Here’s how science explains why cats’ eyes look so strange and scary

Cats have some of the most interesting eyes of all animals. Instead of having round pupils like people, the black parts in the middle of their eyes are vertical. This means they can change quickly and can open and close like a camera’s aperture.

Why do cat eyes stand out? New research shows that it all comes down to how they use their eyes.

Researchers at UC Berkeley looked at 214 different species of land animals and found that the way animals spend their days affects the shape of their pupils. The team published their study on 7 August 2015 in the journal Science Advances.

Pupil shape and size determines how much light gets to the eyes – and is then translated by the brain into a picture of the world around us.

When it’s dark, our pupils get bigger to let in more light and improve our vision. When it’s bright, they get smaller to keep our eyes from getting too stimulated. Cat eyes do the same thing as human eyes, but they do it much better.

Researchers have found that the thin, slit-like pupils of house cats and other predatory animals let them move their muscles more freely and let more light into their eyes.

Cats don’t have round pupils like humans do. Instead, they have thin slits that can change their size by 135 to 300 times between being narrowed and widening.

A press release from UC Berkeley says that the size of a human pupil can only change by a factor of 15.

Cats are mostly active at night, which gives them a big advantage when they are hunting. They can open their pupils very wide, which lets even tiny amounts of light into their eyes so they can see at night. During the day, they can close their pupils down to a tiny slit.

Since most people work during the day and sleep at night, their pupils don’t have to adjust to as many different kinds of light.

Some animals, like sheep, deer, and horses that graze, have horizontal pupil slits that are very strange. Using computer models, Banks found that these animals can see a wider panorama even when they have their heads down to eat.

The study’s author, Martin Banks, a professor of optometry at Berkeley, said in a press release, “The most important thing for these animals to see is predators coming up from the ground, so they need a wide view of the ground with few blind spots.”

The second most important thing is that they need to be able to see where they are going when they see a predator. They have to be able to see well enough out of the corner of their eyes to run quickly and jump over things.

The team found that their horizontally slit pupils cut down on the amount of light from the sun that gets into their eyes, making it easier for them to see the ground.

And after spending hours at the Oakland Zoo, Banks found that the eyeballs of many grazers, like horses, goats, and deer, turn when their head goes down to the ground to keep the line of their pupils parallel to the ground, like in the gif below.

The vertical pupil slits found in cats, snakes, and crocodiles also give these predators a competitive edge, the team found, by allowing them to better approximate their prey’s distance by honing their depth perception and focus on the target. But not all big predators have slits that go up and down.

Larger cats like lions and tigers have circular pupils like human. The team thinks that the difference in shape may be because the pupils are so big.

Since bigger predators, like wolves, are higher off the ground than shorter ones, like cats, their eyes don’t have to do as many tricks to focus on a target.

Because the eye is so complicated, there are still a lot of questions, which makes some researchers doubt the conclusions of the paper.

Ronald H.H. Kroger, a biologist from Lunds University, told The New York Times, “There are so many exceptions to the rules that the authors think they’ve found that there must be a lot more to pupil shape than being a predator or prey or being big or small.”

This study is just one more example of how different and interesting eyes and vision are.

And now you’ll know that the next time your cat gives you the “slit-pupil side-eye,” it’s because it wants to kill you… I hope it was a rat and not you.