Why Do Goats Have Rectangular Pupils? Eyes of Grazers

How the horizontal pupil helps goats see

A goat has its eyes on the sides of its head, and each pupil is a horizontal bar with rounded corners. In bright light it shrinks to a thin slit. In dim light it opens up and becomes more rounded, letting in much more light. Because goat irises are often pale yellow, amber or gray, the dark rectangle stands out, which is why people notice goat eyes so often.

The most detailed explanation comes from a 2015 study in Science Advances by vision scientist Martin Banks of the University of California, Berkeley, and colleagues at Durham University in England. The team compared pupil shapes in 214 land animals and used computer models of the eye to test what each shape does to an image.

They found three main advantages for a horizontal pupil in a grazing animal:

• A wider panoramic view. A pupil stretched sideways lets in more light from the front and back of the animal, so the edges of its visual field are brighter and clearer. That widens the area in which a goat can notice movement.
• Sharper images of the ground. A horizontal pupil keeps horizontal features, such as the ground surface and the outline of a predator moving across it, in sharper focus than vertical ones.
• Less glare from above. The narrow top and bottom of the pupil let in less light from the bright sky and more from the ground, where food and danger are found.

Why do goat eyes rotate?

There is a puzzle. A goat spends a large part of its day with its head down, feeding. If its eyes were fixed in the skull, lowering the head would tilt the pupils toward vertical and lose their advantages.

Goats solve this with a remarkable eye movement. As the head goes down, each eyeball rotates in its socket so that the pupil stays roughly parallel to the ground. Banks and his colleagues filmed goats and sheep and measured these rotations at 50 degrees or more, far beyond anything human eyes can do. The movement is called cyclovergence, or rotational eye movement.

You can see this yourself at a farm. Watch a goat raise and lower its head and its pupils stay level with the horizon, like the bubble in a spirit level.

Pupil shape, lifestyle and height

The 2015 study also found a link between pupil shape, how an animal lives and how tall it is:

Vertical slits help small ambush hunters judge distance to prey just before they pounce, and the effect is strongest when the eyes are close to the ground. That is why a house cat has slit pupils while a lion or tiger has round ones. A goat’s priorities are reversed: it needs to watch the whole horizon, not judge the exact distance to one target.

Other animals with horizontal pupils

Goats share horizontal pupils with many other hoofed plant-eaters, including sheep, horses, red deer, reindeer, antelope and hippos. Llamas and camels have rectangular pupils with a fringed upper edge. In horses, goats and sheep, a small row of dark bumps along the edge of the pupil, called the corpora nigra, may provide some extra shade from the sun.

Some unrelated animals also have horizontal pupils. Toads, many frogs, mongooses and octopuses have them, and the cuttlefish has a curving W-shaped pupil. This shows that pupil shape has evolved many times to suit different ways of life, rather than being inherited from a single ancestor.

How well do goats see?

Goats, like other grazing mammals, trade fine detail for coverage. Their side-facing eyes give them a very wide field of view with only a small blind spot behind the head, but the area where both eyes overlap to give good depth perception is narrower than in people.

Goats also have a reflective layer behind the retina called the tapetum lucidum, which helps them see in dim light and makes their eyes shine in a flashlight beam at night. Their color vision is based on two types of cone cells, like most mammals, so they distinguish blues and yellows but probably confuse reds and greens.

Myths and facts about goat eyes

Myth: goats’ strange pupils mean they have poor eyesight. Goat vision is well suited to its job. It is less sharp than human vision at reading fine detail, but much better at watching a wide sweep of countryside at once.

Myth: rectangular pupils are a sign of something sinister. Goat eyes have been used in horror films and folklore, but the shape is simply an adaptation for a prey animal’s survival.

Fact: pupil shape changes with light. The same goat pupil can look like a thin line at noon and a wide, rounded oval at dusk.