The whites of their eyes

An eye-opening publication in the Journal of Human Evolution garnered some press as noted here in The Economist, Eyeing up the collaboration, and Why eyes are so alluring from LiveScience.com

Michael Tomasello and colleagues at the Max Planck Institute report that whereas great apes like gorillas, chimps and bonobos are influenced by head position when trying to follow another's gaze, humans take cues directly from eye movement. Thanks to the whites of our eyes and other high contrast features, our gaze is easier to follow.

What is the the evolutionary driver for white sclera? One theory addresses the usual mate selection bit: bright white sclera (versus bleary red-veined maybe-he's-flaccid coloration) signal good health and fitness. Another reason, as espoused by Kevin Haley and Daniel Fessler at UCLA, is that the whites of the eyes developed as social cues for cooperative and altruistic behaviors. Haley, as noted in the LiveScience article, believes Tomasello et al.'s findings are consistent with the cooperative eye hypothesis.

In addition to studying eye contact and gaze in the great apes, naked or otherwise, Tomasello and the Max Planck crew have also published some cool stuff on dogs taking social cues from humans, e.g., "Domestic Dogs (Canis familiaris) Are Sensitive to the Attentional State of Humans" in Journal of Comparative Psychology (2003) Vol. 117, No. 3, pp. 257-263 and references therein. Among the cues which dogs note are human head position and eye gaze. Fortuitously, perhaps our high contrast peepers serve our canine pals well.