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Eternal bonds: Great apes exhibit remarkable memory for their long-lost friends

Great apes recognize pictures of group members they haven't seen in more than 25 years and respond more enthusiastically to pictures of their friends, new research suggests

New research shows great apes have an extraordinary social memory, recognizing former group members even after more than 25 years. This finding, which indicates a significant cognitive similarity between great apes and humans, emphasizes the depth and duration of social ties in our animal relatives. The study found the longest-lasting non-human social memories ever recorded. Photo: Johns Hopkins University
New research shows great apes have an extraordinary social memory, recognizing former group members even after more than 25 years. This finding, which indicates a significant cognitive similarity between great apes and humans, emphasizes the depth and duration of social ties in our animal relatives. The study found the longest-lasting non-human social memories ever recorded. Photo: Johns Hopkins University

Great apes recognize pictures of group members they haven't seen in more than 25 years and respond more enthusiastically to pictures of their friends, new research suggests.

The study, published today in the journal Proceedings of the National Academy of Sciences, describes the longest-term social memory ever recorded in a non-human, and highlights how human culture evolved from our common ancestors with the great apes, our closest relatives.

"Chimpanzees and bonobos recognize individuals even after not seeing them for decades," said lead researcher Christopher Krupnier, an assistant professor at Johns Hopkins University who studies animal cognition. "And there is a small but significant pattern of increased attention towards individuals with whom they had more positive relationships. This indicates that it is more than an acquaintance, that they are monitoring aspects of the quality of these social relationships."

Laura Lewis, a biological anthropologist and comparative psychologist at the University of California, Berkeley, added: "We tend to think of the great apes as completely different from us, but we saw in these animals cognitive mechanisms very similar to ours, including memory, and I think that's why this research is so exciting."

The experimental methodology

The research team wanted to investigate the question of how long apes remember their own kind because of their own experiences working with apes - the feeling that the animals recognize them when they visit, even if they haven't visited the apes for a long time.

"We have the impression that they are reacting as if they recognize us and that you are really different from the average visitor to the zoo," Krupnier said. "They are excited to see you again. Therefore, our goal in this study was to ask, empirically, if this is really the case: do they have a durable and long-term social memory for familiar partners?"

Chimpanzees and bonobos recognize details

The team worked with chimpanzees and bonobos, the two closest apes to humans, at Edinburgh Zoo in Scotland, Planckendael Zoo in Belgium, and Kumamoto Sanctuary in Japan. The researchers collected images of great apes that had left the zoos or died, details that the participants had not seen for at least nine months and in some cases up to 26 years. The researchers also collected information about the relationships each participant had with former group members - whether there were positive or negative interactions between them, and so on.

The team invited apes to participate in the experiment by offering juice, and while they drank it, the apes were shown two images side by side - apes they once knew and complete strangers. Using a non-invasive eye-tracking device, the team measured where the apes looked and for how long, hypothesizing that they would look longer at apes they recognized.

Findings and implications

Video: Great apes looked significantly longer at former group members, regardless of how long they had not met them. They looked even longer at their former friends, those with whom they had more positive interactions

In the most extreme case of the experiment, the bonobo Louise had not seen her sister Loretta and niece Erin for more than 26 years at the time of the test. She showed a particularly strong bias of looking at both versus others in eight trials.

Great apes looked significantly longer at former group members

The great apes looked significantly longer at former group members, regardless of how long they had met them. And they looked even longer at their former friends, those with whom they had more positive interactions. Credit: Johns Hopkins University
The great apes looked significantly longer at former group members, regardless of how long they had not seen them. And they looked even longer at their former friends, those with whom they had more positive interactions. Credit: Johns Hopkins University

The results suggest that the social memory of great apes can last beyond 26 years, most of their average lifespan of 40 to 60 years, and may be comparable to that of humans, which begins to fade after 15 years but can last up to 48 years after separation. Such long-term social memory in humans and our closest relatives suggests that this type of memory probably already existed millions of years ago in our common ancestors. This memory probably created a basis for the development of human culture, and enabled the emergence of uniquely human forms of interaction such as trade between groups where relationships are maintained over many years of separation, according to the authors.

The idea that great apes have a memory of the quality of their relationships for many years is another new and human finding of the study, Krupenier said. "The pattern of social relationships that shape long-term memory in chimpanzees and bonobos is similar to what we see in humans, and our social relationships also seem to shape the long-term memory of humans," Lewis said.

Insights and future research

The study also raises the questions of whether great apes miss individuals who are no longer with them, especially friends and family members.

"The idea that they actually remember others and therefore may miss those individuals is a really powerful cognitive mechanism and something that is thought to be unique to humans," Lewis said. "Our research does not establish that they do, but it raises questions about the possibility that they have the ability to do so."

The team hopes the findings will deepen people's understanding of the great apes, all of which are endangered species, while shedding new light on how harmed they can be when they are hunted.

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