Looking the ape in the eye – Cooperative eye hypothesis questioned
The white of our eye is something special. The sclera is devoid of pigment, which is why we can easily follow where our counterpart is looking. Nature has arranged it this way to facilitate this kind of glance-based communication. That is at least the traditional notion among scientists. A team of zoologists based at the University of Duisburg-Essen (UDE) and the Anthropological Institute in Zurich is now challenging this traditional view in a new study. The researchers looked at communicative behavior and eye color in apes and question the proposed connection between the two phenomena. The results have just been published in the international journal 'Nature - Scientific Reports'.*
For a long time, science has assumed that the pronounced contrast between the light sclera and the dark iris in the human eye is an adaptation to effectively convey gaze signals. "Part of this hypothesis is based on the idea that among primates, only humans have white sclerae," says study leader Kai Caspar (UDE). "However, only few comparative data have been available to back up this claim. Therefore, we assessed scleral pigmentation and measured eye contrast values in photos of more than 380 hominoids from 15 species. These included humans, great apes such as chimpanzees and orangutans, and gibbons, the small apes."
Although all hominoids are closely related, they communicate by different means. UDE zoologist Caspar says, "Different from us humans, glances play only a subordinate role in great ape communication, and for the gibbons they seem to have no communicative significance at all. So if the traditional assumption were true, differences in pigmentation should comply to differences in communicative behavior: the lighter the sclera, the more are the eyes used to convey information."
But this is not the case, as the study was able to show. Neither is the white of the human eye unique, nor can a connection be made between scleral color and communicative demands. "The expression of contrast in our eyes is not significantly different from that in some great apes, such as the Sumatran orangutan. Interestingly, however, scleral pigmentation can sometimes be highly variable within the same ape species. In humans there is only plain white. This uniformity is a quite unusual extreme."
The zoologists around Kai Caspar fully reject the common assumption that the lightening of our sclera arose for the purpose of effective communication. Instead, they suspect other evolutionary mechanisms such as genetic drift or sexual selection to be at play: "These may have altered the appearance of our eyes in comparison to that of our closest living relatives."
THe picture shows a female Sumatran orangutan (Photo: UDE/Kai Caspar).
Kai R. Caspar, General Zoology/University of Duisburg-Essen, T. +49 201/183 4310, email@example.com
* Kai R. Caspar, Marco Biggemann, Thomas Geissmann & Sabine Begall: Ocular pigmentation in humans, great apes, and gibbons is not suggestive of communicative functions, Nature Scientific Reports 11, 12994 (2021). https://doi.org/10.1038/s41598-021-92348-z