A chimpanzee looks at the camera from behind a tree branch

UT Joint Study Finds Surprises About Humanity in Chimpanzee Eyes

  • Feb 23, 2023

Scientists have long believed the visible “whites of the eye,” called sclera, were a uniquely human trait. But a new study from researchers at The University of Texas at Austin reveals that sclera are more common in other mammals than previously reported.  

The study analyzed more than 1,000 photographs of 230 wild chimpanzees at the Ngogo Habitat in Kibale National Park in Uganda to assess their sclera colors. It found that 15 percent of the chimpanzees sampled had white sclera, and 41 percent had lighter tan or brown sclera, or sclera with patches of lighter colors. Light and white sclera were most common in infants and often darkened as animals aged. 

In nonhuman animals, the sclera is usually dark, making it harder to determine the direction in which they are looking. Because of this, scientists hypothesized that humans' white sclera aided the evolution of our complex social communication. 

The authors attributed their ability to observe so much variation in sclera color, in comparison to previous research, to a larger sample size. The team also observed 70 species of zoo animals and found that 19 of these populations contained at least one individual with white sclera. 

“Our finding provides a potential mechanism for how white sclera could have evolved in humans,” said Aaron Sandel, assistant professor of anthropology at UT Austin and an author of the study. “If it was somehow beneficial for humans in our ancestral environment to have white sclera, natural selection could have acted on developmental patterns already in place in our ape ancestors.” 

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