Why eye color is so different from animal eyes

“Why isn’t the human eye completely yellow, orange or red like that of some animals?”, Natacha Sourty asks us on our Facebook page. This is our Question of the Week. Thanks to all for your contribution. To answer this question in a very complete way, we invite you to (re)discover the article of Science and Future below, titled “The shape and color of our eyes could be the result of sexual selection” and originally published in July 2021.

The white of the human eye, a trait specific to our species

Whether to challenge each other or to measure the intensity of a bond, humans exchange deep glances, they are also said to look at each other “in the white of the eyes”. It would seem that this white, which corresponds to the external fibrous envelope of the eyeball, called sclera or sclera, is moreover a specific feature of the species. Homo sapiens. Until 2021, scientists agreed that the strong contrast between colored iris and depigmented sclera played a major role in nonverbal communication in humans. A study published in 2021 in Nature by zoologists from the University of Duisburg-Essen (Germany) and anthropologists from the University of Zurich (Switzerland), however, questions this supposition and offers another explanation: the morphology of the human eye would not be the result of an adaptation to the environment, but the culmination of sexual selection.

The Collaborative Eye Hypothesis

It was the zoologist Desmond Morris (born in 1928) who popularized during the 1980s the hypothesis according to which the morphology of the eye in humans is linked to the communicative behavior of the species, and constitutes a criterion of distinction from other primates. The human eye is in fact characterized by very high visibility due not only to its depigmented sclera, but also to its elongated and clearly delimited shape. A study dating from 2001 also perceives this horizontal extension as the result of an evolution, the purpose of which would be to increase the visual field and to allow effective communication by the gaze in human primates.

By look, we must understand the orientation of the eyes, and not that of the head. When a human directs his gaze towards a specific point, he signifies to the other members of his species that he is directing his attention to this point. It is commonly accepted that this kind of communication by the gaze – direct gaze, but even more averted gaze, which exposes more of the sclera – is more developed in humans than in other primates.

This hypothesis, which presumes that the shape of the eye may have facilitated communication in humans, has been referred to as the collaborative eye hypothesis. She defines the human eye as a particularly effective social tool, since it would be able to direct the attention of several individuals, to transmit intentions and to guide actions. It is also based on a strict line of demarcation between the eyes of humans and those of other primates, both in their form and in the social and cognitive role they can play.

What do primate eyes look like?

The collaborative eye hypothesis is primarily based on the assumption that depigmentation of the sclera is a feature that is only expressed in humans. To affirm this, however, previous studies have relied on only a limited number of data. This is why the Swiss and German researchers have extended the field of observation to a corpus of photographs representing more than 380 hominoids belonging to fifteen species, including humans, great apes such as gorillas, chimpanzees and orangutans, and small monkeys (gibbons).

The reasoning is simple: insofar as there is little evidence of the informative role played by the gaze in great apes and since we know that the gaze has no notable communicative value in gibbons, all these species, the eyes should lean towards the dark. To assess the degree of openness and brightness of the eyes of primates, the researchers used several methods: they quantified the width-to-height ratio in order to roughly measure the shape of the eye; they also calculated the amount of sclera exposed, as well as the brightness values ​​to determine the highest ocular contrast (between the darkest and brightest point of the eye) and the relative luminance of the iris compared to to the sclera. But the results show that humans are not the only ones to present depigmented sclera: this is also the case for gorillas (gorilla gorilla), bonobos (pan paniscus) and Sumatran orangutans (Pongo abelii). In small apes, the sclera is mostly darker than the iris, which sometimes even almost completely fills the visible part of the eye. It is then difficult to speak of contrast between iris and sclera.

More contrasting eyes are neither a characteristic of humans, nor even of hominids. Indeed, the researchers failed to identify a pattern explaining the ocular morphology in the two cousin families that are the hominids (humans and great apes) and the hylobatids (little apes). The particular case of the chimpanzee (Pan troglodytes) is indeed an exception: its eyes are the least visible of all hominids, both in terms of exposure and luminance. The degree of luminance also offers inconclusive results and the researchers deduce that it is not a good tool for measuring ocular pigmentation. Instead, they recommend using contrast quantification for further studies.

The human exception

If the depigmentation of the sclera in humans is therefore not an exception among primates, what is unusual is more exactly the extremity that the human eye represents, underline the researchers. Because the scleral depigmentation approaches 100% within our species, while the other species of primates all present variants.
To explain this exception, we must first rule out the hypothesis of the collaborative eye. Firstly because the data are still too incomplete in mammals. Thus, studies that relate to the understanding of the gaze of monkeys are carried out on animals in captivity responding to signals emanating from humans. This situation is far from corresponding to what could occur between congeners, in their natural environment. Then, because humans would actually be able to assess the direction of gaze of their fellow humans without the sclera being visible, when iris and sclera are of identical colors for example (by an artificial process). The researchers deduce that the focus on depigmentation of the sclera as a communication tool could therefore be exaggerated, also because there are other morphological characteristics (such as the elongated outline of the eyeball) that distinguish the human species from others. hominoids.

A trait of sexual selection

After having invalidated the previous presuppositions, the researchers base their conclusions on presumptions that will have to be verified during future work. Given that other hominid species exhibit variable sclera colors, they believe it likely that similar diversity may have occurred in human ancestors. But, according to them, the evolution of eye pigmentation within the human species would rather result from a genetic drift, most certainly supplemented in its effects by sexual selection.

We know that the brightness of the whites of the eyes indicates the age of the individual (in humans as in chimpanzees and bonobos). Clear and visible sclera may therefore have played a more important signaling role in humans, because some apes, such as gorillas and orangutans, avoid looking into each other’s eyes, and because scleral luminosity does not increase. not sexual attraction in some species, such as chimpanzees.

The diversity of iris hues is another characteristic specific to the human species; in other primates, the color of the iris is uniform within the species. We also know that the color of the eyes (which is that of the iris in reality) represents a criterion for choosing a sexual partner in humans. This is why the researchers hypothesize that the color of the iris, just like the uniformly white sclera, could be ocular traits resulting from sexual selection.

Ultimately, the morphology of our eyes, the enhancement of their attractiveness over the course of evolution, would not be intended to help us communicate more effectively in order to strengthen the social group. If we are able to express ourselves with the eyes, it is undoubtedly to enable us to perpetuate the human species.

By Sara de Lacerda

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