What may put some of us prosopagnoses to shame is that apes instantly recognize family and friends they haven't seen in more than two decades, the longest "social memory" ever recorded among non-human animals. The Johns Hopkins University study provides important insights into how human social cognition evolved, with researchers noting how animals seem to recognize the humans they spend time with, even if they are separated by long periods of time.
"We tend to think of great apes as completely different from ourselves, but we do see that these animals possess very similar cognitive mechanisms to us, including memory," said lead author Laura Lewis, a biological anthropologist at the University of California, Los Angeles. "I think that's what's exciting about this study."
The researchers worked with chimpanzees and bonobos at Edinburgh Zoo in Scotland, Plankendal Zoo in Belgium and Kumamoto Sanctuary in Japan to prepare a set of photos featuring a familiar animal and an unfamiliar animal. The apes were free to come and go from areas with juice feeders (one researcher specifically noted that there was no sugar). The window above the juice spout displayed two images unique to each animal, one of a photo of a member of the group that had either died or been moved to another enclosure, and a second photo that the apes had never seen before. Their attention was then measured using non-invasive eye-tracking equipment to determine whether the animals were indeed spending more time looking at "friends" or family members. They did it.
"You get the impression that they're responding as if they know you, that you're really different to them than the average zoo visitor," said senior author Christopher Krupenye, an assistant professor at Johns Hopkins University and an expert on animal cognition. "So our goal with this study was to ask empirically whether that's the case: Do they really have strong and lasting memories of familiar social companions?"
They found that great apes spent more time observing animals that had been part of their social group and with which they had a positive association.
A bonobo named Louise hadn't seen her sister Loretta and nephew Erin in 26 years, but throughout eight trials she consistently focused on images of them compared to unfamiliar animals.
The findings are exciting to researchers because social memory appears to be an important factor in the evolution of our species, given that humans share a common ancestor with these animals.
"This pattern of social relationships that shape long-term memory in chimpanzees and bonobos is similar to what we see in humans, and our own social relationships also appear to shape our long-term memories of individuals," Lewis said.
Of course, the study also raises the question of whether these animals long for the apes they're separated from, and how close that is to the human experience.
"They do remember people, so they may miss those people, which is a really powerful cognitive mechanism and is thought to be unique to humans," Lewis said. "Our study doesn't confirm that they are doing this, but it raises questions about whether they are capable of doing it."
The researchers now hope to expand the study to other primates to see if subtle differences in eye contact can reveal more about the nature of past relationships. A better understanding of these complex connections could also help manage the welfare of animals in captivity.
"This work clearly demonstrates how important and enduring these relationships are and that disruptions to them can be very damaging," Krupenje said.
The research was published in the journal Proceedings of the National Academy of Sciences.