Could the colour of our scleare be connected with our ability to cooperate? Anna Szala from our Centre for Language Evolution Studies is currently researching this, in cooperation with scientists from Leiden, Oakland and Vienna Universities, and with assistance of animals from zoo in Toruń. Juan Olvido, PhD, has prepared a short description of the project.
For a very long time, humans have been touted as having unique eyes, especially because our sclerae (the "white of the eyes") are mostly lacking in pigments. By comparison, chimpanzees - our closest evolutionary relatives - have deeply pigmented eyeballs. Many studies suggest that this may have to do with the evolution tolerance with members of our own species. In a turn of events, recently, marmosets (popularly known as "finger monkeys" due to their tiny size) have also been found to display highly depigmented sclerae - sometimes more than humans. Marmosets are known for being extremely cooperative, forming tightly bound groups in which everyone helps rearing infants. This has led some researchers to think that the loss of scleral pigments may signal tameness, or reduced aggression. In fact, the CLES, who organizes this study, is currently testing whether humans perceive others with lighter sclerae as less aggressive.
However, there could be other reasons why humans think of scleral depigmentation as signaling lack of aggression. We are so accustomed to depigmented eyeballs that seeing anything else looks weird and uncomfortable. Try to think back of the first black and white version of Mickey Mouse, and the more modern one, to get an idea. There's also connotations that we culturally assign to different colorations that do not necessarily respond to biological reality. To get out of this conundrum, testing other species can help us understand whether the association between scleral depigmentation and aggression is rooted in biology.
The Botanical and Zoological Garden of Torun has a pygmy marmoset family that is ideal to test whether they also perceive eyes with depigmented sclerae as less aggressive. To do this, we are showing them movies of things we think they might find interesting - mostly, their favorite foods as well as colorful videos. When the marmosets approach to watch the videos, we quickly switch to a photo of another pygmy marmoset. These photos have been manipulated so that they show exactly the individual but presenting either dark sclerae, light sclerae, or closed eyes. If scleral depigmentation indeed signals lack of aggression, we expect the pygmy marmosets to be less startled, and to make less displays and vocalizations when we show them the photos manipulated to display lighter sclerae.