Intraspecific eye color variability in birds and mammals: a recent evolutionary event exclusive to humans and domestic animals

Abstract

BackgroundHuman populations and breeds of domestic animals are composed of individuals with a multiplicity of eye (= iris) colorations. Some wild birds and mammals may have intraspecific eye color variability, but this variation seems to be due to the developmental stage of the individual, its breeding status, and/or sexual dimorphism. In other words, eye colour tends to be a species-specific trait in wild animals, and the exceptions are species in which individuals of the same age group or gender all develop the same eye colour. Domestic animals, by definition, include bird and mammal species artificially selected by humans in the last few thousand years. Humans themselves may have acquired a diverse palette of eye colors, likewise in recent evolutionary time, in the Mesolithic or in the Upper Paleolithic.Presentation of the hypothesisWe posit two previously unrecognized hypotheses regarding eye color variation: 1) eye coloration in wild animals of every species tends to be a fixed trait. 2) Humans and domestic animal populations, on the contrary, have eyes of multiple colors. Sexual selection has been invoked for eye color variation in humans, but this selection mode does not easily apply in domestic animals, where matings are controlled by the human breeder.Testing the hypothesisEye coloration is polygenic in humans. We wish to investigate the genetics of eye color in other animals, as well as the ecological correlates.Implications of the hypothesisInvestigating the origin and function of eye colors will shed light on the reason why some species may have either light-colored irises (e.g., white, yellow or light blue) or dark ones (dark red, brown or black). The causes behind the vast array of eye colors across taxa have never been thoroughly investigated, but it may well be that all Darwinian selection processes are at work: sexual selection in humans, artificial selection for domestic animals, and natural selection (mainly) for wild animals.Electronic supplementary materialThe online version of this article (10.1186/s12983-017-0243-8) contains supplementary material, which is available to authorized users.

Qualitative morphological diversity of female Pelung Chickens in West Java, Indonesia

“…Domestic species may display coloration change with age for example from dark type to a lighter one, from brown to yellow, from brown to red or from yellow to red. In addition, varied eye coloration also performs in the adult period of domestic species due to carotenoid deposits from diets (Negro et al 2017). Carotenoids are natural food colorants derived from plants which give the yelloworange-red color of many fruits (Mortensen 2006).…”

Short Communication: Phenotypic diversity of male pelung chickens in West Java Province, Indonesia

“…Two types of melanin, eumelanin (responsible for brown/black color) and pheomelanin (responsible for yellow/red color), synthesize the eye pigmentation (Steven 1991). For domestic species, eye color is subject to change due to age and carotenoid from diets (Negro et al 2017) as well as genes (Corti and Vogelaar, 2010;Gwaza et al 2018). Eye color variation may be due to age differences among chickens.…”

“…At hatched, all chicks except for albinos have dark eyes, and they have definitive eye color after sexual maturity (Corti and Vogelaar, 2010). In several wild birds, color transitions of eyes start from dark to lighter colors, for example from yellow to red (Negro et al 2017).…”

In humans, only attractive females fulfil their sexually imprinted preferences for eye colour

“…Still, eye colour is a feature that, being exceptionally salient and-unlike facial hair-not appreciably sex-linked 38 , would be ideal for both males and females to imprint on. We 39 recently investigated eye colour preferences and choices on a sample of over one thousand women of the same ethnicity.…”