Vita gazette – Researchers found that chimpanzees and bonobos could recognise photos of former groupmates more than 25 years after last seeing them in the flesh, with pictures of old friends eliciting an even more positive response.

The monkeys can recognise, through the sole observation of photographs, group mates whom they have not seen for more than 25 years and respond with extreme enthusiasm to the vision of photos in which their friends appear. This is demonstrated by a study published in Proceedings of the National Academy of Sciences journal. The work provides evidence of the longest-lasting social memory ever documented outside humans and highlights how human culture evolved from common ancestors shared with apes.

Senior author Christopher Krupenye, an assistant professor at Johns Hopkins University who studies animal cognition, told CNN the research was inspired by his experience working with apes and sensing that they recognised him even years after their last interaction.

To test this, Krupenye and lead author Laura Lewis, a biological anthropologist and comparative psychologist at the University of California, Berkeley, used photographs of apes who had died or left groups at Edinburgh Zoo in Scotland, Planckendael Zoo in Belgium and Kumamoto Sanctuary in Japan.

The team selected individuals whom the participating apes hadn’t seen for periods ranging from nine months to 26 years, who had high-quality images available on file, and noted what kind of relationship the participants had had with the individuals.

Researchers then left two photographs -one of the apes they had known and another of a stranger- accessible to the apes and used a noninvasive eye-tracking device to measure where they looked and for how long.

Results showed that the apes looked “significantly longer” at those they knew, no matter how long since they last saw them, and even longer at those they had been friendly with.

Researchers believe that apes’ social memory could extend beyond 26 years and even be comparable with humans, who start to forget people after 15 years but can remember them for as long as 48 years, the release added.

Next, the team plans to investigate whether apes can recognise former friends as they look now, rather than when they left the group, and whether other primates, such as gorillas and orangutans, also possess long-lasting social memories.