Chimpanzees are highly social animals, often living in communities with as many as 50 members, though groups of over a hundred are not unheard of. This requires quite a lot of mental processing power to be able to identify each individual, especially in the dark, shadowy rainforests of central Africa. So how do they do it? It turns out that chimps manage facial recognition in a similar way to humans.
Researchers from Kyoto University, Japan, set out to test how chimps search for faces hidden among other objects, and what impacts their ability to do so. They tested three adult female chimps, getting them to select the faces of unknown apes from a randomized selection of other objects such as trees, rocks and trumpets. The apes were easily able to achieve this, and the amount of time it took them to correctly select a face remained roughly the same throughout the experiment, ruling out the possibility of "overlearning." However, as soon as the faces of the chimps were obscured or altered, for example turning them upside down, their ability dropped significantly.
The results of the study, published in Scientific Reports, suggest that the apes use what’s called “holistic processing,” where they make a quick rapid analysis of the information that forms a face. As faces convey all sorts of vital information that is critical for highly social animals, the ability to rapidly recognize faces is of great advantage.
As one of the co-authors of the paper, Masaki Tomonaga, explained to IFLScience, the results imply that the inner features of the face – the eyes, nose and mouth – are most important for chimp facial recognition. Humans too use the inner features, but also look at outer features to further refine the process. This was demonstrated when the chimps were asked to identify chimp faces that had the inner features blurred, a task which they found harder to do.
This suggests that chimpanzees recognize and identify faces in a similar way to how some people believe newborn babies do. Newborns naturally orient towards faces, and some evidence has shown that they might do this by recognizing “three high-contrasting blobs corresponding to the approximate location of the eyes and mouth,” or an inverted triangle layout.
In fact, Tomonaga explained that in other research he has found that chimps also find this positioning important, and demonstrated that chimps identified a house with the windows arranged in this configuration as a face. This processing might also explain why the chimps were able to recognize human faces but not those belonging to monkeys, because their features were less highly contrasted against darker skin.
Another important factor to take into account, however, is the fact that the chimps used in the study were habituated to humans. This could have impacted their ability to recognize human faces over monkey ones, something Tomonaga thinks would be interesting to test in wild apes out in the field. He hopes to see if other species are also able to process faces in the same way.