In 2015, while working as an undergraduate researcher at the North Carolina Zoo, Laura Lewis befriended a male chimpanzee named Kendall. Whenever she visits the chimps, Kendall gently takes her hands and inspects her nails.
Then he disappeared for the summer to study baboons in Africa. When he returned to North Carolina, he wondered if Kendall would even remember his face. Sure enough, as soon as he entered his enclosure, Kendall ran over and motioned to look at his hands.
“I feel like he clearly remembered me after four months,” said Dr. Lewis, now a comparative psychologist at the University of California, Berkeley. “But I don’t have the data to prove it.”
Now he believes. In one study published on Monday, Dr. showed. Lewis and his colleagues found that chimpanzees and bonobos remember the faces of other apes they haven’t seen for years. A bonobo recognizes a face after 26 years — a record for facial memory that surpasses our species.
Dr. Lewis and his colleagues conducted a study on 26 monkeys kept at the Edinburgh Zoo in Scotland, the Kumamoto Sanctuary in Japan and the Planckendael Zoo in Belgium. In each facility, the researchers attached a computer to the monkeys’ enclosure and displayed images of the animals on a monitor. A straw attached to the fence allows the monkeys to drink juice while they look at the pictures.
After giving the monkeys a few months to get used to the unusual setup, Dr. Lewis and his colleagues conducted their experiment. While the animals sipped the juice, the computer displayed pairs of 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 record the animals’ eye movements. If the monkeys had no memory of their old companions, the scientists would expect them to spend an equal amount of time glancing at the same pictures.
But that’s not what the researchers found. The monkeys continued to spend more time looking at their former companions. (Relationship played no part in the results, as unrelated ex-acquaintances also received more attention than strangers.)
A 46-year-old bonobo named Louise at the Kumamoto Sanctuary exhibited the oldest memory. Until 1992, he lived at the San Diego Zoo with his sister and his nephew. He then moved to the Cincinnati Zoo before going to the Kumamoto Sanctuary in 2014. In 2019, Dr. Lewis and his colleagues found that Louise stared longer at the faces of her long-lost relatives than at monkeys she had never met, even after being separated for more than 26 years.
Dr. warned. Lewis that tracking eye movements provides only a limited glimpse into the minds of monkeys. “We can’t fully describe what their memories look like,” he said.
But researchers have found a tantalizing clue that suggests fond memories can stay strong over the years. The monkeys spent less time looking at the faces of animals they had previously encountered, according to ratings submitted by zookeepers.
Dr. thinks Lewis that monkeys can benefit from this strong memory. A female bonobo, for example, will typically leave her mother’s group to join another group throughout her life. If the two groups meet after a few years, he may form an alliance with old acquaintances.
The experiment did not put a limit on the duration of the animals’ memories. They probably remember faces as long as we do. In one study, psychologists asked volunteers to name people in pictures from their high school yearbooks. Their memories began to decline after 15 years, but some volunteers could still correctly name classmates 48 years after graduation.
How many other species have these long-lived memories is hard to say. Jason Bruck, an ethologist at Stephen F. Austin State University in Nacogdoches, Texas, found that dolphins can recognize the calls of other dolphins that they haven’t heard in over 20 years.
Dr. suspects Bruck believes that other long-lived animals that live in groups will also exhibit remarkable memories – if scientists get a chance to test them. “I think all these animals will have lifelong memories,” he said.
Dr. observed. Lewis that chimpanzees, bonobos and humans share a common ancestor that lived about seven million years ago. Early humans may have developed the foundation of long-term memories seen in apes as their societies became more complex.
“In our human evolution, we are faced with environments where we live socially, but not with each other all the time, and populations are more and more dispersed,” he said.
Clive Gamble, an archaeologist at the University of Southampton in England, who was not involved in the new study, agreed with that interpretation. The evolution of language may have fostered long-term social memories, as people told stories about acquaintances they hadn’t seen in years. “We just used our common ancestor, and then turned up the volume,” said Dr. Gamble.
