Hello all, it’s Manasee Wagh, service and news editor at PopMech. Years ago, I had a cat named Cyrus. I used to stare at him staring at me, and wonder what he was thinking. He had beautiful gold-green eyes with vertical black pupils. In the sun, his pupils would narrow to slits, but in dim light, they would grow quite large, almost round.
On a basic level, Cyrus’s eyes worked the same way as my (or any animal’s) eyes: The pupils were the windows that let light hit his retinas, the thin tissue of photoreceptor cells lining the back of his eyes. The cells converted that light into nerve signals that traveled to the back of his brain through an optic nerve. His brain could then interpret the signals into images he could comprehend, like backyard birds and squirrels, which he loved to track with a hunter’s focused gaze. While he also seemed to like looking at me, his vertical pupils were adapted for different needs than my round ones.
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In fact, scientists have determined the evolutionary reasons behind three of the most common types of animal pupils: round, vertical, and horizontal (commonly found in grazing animals). Still, there are plenty of other oddly-shaped pupils scientists are continuing to learn about.
The comprehensive, groundbreaking research on animal pupil shapes by the vision scientist Martin Banks was a side trip from his usual work studying human eyes. While researching binocular disparity (the difference in the location of an object as perceived by the left and right eyes), which comes into play when we focus on objects at different distances, he started wondering about how animal pupils focus.
The Banks team, mostly based at the University of California, Berkeley, looked at 214 terrestrial animal species with the three main kinds of pupils. The researchers considered the ecological niche of each animal, which indicated where it fit into its local environment. Was it predator or prey? Where on the head were its eyes located? What did the animal typically see in its immediate surroundings?
For example, the horse, primarily a prey animal, has horizontal slit pupils in eyes located on the sides of its head. While a horse can’t see as far in a vertical direction (from the sky to the ground), it can take in a wide horizontal expanse of land. In fact, horses can see a panoramic view of their surroundings, giving them an advantage in spotting advancing predators.
But what happens when a horse bends its head to graze? Horizontal pupils would miss any cues of danger in the horse’s surroundings, Banks speculated, so he decided to film goats, horses, and sheep eating. That led to a unique discovery about animal eyes. “When a goat lowers its head, the pupil rotates in the head so its horizontal shape remains parallel to the ground,” he says. “These animals are making rotations of 50 to 60 degrees—way, way more than humans can do. And in opposite directions on the two sides of the head,” Banks says.
On the other end of the spectrum, many wild cats—though not all of them—have vertical eyes, perfect for predatory behavior. “That one didn’t surprise us. It turns out that the advantage you have by a vertical slit pupil depends on your height,” Banks says (see sidebar).
The larger cats—tigers, lions and leopards—have circular pupils, because these provide good binocular vision for judging distances. Some smaller predators that use stealth over speed, such as lynxes, tend to have sub-circular, vertically elongated pupils, a “clever adaptation” that allows short-statured predators to gauge distances along the ground as well as higher objects perched on the ground, like prey. Even shorter cats, like the domestic feline, evolved to have vertical slit pupils because they can’t see as far as taller cats.
Of course, exceptions exist. The Pallas’s Cat, a short, wild breed about the size of a domestic cat, has round eyes, and no one truly knows why.
Look at the ground. Think about your line of sight, from your eyes to the ground. Higher things appear farther away, and the lower objects look near. It turns out that in the upper visual field, your eyes are hyperopic, better designed to see objects off in the distance. In your lower visual field, your eyes are slightly more myopic, meaning they are well-adapted for things that are close.
“We did a calculation of how big an effect we expect in humans. And because we’re so tall, the expected effect was quite small. There’s a little bit of a difference in length between the upper part of the retina and lower part of the retina,” Banks says.
When the eye doctor shines a light on 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 huge difference in the upper and lower parts of their retinas; they are hyperopic in the upper visual field and myopic in the lower visual field. In young chicks, it’s even more obvious. It makes sense, because as a short 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 short animals, a couple of other birds, and the effect was there. And the shorter the animal, the bigger the effect, like the domestic tabby. And the correlation was really striking between the animal height and how big this effect is,” Banks says.
And then there’s the mongoose, with a height typically under two feet. “We didn’t find any animals with frontal eyes (like humans) that had horizontal pupils, except them,” Banks says. “If you ever see one in nature, it just looks wrong. A mongoose is a kind of predator, so by our theory, you’d 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 definite patterns in pupil shape, there are so many other, weirder pupil shapes that Banks has not examined. Why do some geckos have a pupil that looks like a skinny vertical line punctuated by pinholes? How do the crescent-shaped pupils of rays, skates, and some sharks work?
While some theories exist, none of these animals’ pupil shapes are well understood. Banks’s research doesn’t explain why these other pupil types exist, but he hopes other animal-focused researchers will pursue an explanation for each of them.