Why do animals have different pupil shapes?

It’s easy to think of the pupil as a simple hole that dilates in darkness and contracts in light. But if you look across the animal kingdom, you’ll see a wide variety of pupil shapes. For example, a vertical slit in cats and snakes, a horizontal rectangle in goats and horses, and a W-shaped crescent in squid. The shape of an animal’s pupils can tell you a lot about how it sees and what it needs to survive.

In a perfect optical system, the shape of the pupil is not very important. “In an ideal world, the way optics is typically taught, students would be irrelevant, because all the light would be coming to a precise point anyway,” said visual neuroscientist Jenny Reid from the University of Newcastle in the UK.

However, the actual eye is imperfect. Light entering through the pupil causes diffraction and defocusing, and different pupil shapes require different solutions. “It actually turned out to be quite complicated,” Reid said.

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One key factor is depth of field, Marty Banks, professor emeritus of optometry at the University of California, Berkeley, told Live Science. A 2015 study published in the journal Science Advances was the first to systematically explain why the orientation of an animal’s pupils is important for survival.

Depth of field is most easily explained using a camera lens. The camera aperture acts as the pupil. A narrow aperture creates an image where both the far and near parts of the camera are in focus. A large aperture focuses on one object and blurs everything in the foreground and background. But when the pupil is not perfectly round, there is greater variation in focus, which some animal eyes take advantage of.

Ambush predators like cats and snakes have forward-looking eyes and judge distance by comparing small differences between two viewpoints. This is called stereopsis. Because the eyes are aligned horizontally, differences are most clearly visible along the vertical edges. That is, these edges must be sharp.

“How do you check the sharpness? Well, you [narrow] When you dilate your pupils, you create a greater depth of field,” Banks said. “But you only need to do that for vertical contours. So it’s very wise to narrow the pupil horizontally and leave it wide open vertically, because for other contours, such as horizontal contours, where stereopsis is not useful, you can use blur to estimate distance.”

This is why an ambushing predator’s pupils are shaped like slits. Shrinking the pupil horizontally helps with stereopsis, and dilating it vertically helps estimate depth from blur.

However, this is most effective against predators close to the ground. Large predators such as lions and tigers tend to have round pupils. This is because you are looking at the ground at a steeper angle, which reduces the benefit of blurring for estimating distance.

Prey animals have survival priorities, and the shape of their pupils reflects this.

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“For prey animals, field of view is more important than image sharpness,” Banks says. “Most of the predators that approach them are on the ground, so they need to be able to see the panorama along the ground.”

In contrast, the eyes of prey animals such as goats, sheep, and horses tend to be located on the sides of the head, and the pupils are wide horizontally and short vertically in the shape of rods. Its wide shape lets in more light from the front and rear horizontal planes, helping you monitor your surroundings, while the short vertical aperture sharpens horizontal contours. This is basically the opposite of how a predator’s eye works.

But there’s a problem. When a prey animal lowers its head to graze, its horizontal pupils turn to the side, impairing its ability to scan the horizon. But these animals have evolved a surprising solution, Banks discovered. When an animal’s head changes direction, its eyes rotate in their sockets to compensate.

“They developed the ability to move their eyes in opposite directions on each side of their head to keep their pupils parallel to the ground,” Banks says.

There are animals with even more bizarre pupil shapes, especially in the ocean. For example, squid have W-shaped pupils. Scientists still don’t have a clear answer as to why.

“Some people argue that it makes them less visible to other animals,” Banks says. “I’m not sure I believe that a W is somehow harder to see than a circle.”

Other theories suggest that these pupil shapes may help reduce light from above, minimizing scattering and improving contrast. One early theory proposed that this shape could aid in color perception. Cuttlefish have only one photopigment, meaning that despite their dazzling colors and knack for camouflage, they can only see in black and white.

Dr. Banks’ research focuses on terrestrial animals, and he acknowledged that many aquatic animals remain undescribed. Reid suspects the unknown goes much further. “It makes you think about what other abilities animal eyes have that we just don’t think about,” she said.

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