Long believed to be a uniquely human trait crucial for communication, visible white sclera - the "whites of the eye" - occur more commonly in chimpanzees and other mammals than previously reported, finds a new study from researchers at The University of Texas at Austin.
The study, published in the Journal of Human Evolution, analyzed more than 1,000 photographs of 230 individual wild chimpanzees living at Ngogo, a habitat in Kibale National Park in Uganda. The researchers assessed sclera color in animals as young as 1 month to as old as 68 years and found that 15% of chimpanzees in the sample had white sclera, and an additional 41% had some other form of lighter sclera, such as tan or brown sclera, or sclera with patches of lighter color. Light and white sclera were most common in infants under 1½ years old and often darkened as the animals aged. In chimps with dark sclera, 75% exhibited pale irises, which also created contrast that made it easier to observe the direction of the animals' gaze.
In most nonhuman animals, sclera are dark, making it difficult to discern where individuals are looking. Scientists have long hypothesized that humans' white sclera facilitated the evolution of our complex social communication and tended to view instances of lighter sclera in other species as anomalous.
"Psychologists interested in human evolution took that finding and ran with it. They began theorizing how unique aspects of our cognition, like our ability to cooperate and communicate, were made possible by our contrasting eyes," said Aaron Sandel, assistant professor of anthropology at UT Austin and one of the authors of the study. "This idea became so ingrained that some scientists overlooked obvious exceptions to this rule."
Recent studies reported white sclera in other primates but failed to find significant prevalence in chimpanzees, humans' closest living relatives. This new study used a larger sample size than previous research, to which the authors attribute their ability to observe so much variation in sclera color.
"The fact that chimpanzees supposedly have uniformly dark sclera has been used as evidence to support the narrative that they are more competitive than cooperative," said Isabelle Clark, a UT Austin doctoral candidate and co-first author of the paper. "This is the first study to look at such a large sample of wild, individually identified chimpanzees, made possible by the sheer numbers at Ngogo, along with Kevin Lee's skilled and prolific photography. Just like in humans, there is a lot of variation between individuals and perhaps populations of chimpanzees, which may get overlooked if we only consider a small sample."
The team also looked at sclera color in 70 species of zoo animals and found that 19 of these had at least one individual with white sclera.
The study, which included researchers from Arizona State University, Tufts University, University of Michigan and Yale University, could eventually help explain how the human eye developed into its present-day form.
"More species than previously thought have white sclera. Which are those species? What do they have in common? Our finding provides a potential mechanism for how white sclera could have evolved in humans," Sandel said. "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."
