Apes recognise photos of friends and family they have not seen for more than 25 years, researchers have found.
Some even respond enthusiastically to pictures of long-lost comrades, demonstrating the longest-lasting social memory ever documented outside humans.
Professor Christopher Krupenye, from Johns Hopkins University, said it suggested not just familiarity, but that the primates keep track of the nature and quality of specific relationships.
“This work clearly shows how fundamental and long-lasting these relationships are,” he said.
“Disruption to those relationships is likely very damaging.”
Ape recognised sister after 26 years
Professor Krupenye’s team were inspired to research the memories of apes after sensing they recognised them, sometimes even after a long absence.
They worked with chimpanzees and bonobos at three zoos around the world, including Edinburgh Zoo.
The researchers collected photos of apes that had either left the zoos or died, and collected information about the relationships they’d had with those still at the zoos.
The apes they showed the pictures to hadn’t seen them for at least nine months, and in some cases much longer.
A bonobo who took part in the study, named Louise, hadn’t seen her sister Loretta or nephew Erin for more than 26 years and showed a particular interest in photos of them both.
The apes were shown photos of old friends and family members side-by-side with photos of strangers, and the researchers used eye-tracking to judge their interest in them
The apes looked significantly longer at former group mates, no matter how long they’d been apart.
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Could apes miss old friends?
Lead author Laura Lewis, from the University of California, said the study’s results were comparable to how relationships shape the memories of humans.
It could suggest apes may even miss old group mates, too.
“The idea they do remember others, and therefore they may miss these individuals, is really a powerful cognitive mechanism and something that’s been thought of as uniquely human,” she added.
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Researchers hope the findings will shed new light on how deeply affected apes could be when poaching and deforestation separate them from their friends.
They also plan to explore whether these long-lasting social memories are special to great apes, or something other primates experience, and if they have similarly lasting memories for experiences as well as individuals.
The peer-reviewed findings have been published in the Proceedings of the National Academy of Sciences journal.
