Chimps And Bonobos Track Eye Gaze Like Humans

Chimpanzees and bonobos share the contrasting color pattern seen in human eyes, which makes it easy for them to detect the direction of someone’s gaze from a distance.

AsianScientist (Sep. 19, 2019) – Non-human apes can follow one another’s gaze, say an international team of scientists. Their findings are published in the Proceedings of the National Academy of Sciences.

Studies have suggested that the contrast between the white of human eyes—known as the sclerae— and the colorful irises allows others to detect the direction of our gaze. The ability to detect gaze is important as many other human skills, such as social learning, seem to depend on this.

In contrast, as the sclerae of apes’ eyes is often darker than human eyes, researchers have long argued that their gaze is ‘cryptic,’ or hidden. This means that non-human apes would not be able to see where other members of their species are looking.

Now, researchers from the National University of Singapore (NUS), together with collaborators from the University of Saint Andrews, UK, and Leiden University, the Netherlands, have discovered that ape eyes possess the same pattern of color differences as human beings.

The research team compared the darkness of the sclerae, contrasted with irises, of over 150 humans, bonobos and chimpanzees. The researchers found that bonobos, like humans, have paler sclerae and darker irises. Chimpanzees were found to have a different pattern, with very dark sclerae and paler irises. Both of these color patterns show the same type of contrast seen in human eyes and could help other apes find out where they are looking at.

“Humans are unique in many ways, as no other animal can communicate with similar intricate language or build tools of such complexity. Gaze following is an important component of many behaviors that are thought to be characteristically human, so our findings suggest that apes might also engage in these behaviors,” said Mr. Juan O. Perea-García, doctoral student at NUS.

Apart from helping us understand how our ancestors communicated, this study suggests some interesting new research directions. For example, the researchers intend to pursue questions pertaining to why human beings and bonobos evolve in a similar way, despite bonobos being more closely related to chimpanzees.

“We know that some gorillas and orangutans have eye coloration like our own, and some members of these species have eye coloration similar to the chimpanzees, but why is there this variation within a species? We are working with several zoos to find out more,” said Perea-García.

The article can be found at: Perea-García et al. (2019) Scleral Pigmentation Leads to Conspicuous, Not Cryptic, Eye Morphology in Chimpanzees.


Source: National University of Singapore; Photo: Pixabay.
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