into his enclosure, Kendall ran over and motioned to look at her hands.

“The feeling I got was that he remembered me clearly after four months of absence,” said Dr. Lewis, now a comparative psychologist at the University of California, Berkeley. “But I didn’t have the data to prove it.”

Now she believes so. In a study published on Mondayhave Dr. Lewis and her colleagues showed that chimpanzees and bonobos can remember faces of other monkeys that they have not seen for years. One bonobo recognized a face after 26 years – a record for facial memory beyond our species.

Dr. Lewis and her colleagues conducted the study on 26 monkeys kept at the Edinburgh Zoo in Scotland, the Kumamoto Sanctuary in Japan and the Planckendael Zoo in Belgium. At each facility, the researchers rolled a computer to the monkey enclosure fence and displayed images of animals on the monitor. A straw attached to the fence allowed the monkeys to drink juice while staring at the photos.

After giving the monkeys a few months to get used to the unusual setup, Dr. Lewis and her colleagues on their experiment. While the animals drank their juice, the computer showed monkey faces for three seconds at a time. In each pair, one of the faces was a stranger and the other an old companion that the monkey had not seen for years.

The scientists used an infrared camera to film the animals’ eye movements. If the monkeys had no memory of their old companions, the scientists expected them to spend the same amount of time looking at both images.

But that’s not what the researchers discovered. The monkeys consistently spent more time looking at their former companions. (Kindness played no role in the results, as unrelated past acquaintances also received more attention than strangers.)

A 46-year-old bonobo named Louise at the Kumamoto Sanctuary demonstrated the oldest memories. Until 1992, she lived with her sister and cousin at the San Diego Zoo. She then moved to the Cincinnati Zoo before coming to the Kumamoto Sanctuary in 2014. In 2019, Dr. Lewis and her colleagues found that Louise stared longer at the faces of her long-lost relatives than at those of monkeys she had never met, even after being separated for more than 26 years.

Dr. Lewis warned that tracking eye movements only provides a limited glimpse into the monkeys’ minds. “We can’t fully characterize what their memories look like,” she said.

But the researchers did find one tantalizing clue that suggests good memories can remain strong over the years. According to reviews from zoo keepers, the monkeys spent a little more time looking at the faces of animals with which they had once had positive experiences.

Dr. Lewis speculates that monkeys could benefit from these lasting memories. For example, a female bonobo will typically leave her mother’s group for the rest of her life to join another group. When the two groups meet years later, they may be able to form an alliance with old acquaintances.

The experiment places no limit on the duration of the animals’ memory. It’s possible that they remember faces for as long as we do. In one studypsychologists asked volunteers to name people in photos from their high school yearbooks. Their memories began to decline after 15 years, but some volunteers could still name their classmates correctly 48 years after graduation.

How many other species have these long-lived memories is difficult to say. Jason Bruck, an ethologist at Stephen F. Austin State University in Nacogdoches, Texas, has discovered that recognize dolphins the calls of other dolphins that they have not heard for twenty years.

Dr. Bruck suspects that other long-lived animals that live in groups will also exhibit impressive memories – if scientists get the chance to test them. “I think all these animals will have lifelong memories,” he said.

Dr. Lewis noted that chimpanzees, bonobos and humans all share a common ancestor that lived about seven million years ago. Early humans may have built on the foundation of long-term memories seen in apes as their societies became more complex.

“In our human evolution we have faced environments where we live socially but not around each other all the time, and populations are spread further and further apart,” she said.

Clive Gamble, an archaeologist at the University of Southampton in England who was not involved in the new research, agreed with that interpretation. The evolution of language may have strengthened long-lasting social memories, as people told stories about acquaintances they had not seen in years. “We just tapped into our common ancestry and then turned up the volume,” said Dr. Gamble.