And the shorter the animal, the bigger the effect, like the domestic tabby. “We looked at guinea pigs, turtles, other short animals, a couple of other birds, and the effect was there. 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. 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. When the eye doctor shines a light on your eyes, the light reflects off your retina and indicates whether you have myopia or astigmatism. 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. And because we’re so tall, the expected effect was quite small. “We did a calculation of how big an effect we expect in humans. In your lower visual field, your eyes are slightly more myopic, meaning they are well-adapted for things that are close. It turns out that in the upper visual field, your eyes are hyperopic, better designed to see objects off in the distance. ![]() Higher things appear farther away, and the lower objects look near. Think about your line of sight, from your eyes to the ground. The Pallas’s Cat, a short, wild breed about the size of a domestic cat, has round eyes, and no one truly knows why. Even shorter cats, like the domestic feline, evolved to have vertical slit pupils because they can’t see as far as taller cats. 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. The larger cats-tigers, lions and leopards-have circular pupils, because these provide good binocular vision for judging distances. It turns out that the advantage you have by a vertical slit pupil depends on your height,” Banks says (see sidebar). On the other end of the spectrum, many wild cats-though not all of them-have vertical eyes, perfect for predatory behavior. Was it predator or prey? Where on the head were its eyes located? What did the animal typically see in its immediate surroundings? The researchers considered the ecological niche of each animal, which indicated where it fit into its local environment. The Banks team, mostly based at the University of California, Berkeley, looked at 214 terrestrial animal species with the three main kinds of pupils. 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 comprehensive, groundbreaking research on animal pupil shapes by the vision scientist Martin Banks was a side trip from his usual work studying human eyes. Still, there are plenty of other oddly-shaped pupils scientists are continuing to learn about. 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). ![]() While he also seemed to like looking at me, his vertical pupils were adapted for different needs than my round ones. 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. The cells converted that light into nerve signals that traveled to the back of his brain through an optic nerve. 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. In the sun, his pupils would narrow to slits, but in dim light, they would grow quite large, almost round. ![]() He had beautiful gold-green eyes with vertical black pupils. I used to stare at him staring at me, and wonder what he was thinking. Hello all, it’s Manasee Wagh, service and news editor at PopMech.
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