Side-Eye May Be a Common Expression Among Primates Other Than Humans

A photo of a chimpanzee with white on the eyes gives the feeling that something’s slightly off because most primates have a dark sclera, which in humans is the white part of the eye that surrounds the iris and pupil. This means that chimpanzees can also give side-eye as a communication signal.

Chimpanzees Can Give Side-Eye

White sclera evolution was thought to be tied to communication because humans have an impressive ability to tell a lot from just one look. So, side-eye means a person is not happy. Rolling the eyes upwards means something is wrong, and so forth. But what is the situation when it comes to other primates? Chimps and bonobos have very complex social groups and communicate through all kinds of gestures with humans being able to translate half of them. While it was thought that a white sclera was unique to humans, studies have found that this is not actually the case.

Cases of chimpanzees with white sclerae were believed to be anomalies caused by a mutation or an illness, but a new review has determined that eye color was highly variable in a big group of wild chimpanzees in Uganda. Of 230 individuals, 34 had white sclerae, with some of these having them visible even when they were looking straight ahead.

Gorillas Also Have White Sclera

According to the authors of the study, scholarly literature and popular media often regard the white sclera as a characteristic unique to humans. However, their research proved otherwise, as almost one-sixth of the chimpanzees they looked at displayed full or partial white sclera in at least one eye. This feature was more noticeable when the chimpanzees averted their gaze rather than when they looked directly at something, meaning they could give side-eye. The authors noted that even a minimal amount of white or light sclera could enhance the visibility of gaze direction.

The new research is built on existing papers that have shown that some gorillas also have white sclerae. In 2022, an experiment tested whether humans and chimpanzees could discriminate the eye-gaze direction in photos of both species. These findings could mean that how people react to the gaze of conspecifics and how they use their eyes to communicate may have been the reason for the evolution of the white sclera.