Mind & body, Research, Science & environment

Bonobos and chimps show ‘a rich recognition’ for long-lost friends and family

Extensive social memory had previously been documented only in dolphins and up to 20 years. "What we're showing here," said UC Berkeley researcher Laura Simone Lewis, "is that chimps and bonobos may be able to remember that long — or longer."

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A mother and baby chimp hug each other atop a wooden structure in a human-made habitat
“This study is reminding us how similar we are to other species walking on the planet,” said Laura Simone Lewis, a comparative psychologist at UC Berkeley. “And therefore, how important it is to protect them.”

Courtesy of Laura Simone Lewis

Bonobos and chimpanzees — the closest extant relatives to humans — could have the longest-lasting nonhuman memory, a study led by a UC Berkeley researcher found. Extensive social memory had previously been documented only in dolphins and up to 20 years.

“What we’re showing here,” said Berkeley comparative psychologist Laura Simone Lewis, “is that chimps and bonobos may be able to remember that long — or longer.”

Berkeley News writer Jason Pohl first published a story about this study in December 2023. We used his interview with Lewis for this podcast episode.

Read a written version of “Bonobos and chimps show ‘a rich recognition’ for long-lost friends and family”:

UC Berkeley comparative psychologist Laura Simone Lewis has worked with great apes for more than a decade. And she has noticed something about their behavior.

“A lot of us who work with great apes aren’t able to be with the populations that we work with all the time,” said Lewis. “Especially for those of us who are living in the U.S. and working at U.S. institutions. Often, we have to travel to the ape populations that we work with. So that means that we’re spending periods with them, and then periods apart.

“And for a long time, I think a lot of people who study great apes realized that when you leave and come back, they don’t treat you like this new person. They seem to recognize us. They seem to show behaviors that indicate they remember us.”

This got Lewis, a UC President’s Postdoctoral Fellow in Berkeley’s psychology department, wondering: Just how long is the social memory of great apes? Does it last for years, even decades?

“We, as humans, have really long-term social memory,” she said. “We can remember the faces of others, even if we haven’t seen pictures of these people for at least 30 years, if not more like 40 or 50 years.

“I think for a lot of us who study apes, the question is not: Are humans special in that way? But: Do nonhuman apes have those abilities, too?”

“And what Lewis and a team of researchers from Harvard University, Johns Hopkins University and St. Andrews University in Scotland discovered wasn’t how different we are from other apes, but how similar we are.

Lewis and her team decided to study bonobos and chimpanzees, the two species of great apes that are the closest extant relatives to humans.

To begin the project, they identified bonobos and chimps in captivity that had been separated from family members or separated from what we humans would call friends. The populations they worked with were from zoos in Europe and a sanctuary in Japan.

Separations happened for different reasons. Sometimes, their group mates had been relocated to other zoos to prevent inbreeding. Other times, a sibling or elder may have died while they were living together.

To conduct the study, Lewis and her team set up a computer system with sensitive cameras and non-invasive eye-tracking tools. They had photos on screens to show the apes of friends and family they hadn’t seen for up to 10 years.

As someone who works with captive apes, it’s really important to think about ethics and to be very transparent,” said Lewis.

“So, No. 1, this study was completely voluntary. The apes decide to come up to the research window. They get a little bit of diluted juice. It’s completely voluntary. They choose when to come up, and they choose when to leave. And it’s also completely non-invasive. So it’s called non-invasive eye tracking. It’s not harmful at all to them. No harmful effects to them at all.

“But you can imagine, when we were about to show them pictures of groupmates that had died, we were really thoughtful about the effects on the animals that we were working with. We did not want to distress them in any way. So we were really cautious around that. And when we started doing this study, we had two keepers sitting and watching for any signs of stress from the chimps and bonobos, and we didn’t see any.”

What they did see was that some of the apes showed signs of curiosity.

When shown a photo of a former friend or family member next to a photo of a stranger, participants’ eyes stayed significantly longer on the images of those with whom they’d previously lived. This suggested some degree of recognition.

“Typically, they’d be drinking the juice and just kind of drinking it and watching,” Lewis said. “Some of them would stop drinking the juice, pause, pull away and just kind of stare at these images.”

Chimps and bonobos, Lewis said, are obsessed with juice and eat lots of fruit in the wild.

“For them to stop drinking,” she said, “to be so intrigued by what they’re seeing on the screen that they stop drinking juice, it seems like they forget about it almost and are just so pulled in,  drawn into these images, gazing at a screen.

In an extended sample group, a bonobo named Louise hadn’t seen her sister, Loretta, or her nephew, Erin, for more than 26 years. But when researchers showed Louise their images, her eyes homed in on the photos.

I think the story of this study is really beautiful, right?” Lewis said. “We’re showing that these animals have a rich recognition of each other. You know, we don’t know exactly what that representation looks like, but we know that it lasts for years.

“We know these individuals, for them, their social relationships are incredibly meaningful in their lives. Just as our social relationships as humans are incredibly meaningful for our well-being and survival. And I love what this study is showing us is not how different we are from apes, but how similar we are to nonhuman apes.

Up until this point, the longest documented social memory in a nonhuman animal that went beyond just a few years has been found only in dolphins. Studies have found that dolphins can recognize vocalizations for up to 20 years.

And what Lewis and her team’s research shows is that chimps and bonobos may be able to remember that long — or longer.

The next step, Lewis says, is to figure out what that memory looks like.

“Is it kind of just a low-level recognition?” she said. “Is it kind of this rich memory of the relationships that we had with these individuals, the times they’ve spent together, hardships they may have faced together, things like that?

“We know they recognize these individuals they haven’t seen for years. But what the content of that memory looks like, we don’t know yet. I think we’ll get closer in future studies to follow up.”

You can read more about these findings in the journal, Proceedings of the National Academy of Sciences, where the study was published in December 2023.

This study was first written about by my colleague, Jason Pohl. You can read his story on Berkeley News at news.berkeley.edu.