It's the Eye of the Tiger… and the Gator, the Deer, the Housecat

The shape of animal eyes is influenced by whether it is an ambush hunter or the huntery's prey. JonathanKnowles/DavidMaitland/GettyImages

Ever wonder how housecats are able to pounce so precisely on unsuspecting prey? Or why goats have those weird, horizontal pupils? A recent Science Advances journal study looked for the answers in the eyes of 214 species including both mammals and reptiles.

The study specifically examined terrestrial animals for information about their ecological niche, highlighting their foraging mode and what time of day they're active. Researchers then combined that data with information on pupil shape, and some fascinating information came into focus.

The study found that animals with horizontally elongated pupils – think goats, sheep and deer – are more likely to be prey. Animals with vertically elongated pupils – housecats and foxes, for instance, and crocodiles – are likely to be ambush predators that are active both day and night. There's actually a reason why predator and prey species evolved to have different pupil shapes; certain shapes help animals hunt, while others help animals stay alive.

The animal study grew out of a prior examination of how human eyes work. “We did a study in 2012 where we showed that humans' brains complement the use of stereopsis and blur to estimate distance,” says Martin Banks, professor of optometry and vision science at the University of California, Berkeley, and an author of the new study. “In writing that paper, we thought, ‘Are there any other implications?'”

Vertical Pupils for Hunters

It's essential for ambush predators to accurately determine the distance to their prey, and vertically slit pupils help them do this in several ways.

First, slit pupils can dilate and constrict at a greater range than circular pupils. “The amount of light that gets into the eye is proportional to the area of the pupil," says Banks, explaining how the eyes help ambush predators hunt at night. "A slit pupil is able to undergo a bigger change in area. That's particularly true for vertical ones.”

In addition, vertical slits allow ambush predators to estimate the distance to their prey – and now we're getting to the visual qualities of stereopsis and blur: ways the eyes and brain work together to determine distance. “You've got a left-eye viewpoint and a right-eye viewpoint," says Banks. "The brain compares the two images, and that's stereopsis. [Understanding] blur is less obvious, but it is the same kind of cue.”

Banks suggest thinking of two candles in a dark room to better understand blur. You focus on the closer candle, so the light rays enter your eye through the lens and focus into a point on your retina. This brings the candle into focus. The candle that's farther away is blurry because while the light rays are entering your eye, they aren't combining. “The farther away that candle is, the more unfocused the blur is. So the amount of blur is a cue to how far away that candle is from the first one,” Banks says.  

Vertically slit eyes have more pupil area in a non-dilated state with which to take in information, and can more easily process blur. With tiny pupils, the blur – and the cues to distance that blur provides the brain – goes away.

There's a reason, though, why housecats have vertically slit pupils but their bigger feline relatives don't. Larger predator animals like tigers, lions and even humans have circular pupils because efficiently processing blur, says Banks, only significantly benefits short animals that hunt close to the ground – those that particular lay in wait and ambush prey, rather than chase them over far distances.

Goats' horizontal pupils serve them well due to the animals' herbivorous traits.

Horizontal Pupils for the Hunted

Next, the researchers found that terrestrial animals with horizontal pupils and laterally placed eyes (on the sides rather than the front of their heads) are likely to be prey. Not only that, they are often herbivores – grazers, who keep their heads down where the food is. As it turns out, horizontal slits are a good thing, evolutionally speaking, for those head-down eaters.

“The prey animals ... are very concerned about what's around them because a potential predator could come from behind. It could come from the front. It could come from the left,” Banks says. “So they have to see panoramically around themselves, and primarily on the ground. The horizontal slits are an adaptation ... so they can see better around themselves.”

So, next time you see a cat pounce or a sheep graze, you'll know it's not just instinct – it's all in the eyes.