Though our evolutionary paths diverged in the distant past, chimpanzees and bonobos are our closest living relatives and share numerous similarities with us, including what appears to be an impressively long social memory. In a study published earlier this month, researchers discovered that chimps and bonobos can recognize the faces of apes they were friendly with in the past, even after years or even decades apart.
Laura Lewis, a postdoctoral fellow at the University of California, Berkeley and the study’s lead author, hypothesized that chimpanzees could remember their former companions after a lengthy separation based on an experience she had while conducting undergraduate research at the North Carolina Zoo. She became friendly with a male chimp named Kendall, who loved looking at her hands. When Lewis returned after spending four months away, Kendall seemed to recognize her and immediately wanted to see her hands.
It was already known that great apes can remember important details, such as the location of their favorite fruit trees, but this was the first research to look at their social memories specifically. The study involved showing chimpanzees and bonobos two side-by-side photos of apes for three seconds. One photo showed a stranger, while the other showed an ape that the study subject had previously lived with but hadn’t seen for at least a year. The chimpanzees and bonobos chosen for the study live in zoos or ape sanctuaries in Scotland, Belgium, and Japan.
With the apes happily sipping fruit juice to encourage them to keep their heads still, the researchers used eye-tracking technology to record how long the study subjects looked at the photos and found that they spent around a quarter of a second longer looking at their former companions than the unfamiliar apes—a reaction similar to what you might experience if you unexpectedly saw an old high school classmate walking down the street. There also seemed to be a noticeable increase in the length of their gaze when the photo depicted a close companion or relative. For example, a bonobo named Louise appeared to recognize her sister and nephew despite a 26-year separation.
Do you remember me?
- In this study, chimpanzees and bonobos surpassed the previous record for social memory in non-human animals. Captive dolphins have been found to recognize the whistles and vocalizations of other dolphins even after 20 years apart.
- However, some researchers who were not involved in the study cautioned against concluding that the results necessarily indicate recognition but are instead evidence of familiarity—a notable distinction.
- Perhaps the study's most interesting finding is the suggestion that social memory was developed very early in our evolutionary history, as the divergence of humans from chimpanzees and bonobos likely occurred around six or seven million years ago.