Animal-deep memory of friends

Animal-deep memory of friends

Chimpanzees and bonobos look particularly long in photos with the faces of once-familiar individuals. © Fumihiro Kano

Unforgotten for decades: Chimpanzees and bonobos can recognize former group members in photos that they have not seen for more than 25 years, according to an experimental study. They react particularly intensely to individuals with whom they were particularly friendly. The results suggest that the similarly strong long-term social memory in humans and great apes goes back to our common ancestors, say the scientists.

“This photo shows my grandfather, who died 25 years ago.” We usually recognize the faces of people who were once close to us without any problems, even after a long time. But to what extent is this pronounced social memory a special feature of humans? In principle, it is already known that even some highly developed animal species can remember certain members of their species. The previous long-term record for social memory was held by dolphins: they can still recognize the calls of certain individuals after 20 years. However, the potential of our closest relatives in the animal kingdom has so far remained unclear. Scientists led by Laura Lewis from the University of California at Berkeley have now closed this gap through an experimental study on chimpanzees and bonobos.

They examined groups of these great apes that are kept in the Edinburgh Zoo, the Belgian Planckendael Zoo and the Kumamoto Sanctuary in Japan. For the study, the researchers first collected photos of monkeys that had either previously left the respective group or had died. The period of absence ranged from nine months to many years to the extreme case: 26 years. The scientists also collected information about each animal study participant's relationships with former group members. Even among the great apes, some animals are particularly friendly with each other - but there are dislikes between others.

Monkey memories in the mirror of looks

A total of 26 chimpanzees and bonobos were invited to take part in the experiments by offering drinks: they could sip fruit juice from a dispenser. He was in front of a screen on which two photos were presented to the animals in parallel. One showed a former group member, while the other showed a completely unknown monkey. To capture the attention that each of these images attracted, the researchers tracked the eye movements of the animal subjects using an eye tracking system. This method is also used in humans to capture information about the effects of certain visual stimuli. As a rule, we always focus a little longer on impressions that seem comparatively important to us.

When evaluating the eye tracker data in connection with the information about the history of the respective animal's acquaintance, it became clear that the monkeys looked at the respective photo of the former group member for significantly longer than the control picture - no matter how long they had looked at the respective animal hadn't seen anymore. In some cases, the animals even stopped drinking altogether when images of a once-familiar animal appeared on the screen. The scientists report that they sometimes seemed to be hypnotized. The long-term memory record was set by the bonobo lady Louise: she was shown pictures of her sister Loretta and her nephew Erin, whom she had not seen for 26 years. During various experimental runs, Louise looked meaningfully intently at the images of these two individuals.

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The researchers were also able to demonstrate an effect when it came to the personal relationship with the former group member: “We found a pattern of greater attention towards known individuals with whom the animal in question had a rather positive relationship. “This suggests that the monkeys perceive more than just familiarity, but also consider aspects of the quality of a social relationship,” says senior author Christopher Krupenye of Johns Hopkins University in Baltimore. The study results now prove that chimpanzees and bonobos have a similarly developed social memory as humans, the researchers conclude. According to them, this suggests that this type of memory probably existed in our common evolutionary ancestors, who lived around six to nine million years ago.

However, one can only speculate about what exactly goes on in the animals when they remember once-familiar members of their species. Specifically, the study raises the question of whether the monkeys miss individuals they are no longer with. “Our study doesn't prove that they do this, but there is at least a possibility that they are able to do this,” says Lewis. The scientists now want to remain true to the interesting research topic: Among other things, they plan to explore whether particularly long-lasting social memory can also be found in primates other than great apes. They also want to investigate in more detail the extent to which social memory in great apes is linked to specific experiences with other species.

Source: Johns Hopkins University, University of California – Berkeley, specialist article: Proceedings of the National Academy of Sciences, doi: 10.1073/pnas.2304903120

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