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Monkeys Old friends they haven’t seen in decades can be recognized, according to new research, the longest social memory ever documented outside of humans.
Researchers found that Chimpanzees and bonobos They were able to recognize photos of their former group mates more than 25 years after they had last seen them in the flesh, with photos of old friends eliciting a more positive response, according to Stady Published Monday in the Proceedings of the National Academy of Sciences.
Senior author Christopher Krupeni, an assistant professor at Johns Hopkins University who studies animal cognition, He told CNN that the research was inspired by his experience working with monkeys and their feeling that they recognized him even years after their last interaction.
In order to test this, Krupeni and lead author Laura Lewis, a biological anthropologist and comparative psychologist at the University of California, Berkeley, used photographs of apes that had died or left groups at the Edinburgh Zoo in Scotland, the Blankendael Zoo in Belgium and the Kumamoto Reserve in Japan.
The team selected individuals whom the participating monkeys had not seen for periods of time ranging from nine months to 26 years, had high-quality photographs available on file, and observed what kind of relationship the participants had with the individuals.
Next, the researchers left two photos — one of a monkey they knew and one of a stranger — within the monkeys’ reach, and used a non-invasive eye-tracking device to measure where they looked and for how long.
The results showed that the monkeys looked “significantly longer” at people they knew, regardless of how long it had been since they had last seen them, and even longer at those they were friendly with.
Krupini likens the experience to bumping into someone from high school on the street after not seeing them for years.
“It’s a very familiar experience for humans as well,” he told CNN.
One bonobo, Louise, had not seen her sister Loretta or her nephew Erin for 26 years at the time of the test, but she “displayed a strikingly strong bias toward both over the course of eight trials,” according to a press release.
The statement added that researchers believe that the social memory of monkeys can extend beyond 26 years and can be compared to the memory of humans, who begin to forget people after 15 years but can remember them for up to 48 years.
“This seems to be approaching a kind of lifelong memory for these animals,” Krupeni told CNN.
This is also a new record for the length of social memory in non-human animals, after previous research showed that dolphins remember individuals they have not seen for 20 years, according to the researchers.
The results show that social memory is a general ape trait that we share with our closest relatives, rather than something that evolved separately in humans, Krupeni said.
The study also raises the possibility that monkeys may be able to lose loved ones.
“The idea that they remember others, and therefore may miss these individuals, is actually a powerful cognitive mechanism and something that is thought to be unique to humans,” Lewis said in the press release.
“Our study did not determine that they do, but it raises questions about the possibility that they have the ability to do so.”
The authors hope the study will raise awareness about how poaching and deforestation affect ape communities by separating their group mates, and strengthen conservation efforts.
“We hope these findings will provide people with more empathy for our closest living cousins,” Lewis told CNN, stressing that bonobos may become extinct in our lifetime.
Next, the team plans to investigate whether apes can recognize their former friends as they appear now, rather than when they left the group, as well as whether other primates such as gorillas and orangutans also have long-term social memories.
“I would be very surprised if we didn’t see similar effects in other monkeys,” Krupeni said, adding that their method could be used to study social memory in other animals, such as sheep and dogs.
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