White chimpanzee update#

This is likely because humans use eye-gaze in key social activities (including learning languages, coordinating to complete complex tasks and transmitting cultural information), indicating that white sclerae may have evolved to enhance human-specific communication.

Kano, Kawaguchi and Yeow’s findings indicate that white sclerae do indeed help both humans and chimpanzees to discriminate the direction of eye-gaze, even though only humans have white sclerae. Moreover, artificially altering the eyes in the chimpanzee images so that they were more human-like – that is, had a light-colored sclera and a darker iris – enabled both humans and chimpanzees to better discriminate the eye-gaze directions of the chimpanzees. The experiments found that both humans and chimpanzees were better able to discriminate the directions of eye-gaze from the images of humans than those of chimpanzees, particularly when the images were smaller or more shaded. Here, Kano, Kawaguchi and Yeow presented human and chimpanzee participants with images of other humans and chimpanzees on a computer screen and asked them to indicate the direction of eye-gaze in each image. However, there is a lack of experimental evidence as to whether white sclerae actually helps humans to distinguish the direction of eye-gaze. Some researchers believe that the white sclera of the human eye may have evolved to make it easier for other individuals to detect the direction of our gaze. The front of the human eye has a large white area known as the sclera that surrounds the darker colored iris and pupil in the center.Ĭompared to us, chimpanzees and other nonhuman great apes have sclerae that are much darker in color or at least not as uniformly white as human eyes. Editor's evaluationįrom an early age, we are able to detect the direction others are looking in (known as eye-gaze) and make eye contact with each other to communicate.

White chimpanzee update#

Our findings thus support but also critically update the central premises of the gaze-signaling hypothesis. Uniform whiteness in the sclera thus facilitates the visibility of eye-gaze direction even across species. Also, participants of both species discriminated the eye-gaze directions of chimpanzees better when the contrast polarity of the chimpanzee eye was reversed compared to when it was normal namely, when the chimpanzee eye has human-like white sclera and a darker iris. We found that both humans and chimpanzees discriminated eye-gaze directions of humans better than those of chimpanzees, particularly in visually challenging conditions. We varied the level of brightness and size in the stimulus images to examine the robustness of the eye-gaze directional signal against simulated shading and distancing. This study tested the ability of human and chimpanzee participants to discriminate the eye-gaze directions of human and chimpanzee images in computerized tasks. However, experimental evidence is still lacking. The gaze-signaling and related cooperative-eye hypotheses posit that humans evolved unique external eye morphologies, including uniformly white sclera (the whites of the eye), to enhance the visibility of eye-gaze for conspecifics. Hallmark social activities of humans, such as cooperation and cultural learning, involve eye-gaze signaling through joint attentional interaction and ostensive communication.