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Scleral pigmentation leads to conspicuous, not cryptic, eye morphology in chimpanzees

Researchers of the National University of Singapore and Leiden University have discovered that chimpanzees and bonobos share the contrasting colour pattern seen in human eyes, which makes it easy for them to detect the direction of someone’s gaze from a distance.

Juan Olvido Perea-García, Mariska E. Kret, Antónia Monteiro, and Catherine Hobaiter
04 September 2019
PNAS - Abstract

Humans are unique in many ways. No other animal communicates with language or builds tools of such complexity. One of the things which is thought to make humans unique is the contrasting colours of our eyes. Researchers have suggested that the contrast between the white of human eyes, known as the sclerae, and the colourful irises allows others to detect the direction of our gaze. Many of our other skills, such as social learning, seem to depend on this. The sclerae of apes’ eyes is often darker. Because of this, researchers have long argued that their gaze is “hidden” or cryptic. This means that other apes would not be able to see where other members of their species are looking. 

A team involving Ph.D. student Juan O. PEREA-GARCÍA and Prof Antónia MONTEIRO, both from the Department of Biological Sciences, NUS together with collaborators from the University of St Andrews, United Kingdom and Leiden University, The Netherlands, has discovered that ape eyes have the same pattern of colour differences as human beings. This suggests that they could also follow each other’s gaze.

Before humans had language, our ancestors might have used the gaze of those around them to help communicate dangers or other useful information. They might not have been able to say “Look over there!”. However, a look in the direction of the predator might be sufficient, as long as it was possible to follow the direction of their gaze. In humans, the contrast between the white and colourful parts of our eyes lets us do this easily. It turns out that our closest primate cousins, bonobos and chimpanzees, also have a strong contrast between the same two areas of their eyes. Bonobos, like humans, have paler sclerae and darker irises, but chimpanzees have a different pattern. They have very dark sclerae, and paler irises. Both of these colour patterns show the same type of contrast seen in human eyes and could help other apes find out where they are looking. Gaze following is an important component of many behaviours that are thought to be characteristically human, so this finding suggests that apes might also engage in these behaviours.

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

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

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