Kai R. Caspar: Self-domestication and scleral depigmentation – lessons from primates and other mammals

Determinants of external eye pigmentation in mammals have received little study so far. Despite that, the human “white of the eye”, which results from a depigmented bulbar conjunctiva and bright sclera, has traditionally been described as exceptional and various evolutionary hypotheses were invoked to explain its emergence. Among the factors discussed to explain the human ocular phenotype is self-domestication. This concept assumes that via pleiotropy, selection against aggression in wild animals can bring about a syndrome of traits otherwise known from domesticated ones, including depigmentation. Integumental pigmentation defects are indeed universally observed in domestic mammals (though not in humans) and conjunctival depigmentation has been equated with this phenomenon. However, so far no comparative studies between domesticated and wild mammal taxa have been conducted to validate this claim. Here I present preliminary data from such an analysis in 13 lineages of domestic mammals and their close wild relatives, which suggest that effects on the ocular pigmentation are noticeable but often minor and suggest that (self-)domestication cannot be invoked to explain conjunctival phenotypes in mammals, including (human) primates.

I argue for considering ecological factors and genetic drift rather than self-domestication or communicative demands to explain ocular phenotypes in primates and other mammals. More generally, I call for a cautious application of the self-domestication concept by discussing the example of African mole-rats (Bathyergidae), a group of rodents that does exhibit traits commonly identified with the domestication syndrome, but which can nevertheless be parsimoniously characterized as ecological adaptations. The talk closes with final reflections on the evolution of human ocular appearance and whether it might require explanations that go beyond principles applicable to other mammals.