Eye To Eye

When a camera—or an eye—focuses on something close to it (see The Workings of an Eye), other objects slightly nearer or farther away appear a little blurry. You can see this by closing one eye, holding two fingers at different distances from your face, and focusing on one finger and then the other. This effect, called blur, helps a viewer judge the distance to a nearby object. 

Banks and his colleagues found that a vertical slit aperture enhances the effect of blur. In the case of an animal’s eyes, that means vertical pupils probably help a cat or other predator determine exactly how far away its prey is, so it can pounce just the right distance. The pupil shape is an adaptation, a trait that helps the animal survive in its environment.

Banks’s team also noticed that animals with vertical pupils usually have eyes on the front of their face rather than on the sides of their head. This allows predators to focus both eyes on a single object, a capability called binocular vision. Each eye sees a slightly different overlapping image. The brain combines the two images to create the perception of depth. Like blur, binocular vision helps viewers judge the distance to an object—and it's even more powerful in vertical pupils.

A camera—or an eye—can’t focus on everything at once. Let’s say it focuses on something close to it (see The Workings of an Eye). Other objects that are closer or farther away look a little blurry. To see how this works, close one eye. Hold two fingers at different distances from your face. Focus on one finger, and then the other. This effect is called blur. It helps a viewer judge the distance to a nearby object.

The scientists looked at their photos. They found that a vertical slit aperture increases blur. For a cat or other predator, vertical pupils may come in handy. They could help the animal tell exactly how far away its prey is. Then the predator can pounce just the right distance. The pupil shape is an adaptation, a trait that helps the animal survive in its environment.

Banks’ team spotted another clue. Most animals with vertical pupils have eyes on the front of their face, not on the sides of the head. This allows predators to focus both eyes on one object. In other words, they have binocular vision. Each eye sees a slightly different overlapping image. The brain combines both. This gives depth to the image. Like blur, binocular vision helps viewers judge the distance to an object. And it’s even stronger in vertical pupils.

Unlike predators, prey animals such as sheep, goats, deer, and horses have eyes on the sides of their heads and horizontal pupils. These features maximize the animals’ field of view, the area they can see around themselves (see Field of View). The animals don’t know what direction a potential predator might come from. “So they have to watch all around: in front, to the sides, and behind,” says Banks. 

Once an animal spots a predator, it needs to run away. But running with eyes on the sides of the head is tricky. “For us, it would be like turning your head to the side and then running forward as fast as you can while looking out of the corner of your eye,” says Banks. Horizontal pupils give animals with eyes on the sides of their head a much better view to the front than other shapes would.  

With prey animals such as sheep, goats, deer, and horses, it’s the opposite. They have eyes on the sides of their heads and horizontal pupils. This gives them a bigger field of view, the area they can see around themselves (see Field of View). The animals don’t know where a predator might come from. “So they have to watch all around: in front, to the sides, and behind,” says Banks.

An animal spots a predator. Now it needs to run away! But running with eyes on the sides of the head is tricky. “For us, it would be like turning your head to the side and then running forward as fast as you can while looking out of the corner of your eye,” says Banks. For animals with eyes on the sides of the head, horizontal pupils help solve this problem. They give the animals a better view to the front than other shapes would.

Banks and his team wondered if animals like sheep and goats—which spend a lot of time grazing—lose the benefits of their horizontal pupils when their heads are lowered to the ground. So Banks visited a petting zoo to study the animals as they grazed. “I watched the goats,” he says. “When they pitched their heads down, the eyes rotated so the pupils stayed horizontal and parallel to the ground.” Other colleagues checked on sheep, horses, moose, and deer, and found the same results: Horizontal pupils are so helpful to these animals that the eyes actually twist in the sockets to keep them that way during grazing. 

Banks and his team wondered about animals like sheep and goats. These animals spend a lot of time grazing. Do their horizontal pupils still help when their heads are lowered to the ground? Banks visited a petting zoo. He studied the animals as they grazed. “I watched the goats,” he says. “When they pitched their heads down, the eyes rotated so the pupils stayed horizontal and parallel to the ground.” Other team members watched sheep, horses, moose, and deer. They found the same results in all of these animals. During grazing, the animals’ eyes twist in the sockets. That way, the pupils stay horizontal.

WHO'S WHO? ANSWERS: TOP, LEFT TO RIGHT: GECKO, MANTIS SHRIMP, LLAMA, SHEEP. BOTTOM, LEFT TO RIGHT: HUMAN, CUTTLEFISH, CAIMAN, PARROT.

WHO'S WHO? ANSWERS: TOP, LEFT TO RIGHT: GECKO, MANTIS SHRIMP, LLAMA, SHEEP. BOTTOM, LEFT TO RIGHT: HUMAN, CUTTLEFISH, CAIMAN, PARROT.