The pupils of animals, including us humans, come in a variety of shapes but few seem weirder than the horizontal rectangular pupils of goats.
Goats developed this odd pupil shape to better see predators while they are grazing. Their ability to spot danger is further enhanced by the location of their eyes. Other prey animals have similar pupils, while predators can have pupils that are circles, vertical slits, or something in between.
Each type of animal has a pupil shape that is best suited to its environment and behavior. Let’s find out why.
Why Do Some Animals Have Horizontal Pupils?
Scientists have identified several pupil shapes. Most are related to an animal’s place in the food chain.
Goats and other grazing prey animals have horizontal pupils. Their eyes are also located quite high on the sides of their heads. The combination of these traits helps them scan for stalking predators all around them while they have their heads down eating grass. Their vision is usually attuned to spotting objects on the horizon, though their overlapping field of vision is minimal. Instead, they rely more on viewing as much as possible; the placement of their eyes and the shape of their pupils give them a vision field of around 320–340 degrees. This is also known as a panoramic view.
We often notice a goat’s strange pupils, but they are not unique. Other grass-eating prey animals have similar eyes, you just may not see them as well because their irises may be darker.
Horses have them, and so do sheep, deer, and antelope. Many of these animals have a blind spot directly behind their head. However, their instinct to stay in herds helps them overcome this issue, as there is always at least one animal on the lookout for predators.
Why Do Predators Have Vertical Pupils?
Predators, especially ambush predators like cats and foxes, have vertical slit pupils. They also usually have binocular vision, which means their eyes are toward the center of their head and their field of vision overlaps extensively between both eyes. This type of pupil shape and eye placement helps predators determine the distance between themselves and their prey.
As we see with domestic cats, their pupils can become round and large in the dark to help them see better to hunt in low light.
Some animals, and us humans, have pupils that are round all the time.
An animal’s pupil shape is mostly determined by the way that it acquires food. Predatory animals tend to have eyes with circular or vertically slit pupils and binocular vision with a great deal of overlapping visual fields between both eyes. Prey animals often have eyes with rectangular horizontal pupils and very low visual field overlaps but a better degree of vision to gauge the environment around them for predators.
The next time you see a goat’s eyes, remember that they’re designed to keep them safe while foraging for food. They share this eye type with other prey animals like sheep for the same reason.
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Featured Image Credit: JACLOU-DL, Pixabay