Ecological factors are likely drivers of eye shape and colour pattern variations across anthropoid primates

Abstract

External eye appearance across primate species is diverse in shape and colouration, yet we still lack an explanation for the drivers of such diversity. Here we quantify substantial interspecific variation in eye shape and colouration across 77 primate species representing all extant genera of anthropoid primates. We reassess a series of hypotheses aiming to explain ocular variation in horizontal elongation and in colouration across species. Heavier body weight and terrestrial locomotion are associated with elongated eye outlines. Species living closer to the equator present more pigmented conjunctivae, suggesting photoprotective functions. Irises become bluer in species living further away from the equator, adding to existing literature supporting a circadian clock function for bluer irises. These results shift the current focus from communicative, to ecological factors in driving variation in external eye appearance in anthropoid primates. They also highlight the possibility that similar ecological factors contributed to selection for blue eyes in ancestral human populations living in northern latitudes.

Scleral appearance is not a correlate of domestication in mammals

“…It should also be pointed out that depigmented conjunctivae in different primate taxa may have emerged due to different evolutionary pressures (or a lack thereof), given the in parts great phylogenetic and morphological disparities between them. Our preliminary data on conjunctival pigmentation as a correlate of eye size also raise additional questions about the evolution of the human ocular phenotype: While the transparent conjunctivae of marmosets [7, 12] resemble the ocular phenotype found among other small mammals, such as lagomorphs, the eyes of humans are in striking contrast to those of mammalian species of comparable body and eye size such as ungulates and large-bodied carnivorans [47]. If we assume that conjunctival pigmentation does indeed adaptively shield exposed ocular epithelia from UV radiation, how do humans (and some other great apes such as Sumatran orangutans - [6, 12]), which are strictly diurnal animals and evolved at low latitudes, compensate for its reduction?…”

“…Our preliminary data on conjunctival pigmentation as a correlate of eye size also raise additional questions about the evolution of the human ocular phenotype: While the transparent conjunctivae of marmosets [7, 12] resemble the ocular phenotype found among other small mammals, such as lagomorphs, the eyes of humans are in striking contrast to those of mammalian species of comparable body and eye size such as ungulates and large-bodied carnivorans [47]. If we assume that conjunctival pigmentation does indeed adaptively shield exposed ocular epithelia from UV radiation, how do humans (and some other great apes such as Sumatran orangutans - [6, 12]), which are strictly diurnal animals and evolved at low latitudes, compensate for its reduction? So far, this interesting question has attracted little scientific attention (but see [3] for relevant discussions on the distribution of limbal stem cells in the human eye).…”

“…In any case, it is crucial to assess ocular pigmentation in domesticated and closely related wild mammals before one can make informed conclusions about whether conjunctival depigmentation in species such as humans, bonobos, and marmosets is indeed a correlate of self-domestication. It is also important to point out that the evolution of scleral appearance may be guided by various other factors aside from self-domestication, including communicative demands that may relate to irido-scleral contrast as well as the need for efficient photoprotection of the external eye [3, 4, 6, 12]. Due to scaling effects and energetic constraints, larger-bodied mammals expose larger portions of the bulbar conjunctiva during glancing and rely on movements of the eyeball rather than the head to visually scan their surroundings [10].…”

Is there no beauty in sexually dimorphic eyes? Facial attractiveness and White Europeans ocular morphology—Brief communication

“…Compared to most other primate species, human ocular morphology stands out; our eyes are "wide and white" with unusually horizontally elongated eye fissures and uniformly pale sclera [ 1 , 2 ]. On the intraspecific level, the peculiarity of human eyes is further reflected in sex differences regarding eye morphology observed, especially in Whites.…”

Photopic factors influence external eye appearance in parrots (Psittaciformes)

“…Whereas Kobayashi and Kohshima (1997; focused on social functions as drivers of variation in external eye appearance, they speci cally rejected ecological alternatives such as the anti-glare theory (Duke-Elder, 1985), which proposes that pigmentation patterns in and around the eye serve to enhance visual functions in uneven lighting. More recently, researchers have revisited these potential ecological functions of external eye appearance in primates (Perea-García et al 2021), with results providing support for ecological functions (Perea-García et al, 2022). At the infraorder level (anthropoid primates), iris color was found to change depending on the distance of the species' distribution from the equator (Perea-García et al, 2022).…”