I Know That Face! Chimpanzees and Gorillas Can Spontaneously Differentiate Photos of People They Know from Those of Unfamiliar People

Lydia Hopper, Director of Behavioral Management, Research Animal Resources, Johns Hopkins University

Flipping through photos on your phone, your eye will likely be drawn to those of your friends and family. And being able to quickly differentiate people you know from strangers is an important component of how we navigate our daily lives, from maintaining relationships to avoiding potential danger. Primates also live in rich social worlds and those living in captivity also develop relationships with the people who care for them. Therefore, Lydia Hopper and her colleagues wanted to test if zoo-housed apes could spontaneously distinguish their care staff from unfamiliar people. They assessed whether chimpanzees and gorillas would show different attentional biases toward unfamiliar people compared with familiar people. To do so, they showed the apes photos of their care staff and photos of unfamiliar people, presented on a touchscreen, to see which images captured the apes’ attention more quickly. Hopper and colleagues found that the chimps and gorillas differentiated their familiar care staff from unfamiliar people—even when the people’s faces were partially occluded by a face mask. This pattern was consistent for all the animals. The study revealed that primates can spontaneously classify people based on familiarity, from their faces alone. These results show not only that the apes differentiated people with whom they had a relationship over people they did not, but that they made these distinctions rapidly (the photos were only presented on the screen for 300 ms), from two-dimensional greyscale photographs, and from only viewing the person’s face, without behavioral or other context cues, or with any training.