Why do animal pupils have different shapes? – Tips & Results

Hi everyone, this is Manasee Wagh, Service and News Editor at PopMech. Years ago I had a cat named Cyrus. I would always stare at him the way he stared at me and wonder what he was thinking. He had beautiful golden green eyes with perpendicular black pupils. In the sun his pupils would narrow to slits, but in low light they would become quite large, almost round.

At a basic level, Cyrus’ eyes worked just like mine (or any animal’s eyes): the pupils were the windows that let light fall on his retina, the thin tissue of photoreceptor cells that lined the back of his eye. The cells converted this light into nerve signals that traveled through an optic nerve to the back of his brain. His brain could then interpret the signals into images he could understand, like backyard birds and squirrels, which he liked to track with a hunter’s focused gaze. While he also seemed to enjoy looking at me, his vertical pupils were adjusted to different needs than my round ones.

👁 Science explains the world around us. We’ll help you unravel its mysteries.

In fact, scientists have identified the evolutionary reasons behind three of the most common types of animal pupils: circular, vertical, and horizontal (commonly found in grazing animals). Still, there are many other oddly shaped students that scientists continue to learn about.

The comprehensive, groundbreaking Research on pupil shapes in animals from the vision scientist Martin Banks was a detour from his usual work of examining human eyes. While exploring the binocular disparity (the difference in an object’s position as perceived by the left and right eyes) that comes into play when we focus on objects at different distances, he began to wonder how the pupils of focus on animals.

The Banks team, which is primarily based at the University of California, Berkeley, studied 214 terrestrial animal species using the three main types of pupils. Researchers looked at each animal’s ecological niche, which indicated where it fit into its local environment. Was it predator or prey? Where were his eyes on his head? What did the animal typically see in its immediate vicinity?

For example, the horse, primarily a prey animal, has horizontal slit pupils in its eyes, located on the sides of its head. While a horse cannot see that far vertically (sky to ground), it can see a wide horizontal area of ​​land. In fact, horses can have a panoramic view of their surroundings, giving them an advantage when spotting advancing predators.

But what happens when a horse tilts its head to graze? Horizontal pupils would miss any cues of danger in the horse’s environment, Banks speculated, so he decided to film goats, horses and sheep eating. This led to a unique discovery about animal eyes. “When a goat lowers its head, the pupil in the head rotates so its horizontal shape remains parallel to the ground,” he says. “These animals make turns of 50 to 60 degrees – much, much more than humans can do. And in opposite directions on either side of the head,” says Banks.

From left to right, the top line shows the eyes of a white lion, a gecko, a goat, and a lion. The bottom row shows the eyes of a stingray, a mongoose, a cat, and an octopus.

Jesse Southernland/Staff

On the other end of the spectrum, many feral cats—though not all—have vertical eyes that are perfect for predatory behavior. “That didn’t surprise us. It turns out the benefit you get from a vertically slit pupil depends on your height,” says Banks (see sidebar).

The larger cats – tigers, lions and leopards – have circular pupils because they allow good binocular vision for judging distances. Some smaller predators that use camouflage over speed, such as lynxes, tend to have sub-circular, vertically elongated pupils, a “clever adaptation” that allows small predators to estimate distances along the ground, as well as higher objects on the ground, such as prey . Even smaller cats, like the house cat, have vertically slit pupils because they can’t see as far as larger cats.

Of course there are exceptions. That Palla’s cata small, feral breed about the size of a house cat, has round eyes, and no one really knows why.


Try this:

look at the floor Think about your line of sight, from your eyes to the ground. Higher things appear further away and the lower objects look close. It turns out that your eyes are hyperopic in the upper visual field and are better designed to see objects in the distance. In your lower field of vision, your eyes are a bit more myopic, meaning they’re good at things up close.

“We calculated how large the effect is that we expect in humans. And because we’re so big, the expected effect was pretty small. There’s a small difference in length between the top of the retina and the bottom of the retina,” says Banks.

When the optometrist illuminates your eyes, the light reflects off your retina and indicates whether you have myopia or astigmatism. It’s an annoying but harmless test. It turns out that similar tests done on chickens show a big difference in the top and bottom parts of their retinas; they are hyperopic in the upper visual field and myopic in the lower visual field. It is even more evident in young chicks. This makes sense because as a small animal you need to see objects close to the ground, but you also need to compensate to see things farther away.

“We looked at guinea pigs, turtles, other small animals and a few other birds, and the effect was there. And the smaller the animal, the greater the effect, as with the domestic tabby. And the correlation between animal size and the size of this effect was really striking,” says Banks.


And then there’s the mongoose, with a height usually under two feet. “We didn’t find any animals with frontal eyes (like humans) that had horizontal pupils, except them,” says Banks. “If you ever see one in nature, it just looks wrong. A mongoose is a type of predator, so by our theory you would expect frontal eyes and vertical pupils.” Scientists need to learn more about how the mongoose fits into its ecological niche, he says.

While the team found specific patterns in the pupil shape, there are so many other, stranger pupil shapes that Banks hasn’t examined. Why do some geckos Do you have a pupil that looks like a thin vertical line broken up by small holes? How does it work crescent-shaped pupils of rays, skates and some sharks work?

Although there are some theories, none of these animals’ pupil shapes are well understood. Banks’ research doesn’t explain why these other types of students exist, but he hopes other animal researchers will look for an explanation for each.

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