A bonobo named Louise at a sanctuary in Japan helped document the longest-lasting social memory recorded outside our species. When shown a photograph of her sister Loretta, whom she had not seen in more than 26 years, Louise stared intently at the familiar face and barely glanced at a stranger beside it, suggesting apes can retain memories of family and friends for much of their natural lifespan.The findings come from a Johns Hopkins University team led by comparative psychologist Christopher Krupenye and lead author Laura Simone Lewis, and the results were published in the Proceedings of the National Academy of Sciences. The experiment used infrared eye-tracking cameras to measure exactly where 26 chimpanzees and bonobos living across zoos and sanctuaries in Europe and Japan focused their attention when shown paired photographs on a screen. In each pair, one image showed an ape the subject had once lived alongside, sometimes a relative and sometimes simply a former groupmate, while the other showed a stranger.What the eye-tracking data revealedThe apes did not need to be trained to respond to the photographs, and the team measured where their gaze lingered over a three-second window. Across the group, chimpanzees and bonobos looked longer at the faces of former groupmates than at strangers, spending 11% to 14% more time on the familiar faces even though some had been out of sight for decades. Louise, at 46 years old, produced the standout result in the entire dataset. She had lived with her sister Loretta and nephew Erin before being transferred from the San Diego Zoo to Japan’s Kumamoto Sanctuary in 1992. When Lewis and her colleagues showed her the photographs in 2019, she focused on their faces across eight trials and largely ignored an unfamiliar bonobo beside them.Her result stood out against the wider dataset, in which the full group of 26 apes already showed a general tendency to look longer at familiar faces than at strangers.That single case suggests non-human social memory may last far longer than previously assumed. Louise’s recognition of Loretta represents the longest-lasting nonhuman social memory ever documented, according to the study’s authors, surpassing the previous record set in a 2013 study published in Proceedings of the Royal Society B, which found bottlenose dolphins could recognise the calls of former podmates after twenty years apart. Because chimpanzees and bonobos typically live 40 to 60 years in captivity, this suggests they may be capable of remembering individuals for much of their lives.A pattern shaped by old friendships, not just kinshipLooking more closely at the data, memory strength was not simply about blood relations. Apes lingered longest not only on family members but also on individuals with whom they had shared what the team described as clearly positive relationships in the past, including time spent grooming each other or resting in close proximity. That pattern suggested the apes were not just recognising a familiar face but retaining something closer to a memory of the relationship itself. Johns Hopkins University’s own account of the research traces the idea back to Krupenye’s fieldwork, where primatologists who revisit the same zoos and sanctuaries for years often sense that the animals still remember them.
A bonobo (Pan paniscus) sitting in a forested habitat. Credits – Wikimedia Commons
Why the discovery matters beyond the labOutside experts have said the result could mark an important step in understanding memory in great apes, placing long-term social memory among the abilities of our closest living relatives. The findings may also have practical implications. If apes carry detailed memories of former companions for decades, then decisions to separate related or bonded individuals between zoos, sanctuaries and breeding programmes carry a psychological weight that has often gone unacknowledged. A summary of the findings from the University of California, Berkeley, quotes lead author Laura Simone Lewis noting that the finding matters for great ape welfare more broadly, particularly given how often family members and close companions are moved apart during their lifetimes. For Louise, now in her fifth decade of life, the moment of recognition offered researchers a rare look at long-term social memory in a great ape.
