The terrifying function of the eyes

This post is also available in Dutch.

A recent study showed that painting eyes on the buttocks of cattle can save them from predators. Why is this the case, and what’s special about the eyes in communication anyway?

An organ of perception and communication

The eye is a fascinating organ. Its main function is to gather information about the visual world. However, unlike other sensory organs (ears, nose, etc.), the eyes can also send signals. In humans, the eyes can indicate the direction of gaze, attention, emotions, and the level of interest – all of which play a role in regulating social interactions. In non-human animals, the eyes can have an important social dimension too. One example are gorillas – when they want to make peace after a fight, gorillas first try to establish eye contact with the other individual. On top of facilitating bonding with peers, the eyes serve another function in some animals – protecting them against predators.

This may (or may not at all) ring a bell to you. Have you seen butterflies and moths that have eyespots – circular patterns of contrasting colors – on their wings? If so, have you wondered why they have these eyespots? This is because eyespots work as a deterrent to predators by mimicking the eyes of the predators themselves.

Eyespots help butterflies avoid predators

Painting eyespots to protect cattle

The strategy of having an extra pair of “eyes” can also be used to protect cattle from predators. A recent study showed that painting eyespots on the buttocks of cattle can save them from the attack of lions and leopards. The project was conducted in Botswana over the course of 4 years. In the experiment, the cattle were divided into three groups: the ones with unmarked backside, those marked with crosses, and the ones with eyespots. The results suggested that the group with eyespots was less likely to be killed by predators than other groups.

Image from Radford et al. (2020) showing cattle in the three conditions: marked with eyespots, cross-marked and unmarked

Human eyes have unique features

While this recent study suggests that the eye is a conspicuous signal, there is other research that directly studied features related to the anatomy of the eye in primates. In a 1997 study, Japanese researchers measured three features in the eyes of 88 primate species, including humans. Based on pictures, they measured the index of exposed sclera in the eye outline (sclera is the white part of the eye), the width to height ratio of the eye outline, and the color of sclera. They found human eyes having the largest exposed sclera and being extraordinarily elongated. Humans also had the most pronounced contrast between sclera and skin color compared to other primates. Such evolution in human eyes might arise from adaptations to the environment. The elongated eyes afforded more horizontal scanning while the extremely pale sclera enabled detecting the eye gaze, which in turn facilitated communication and cooperation. So, while most primates have “gaze-camouflaging eyes” – such that the anatomy of the eyes is masking gaze direction – humans have “gaze-signaling eyes”.

It is remarkable that an organ, which initially seems to have an obvious function (seeing), is actually responsible for so much more. We have seen that humans and other animals use the eyes as a means of communication to trigger both positive and negative behaviors in others.


Credits
Original language: English
Author: Julija Vaitonyte
Buddy: Kim Beneyton
Editor: Marisha Manahova
Translator: Wessel Hieselaar
Editor translation: Floortje Boowkamp

Images
Eyespot experiment on cattle in Botswana by Cameron Radford via Wikimedia Commons
Photo by Gabriela Piwowarska via Pixabay

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