Great apes and chimpanzees, our closest living relatives, can recognize groupmates they haven’t seen in over two decades, a new study shows.
It’s evidence of what’s believed to be the longest-lasting nonhuman memory ever recorded.
The findings also bolster the theory that long-term memory in humans, chimpanzees, and bonobos likely comes from our shared common ancestor that lived between 6 million and 9 million years ago.
For the study, published in the Proceedings of the National Academy of Sciences, the team used infrared eye-tracking cameras to record where bonobos and chimps gazed when they were shown side-by-side images of other bonobos or chimps. One picture was of a stranger; the other was of a bonobo or chimp that the participant had lived with for a year or more.
“Chimpanzees and bonobos recognize individuals even though they haven’t seen them for multiple decades,” said senior author Christopher Krupenye, an assistant professor at Johns Hopkins University who studies animal cognition.
“And then there’s this small but significant pattern of greater attention toward individuals with whom they had more positive relationships. It suggests that this is more than just familiarity, that they’re keeping track of aspects of the quality of these social relationships.”
Participants’ eyes lingered significantly longer on images of those with whom they had previously lived, suggesting some degree of recognition. In one case, a bonobo named Louise had not seen her sister, Loretta, or nephew, Erin, for over 26 years. But when researchers showed Louise their images, her eyes homed in on the photos.
“These animals have a rich recognition of each other,” says Laura Simone Lewis, a postdoctoral fellow in the psychology department at the University of California, Berkeley.
What’s more, participants looked longer at individuals with whom they had had more positive, as compared with antagonistic, relationships. In other words, they seemed to recognize friends more than foes.
“We don’t know exactly what that representation looks like, but we know that it lasts for years,” she says. “This study is showing us not how different we are from other apes, but how similar we are to them and how similar they are to us.”
The findings expand what was known about long-term memory in animals and also raise questions central to evolutionary biology and psychology. Chief among them: How did humans develop such good long-term memory?
Understanding the links between our vivid, episodic memory and the recall of other animals has long been a research puzzle. Previous studies have shown that ravens, for example, remember people who tricked them and can recall social relationships in uncanny ways. Social memory beyond just a few years had previously been documented only in dolphins, which studies have found can recognize vocalizations for up to 20 years.
“That, up until this point, has been the longest long-term social memory ever found in a nonhuman animal,” Lewis says of the dolphin research. “What we’re showing here is that chimps and bonobos may be able to remember that long—or longer.”
Lewis’s project was one born from a longtime observation among primate researchers, who often go months or years between seeing the animals they study. When they returned, bonobos and chimpanzees acted as if they were picking up right where they left off. So the researchers decided to see if that memory hunch was true.
To get answers, the team began what at times was equal parts genealogy and scrapbooking.
First, they needed to identify bonobos and chimps that had been separated from what we might view as friends or family. Sometimes, their groupmates had been relocated to other zoos to prevent in-breeding. Other times, a sibling or elder may have died while they all lived together.
With a list of pairs in hand, sprinkled across zoos in Europe and Japan, the researchers needed to track down photos to show the participants. It couldn’t be just any snapshot, however. They needed a quality image taken from around the time that the pair last saw one another. This was somewhat easy for the animals that were separated recently in an era rich with high-quality photos. It proved much trickier for others, like Louise’s relatives, who were separated circa 1995.
The team ended up being able to show images to 26 bonobos and chimpanzees.
After setting up a computer system with sensitive cameras and non-invasive eye-tracking tools, participating animals were allowed to enter the room voluntarily. Their compensation? A bottle filled with diluted juice. (Bonobos and chimps love fruit juice and eat lots of fruit in the wild.)
As they sipped, the screens in front of them alternated between pairs of images. The cameras monitored where the animals’ eyes wandered. And the computer logged the time spent on each image down to a fraction of a second—data the team would comb through months later.
“It was a really simple test: Do they look longer at their previous groupmate, or are they looking longer at the stranger?” Lewis says. “And we found that, yes, they are looking significantly longer at the pictures of their previous groupmates.”
Lewis says she and others were especially concerned about how the participants might react when they were shown an image of a relative they hadn’t seen in years. As the project began, zookeepers monitored the animals for signs of stress. But they didn’t show any markers of agitation. Instead, when images of a once-close relative appeared on the screen, the participants would sometimes stop drinking the juice entirely, seemingly mesmerized by the image.
The study showed that something is happening with the mind in recognizing the images. What’s unclear is what kind of memories they were. Could they have been rich, episodic narratives like humans have? Might there have been some fleeting curiosity about why they saw this? Can they extrapolate what those relatives might look like today?
“This study is reminding us how similar we are to other species walking on the planet,” Lewis says. “And therefore, how important it is to protect them.”
Additional coauthors are from Harvard University, Johns Hopkins University, Kyoto University, the University of Antwerp in Belgium, and the University of Konstanz in Germany.
Source: Jason Pohl for UC Berkeley
