Whether it is a sea of faces at a school reunion or distant family at a wedding, our ability to remember people we met years ago can come in handy. Now it seems our evolutionary cousins have a similar skill.
Researchers have found bonobos and chimpanzees can recall peers they spent time with in the past, even if they have been separated for decades. What is more, this recognition appears to be influenced by whether they got on well with each other – or not.
“These results represent some of the longest long-term memory ever found in nonhuman animals. It is also one of the very first studies to show that apes’ memories may be shaped by their social relationships,” said Dr Laura Lewis, the first author of the research who is based at the University of California, Berkeley.
“It is surprising because the length and nature of this social memory is so similar to our own human long-term memory.”
Lewis added that previously the longest known memory in non-human animals was in dolphins, who can remember the vocalisations of others for 20 years, while bonobos had been found to remember vocalisations of previous group mates for up to five and a half years.
Writing in the Proceedings of the National Academy of Sciences, Lewis and colleagues reported how they made their discovery by showing 26 bonobos and chimpanzees side-by-side pictures of other members of their species.
Two images were shown at a time, one depicting an individual they had not met before, and the other showing a former group mate who had either died or been moved to another location at least nine months before. During the process the team tracked the gaze of the apes.
The results suggest that, on average, the apes spent longer looking at images of former group mates than strangers, although this finding was most robust for the 12 apes at Kumamoto sanctuary in Japan, who are most used to screen-based eye-tracking experiments.
“In the most extreme case, bonobo Louise had not seen her sister Loretta nor nephew Erin for over 26 [years] at the time of testing,” the team wrote. “Strikingly, she showed a robust attentional bias toward both Loretta and Erin.”
The results also suggest the apes spent longer looking at former group mates if their relationship had generally been positive rather than negative.
“The positive relationships that apes have with others are characterised by spending more time in close proximity to each other and grooming each other. Thus, it could be that these relationships are more salient to apes even after years of separation,” said Lewis.
Lewis said further work was required to understand why this type of long-term memory may bring evolutionary benefits to the apes, but suggested one factor could be that when females reach reproductive age, they leave the groups in which they are born to join neighbouring groups, primarily to avoid inbreeding.
“So it could be the case that having long-term memory for others aids in these types of social dynamics where they don’t see individuals for long periods of time,” said Lewis.
Dr Christopher Krupenye from Johns Hopkins University, another author of the research, said that while the experiments suggested apes were just as good at remembering relatives as non-relatives, that could be down to the size of the sample, or reflect that relationships outside family also matter – pointing to strong bonds and even alliances among non-relatives.
“Just having a rich model of your social world is critical, regardless of whether the individual is [family],” he said.
