A study published on Dec. 18 to the scientific journal PNAS showed evidence that apes, including bonobos and chimpanzees, can recognize old zoo-mates they haven’t seen in years.

Apes’ attentional biases not only appear to reflect basic features of familiarity but may also track complex properties of their social relationships

How was the study conducted?

The study included 26 bonobos and chimpanzees in several zoos and sanctuaries across Europe and Japan. An ape would voluntarily walk into a room where a bottle of diluted juice was set up in front of a monitor.

While the apes drank the juice, they looked at side-by-side photos of a previous group-mate and a stranger of the same gender. An infrared eye-tracker recorded the apes’ eye movements, and analysts found that apes looked at the previous group-mate for a significantly longer amount of time than at the stranger.

Laura Simone Lewis, lead author of the study, said, “It was a really simple test: Do they look longer at their previous group-mate, or are they looking longer at the stranger?” per UC Berkeley.

She added, “And we found that, yes, they are looking significantly longer at the pictures of their previous group-mates.”

In one case, a 46-year-old bonobo, Louise, “showed a robust attentional bias toward” photos of her sister Loretta and her nephew Erin, the study explained. Though she hadn’t seen either of these apes for over 26 years, she looked at Loretta and Erin longer across eight different trials.

Apes are more likely to remember positive relationships

The study showed that test subjects were more likely to look at apes they’d previously had positive relationships with for a longer period of time, while they were likely to not focus on the faces of apes that had scared or hurt them.

Time spent apart didn’t impact the apes’ recognition of previous group-mates. The study compared this to human social memory, “which begins to decline after ~15 y(ears) but can persist 48 y(ears) beyond separation.”

The study added that understanding apes’ ability to remember social interactions will help scientists identify when this ability first appeared in human evolution.

Ape memory versus dolphin memory

Before this study, the longest nonhuman social memory was believed to belong to dolphins. A study reported on by Science explained that dolphins each have a unique whistle.

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The lead author of the study, Jason Bruck, recorded over 71 whistles from 43 dolphins over five years. He played several whistles to each dolphin through a submerged speaker.

“They don’t pay much attention to signature whistles of dolphins they don’t know,” Bruck said. But when he played the whistle of another dolphin they used to know, “They will hover around it, whistle at it, seemingly try to get a response,” Bruck explained.

Lewis compared this new study with previous dolphin research. She said, “That, up until this point, has been the longest long-term social memory ever found in a nonhuman animal,” per UC Berkeley. “What we’re showing here is that chimps and bonobos may be able to remember that long — or longer.”

A biological anthropologist at the University of Utah, Rachna Reddy, said this study on apes opens “a window into the incredibly rich interior emotional life of chimpanzees,” per Science.

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