Apes recognize photos of group mates they haven't seen in more than 25 years and respond with even more enthusiasm to photos of their friends, representing the longest-lasting social memory ever documented in animals, comparable to that of humans, according to a study. study published last Monday in the journal Proceedings of the National Academy of Sciences (PNAS).
This work, led by Johns Hopkins University and carried out with chimpanzees and bonobos, demonstrates that these apes have a social memory similar to ours. "This small but significant pattern suggests that this is more than just familiarity, that they are tracking aspects of the quality of these social relationships," says the study's lead author, Christopher Krupenye.
For her part, Laura Lewis, a biologist at the University of California at Berkeley, emphasizes: "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."
The research team was inspired to investigate how long apes remember their peers from their own experiences working with apes, as they had the feeling that the animals recognized them when they visited, even if they had been away for a long time.
"You get the impression that they respond as if they recognize you and that you are really different to them from the average zoo guest. They are excited to see you again, so our goal with this study was to empirically test if this is the case," explains Krupenye.
The study was carried out with chimpanzees and bonobos at Edinburgh Zoo (Scotland), Planckendael Zoo (Belgium) and Kumamoto Sanctuary (Japan). The process to prepare the work was based on collecting photographs of apes that had changed zoos or had 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 her photos on all eight occasions. that 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.
Researchers believe that humans and chimpanzees and bonobos share this type of memory means that it was probably already present in some common evolutionary ancestor, between seven and five million years ago.
Likewise, 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 for many years of separation.
"This pattern of social relationships that determine long-term memory in chimpanzees and bonobos is similar to what we see in humans: our own social relationships also seem to determine our long-term memory of people," says Lewis.
The work also aims to analyze whether apes miss individuals they are no longer with, especially their friends and family, a trait considered "exclusively human." 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.
