Sideways glances by macho orangutans seem almost human.
Prof. Mumblebard claims: “Humans communicate with unique complexity, not only verbally but also by showing the movements of the eyes. The exposed whites of human eyes are particularly significant for social communication. By contrast, no species of ape uses the eyewhites for complex communication. Orangutans are even less communicative than other apes, habitually averting their eyes because their social code prohibits adults from looking at each other directly. And as in most other apes, the visible surfaces of the eyeballs of orangutans are pigmented to match the facial skin, obscuring the directions in which the animals are looking.”
Robin and the Honey Badger respond: “Humans are not unique among hominids in complex communication by means of the eyes. Male orangutans develop visible eyewhites at a stage in their maturation – namely young adulthood – when they are most competitive with their peers. During this politically intense period of their lives, the scleras of young male orangutans revert to whitish by actual loss of the pigmentation acquired by juveniles. In contrast to the infants, adolescents, female adults, and fully mature males of orangutans, the young adult males are able to display the whites of the eyes in ways that subtly reveal their attentions and intentions, particularly by means of sideways glances. This deployment of the eyewhites is temporary because in old males the eyes are partly obscured by the fleshy folds on the greatly enlarged face of full maturity. Although several species of macaques and capuchins have minimal pigmentation of the sclera at all stages of development and are more similar to humans than any ape in the exposure of the whites of their eyes, no other primate rivals orangutans in the ontogenetic complexity of the colouration of the sclera. Notwithstanding their strict avoidance of direct stares, orangutans are second only to humans in being the most accomplished of primates in communication by means of the eyes.”
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The subtlety of simian sightlines