It's the 'Longest-Lasting Nonhuman Memory'

Long-term memory goes back a long, long time: perhaps some 7 million years, according to new research on humans' closest living relatives. Researchers led by Laura Simone Lewis, a comparative psychologist at the University of California, Berkeley, showed 26 bonobos and chimpanzees side-by-side images of strangers and former groupmates, including deceased members and those relocated to other zoos, some of whom hadn't been seen by the participants in more than 25 years. The bonobos and chimps' eyes lingered significantly longer on the images of their former groupmates than those of the strangers, suggesting a degree of recognition. Indeed, this is thought to be "the longest-lasting nonhuman memory ever recorded," per a release.

Researchers had noticed bonobos and chimps seemed to recognize them even after an absence of years. To test their recognition, they collected high-quality photos of relatives and friends who were likely to have made an impact. One bonobo appeared to recognize her sister and nephew, whom she hadn't seen in more than 26 years, per the release. In some cases, the bonobos and chimps were so captivated by the images of their former groupmates that they stopped drinking the fruit juice that served as their reward during the test. Previous research found dolphins could recognize vocalizations for up to 20 years, but "what we're showing here is that chimps and bonobos may be able to remember that long—or longer," says Lewis, lead author of the study published Monday in PNAS.

It's similar to when humans see an old high school classmate, study co-author Christopher Krupenye, an assistant professor at Johns Hopkins University, tells the Washington Post. "You might do a double take, and it might catch you off guard," he says. As bonobos and chimps "seem to have long-term social memory" just like us, we can assume "our last common ancestor that lived somewhere between 5 to 7 million years ago also likely had long-term social memory that would've aided them in doing more and more complex things," Lewis says in a video. She and colleagues next hope to explore what's happening in the mind of the bonobos and chimps. "Could they have ... rich, episodic [memories] like humans have?" asks the release. "Can they extrapolate what those relatives might look like today?" (More memory stories.)