Apes don’t forget their friends

Apes recognize members of their group even if they have not seen each other for more than 25 years, which represents the longest-lasting social memory ever documented in animals, comparable to that of humans, according to a study. study published in ‘Proceedings of the National Academy of Sciences’ (PNAS).

The study, led by Johns Hopkins University and carried out with chimpanzees and bonobos, shows that these apes have a social memory similar to ours, and that they not only recognize photos of former group mates whom they have not seen in decades, but who, in addition, are excited when the images they see are those of their friends.

“We like to think of ourselves, humans, as unique and special creatures with incredible intellectual capacities very different from those of any other animal on Earth, but this study shows us how similar we are to chimpanzees and bonobos,” emphasizes Laura Lewis, a biologist at the University of California at Berkeley.

The team conducted the study with chimpanzees and bonobos at Edinburgh Zoo (Scotland), Planckendael Zoo (Belgium) and Kumamoto Sanctuary (Japan).

The researchers collected photographs of apes that had changed zoos or died, individuals that the participants had not seen for at least nine months and, in some cases, up to 26 years.

Afterwards, they were offered a juice and, while drinking it, they were shown two photographs, one of an ape they had met and another of a stranger. Using a non-invasive eye-tracking device, they measured where the apes looked and for how long.

They found that the apes looked much longer at their former group mates – regardless of how long they had been apart – and that they looked longer at their former friends, with whom they had had more positive interactions.

In the most extreme case, Louise, a bonobo who had not seen her sister Loretta or nephew Erin for more than 26 years at the time of testing, showed a surprisingly robust gaze bias toward their photos on the eight occasions in which she They showed him the photographs.

The results suggest that the social memory of great apes could last more than 26 years, most of their 40 to 60 year average lifespan, and could be comparable to that of humans, which begins to decline at age 15. but it can persist up to 48 years after separation.

An evolutionary inheritance

For the team, the fact that humans and chimpanzees and bonobos – our closest relatives – share this type of memory means that it was probably already present in some common evolutionary ancestor, between 7 and 5 million years ago.

The authors believe that this memory laid the foundations for the evolution of human culture and allowed the emergence of exclusively human forms of interaction, such as trade, where relationships are maintained over many years of separation.

In addition, the idea that apes remember information about the quality of their relationships is another novel finding and unique to humans, emphasizes Christopher Krupenye, lead author of the study and professor at Johns Hopkins University.

“This pattern of social relationships that determine long-term memory in chimpanzees and bonobos is similar to what we observe in humans: our own social relationships also seem to determine our long-term memory of people.”says Lewis.

The work has also analyzed whether apes miss individuals they are no longer with, especially their friends and family, a trait considered “exclusively human”Lewis points out.

However, although the study does not determine that they do so, it does not rule out either, he acknowledges. “That possibility is there,” suggests the biologist.

From now on, the team will try to find out whether these long-lasting social memories are unique to great apes or have them in other primates and determine how much memory apes have and whether, for example, they have long-lasting memories of both experiences and individuals.