New Study Reveals Why Cats Have Vertical Slit-Shaped Pupils While Other Animals Have Horizontal

I’m not sure how often people think about animals’ pupil shapes, but when you do think about it, you’ll realize that pupils come in a variety of shapes. Some animals, such as cats, have vertical slit-shaped pupils, while others, such as sheep, have horizontal bars. Why is this?

A recent study suggests it has to do with their place in the food chain.

Martin Banks from the University of California in Berkeley and his colleagues observed the eyes of more than 200 land animals and found their status as predator or prey was correlated to the shape of their pupils. Their findings were published in Science Advances in August.

So what do we know about a cat’s pupil?

Depending on the light, a domestic cat’s pupil changes from that vertical slit we’re used to seeing to almond to almost completely round, like a human’s. Their pupils can expand and act like built-in night vision goggles.

The slit is how they can hunt in the dark and also in daylight. Banks’ study shows the slit-shaped pupil is most commonly found in animals that hunt by day and night, and especially among predators that ambush their prey like cats, snakes and crocodiles. On the other hand, animals that chase down their prey, such as cheetahs and wolves, usually have circular pupils.

You might wonder why being an ambush predator determines pupil shape. Banks says that the slit helps cats judge distance, allowing them to be great at things like pouncing on mice. Their brain compares the different images transmitted from left and right eye to estimate distance. With this being said, tiny pupils deliver the sharpest images.

Now let’s take a look at the horizontal bar-shaped pupil.

In contrast to the cat’s pupil, animals like sheep have a similar slit, except it’s horizontal.

Banks found that most land animals with these pupils were herbivores and had to keep a frequent eye out for predators. Being horizontal, their pupils permit them to scan the horizon surrounding them at all angles.

While sheep graze, they tilt their heads facing down. As they face down, their eyeballs roll up, keeping their pupils parallel to the horizon.

Banks believes a benefit of having pupils like this is to reduce glare from the sun overhead.

Then of course there are the round pupils that us humans have.

“There’s more demand on our eyes,” Banks says. Which may be that we sacrifice certain light-controlling mechanisms of the cat, for eyes that are able to pick up colors and intricate details.

Banks believes that height might also have to do with our pupil shape. Taller species, such as lions tend to have round pupils. Banks thinks it may be because the blurry, shallow depth of field trick domestic cats use to judge distance is most successful at close range. The distance between a cat ready to pounce on a mouse is much shorter than that of a lion ready to attack an antelope.

A lot of work for tiny little eyeballs, huh? You learn something new every day!