New York: Scientists have unravelled the mystery behind why cats’ eyes have creepy vertical slits for pupils, suggesting the reason may lie in felines’ preferred mode of hunting.

Vertical-slit pupils are most common among nocturnal predators that ambush their prey, researchers said. This pupil shape, most likely, provides the sharpest way to gauge distance for a prey-snatching leap, the study found. Martin Banks, a researcher at the University of California, Berkeley, and his team assembled a database of 214 land-based animal species.

They analysed the pupil shape of each species in relation to the animal’s foraging habits and day or nighttime activity. Researchers found that ambush predators, like many cats and snakes, were most likely to sport vertical-slit pupils, particularly when those animals were active at night. Ambush hunters need to be very good at gauging depth so they can effectively leap out at their prey, Live Science reported.

There are two ways to gauge depth without moving. In one method, stereopsis, the brain compares the distance between the two images returned by each eye to gauge depth. The other method, blur, takes advantage of the fuzziness of objects behind and in front of the spot on which one is focusing.

The side-to-side displacement used in stereopsis is easier to gauge using vertical lines and contours than it is with horizontal ones. The vertical pupil, therefore, provides the best view for stereopsis, researchers said.

To judge horizontal distances, though, cats and other slit-pupil predators likely use the blur method, Banks said. The pupil must open wide in order to maximise blur. And to maximise blur for horizontal lines, the pupil must open wide from top to bottom. This means the ideal shape is narrow horizontally and wide vertically - precisely the arrangement of a cat’s eye, Banks said.

The study was published in the journal Science Advances.

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