Chimpanzees use tools, maintain friendships and enmities, help each other and even treat their wounds. To the long list of abilities that were long thought to be uniquely human until a chimpanzee was observed, another is now added: chimpanzees, like humans, can remember the faces of former friends, even if they haven’t seen them in decades .
The long-term social memory of chimpanzees and bonobos is as good as that of humans, writes a team led by psychologist Laura Lewis from the University of California in a study that has just been published in the scientific journal PNAS has appeared.
The scientists came up with the idea of investigating how long great apes remember other members of their own species after separation from their own experiences with the animals. Many of the team had the impression that “their” monkeys recognized them when they returned after a long absence. “You get the impression that they react differently than to normal zoo visitors,” says Christopher Krupenye of John Hopkins University, according to a university press release. “They’re excited to see you again.”
After four months of separation, the chimpanzee Kendall finally wanted to hold the researcher’s hand again
Laura Lewis had a similar experience with a chimpanzee named Kendall at the North Carolina Zoo who wanted to hold her hand whenever she entered the enclosure. When the scientist was unable to visit Kendall for four months because of a research project in Africa and then returned, the chimpanzee ran straight up to her and demanded to hold her hand.
For their study, the researchers tested 26 monkeys that live in three different places: the Edinburgh Zoo in Scotland, the Kumamoto Sanctuary in Japan and the Planckendael Zoo in Belgium. Before the actual experiment began, the scientists collected photos of conspecifics that the chimpanzees and bonobos participating in the study had not seen for at least nine months. In addition, they found out whether the relationship between the test monkeys and the missing group members was friendly or not.
In the actual experiment, the scientists then showed each monkey two photos at the same time: one showed a former group member, the other showed a strange monkey that the animals had never seen before. At the same time, the researchers recorded the animals’ eye movements with an infrared camera. The analysis showed that the chimpanzees and bonobos looked at pictures of conspecifics with whom they had previously lived for much longer than those of unfamiliar monkeys. The researchers interpreted this as a sign of recognition.
They also noticed a difference in how the animals reacted to photos of former friends compared to pictures of other monkeys they knew but had no special relationship with. The friends’ faces looked at the animals much more closely. “This suggests that the monkeys not only remember former companions, but also what kind of relationship they had with them,” says Krupenye.
Most impressive was the memory of a female bonobo named Louise from the Kumamoto Sanctuary. Louise had not seen her sister Loretta and her nephew Erin for more than 26 years at the time of the experiment. Still, she remembered both of them. To be absolutely sure, the researchers repeated the test eight times. The result was the same every time.
This means that monkeys’ ability to remember the faces of former friends is in the same range as that of humans. In Homo sapiens, memory begins to fade after about 15 years, according to the study. However, some people still remember the appearance of people with whom they had a close relationship up to 48 years after the breakup.
The question remains whether chimpanzees and bonobos miss their friends, just as humans do, when they are separated from them. It is at least reasonable to assume that such a separation affects the monkeys.