![]() The effect works best in daylight, when the cat can contract its pupil to its narrowest. Impossible, right? Not if it is slit-shaped, Banks says: “We think the vertical slit is a really clever adaptation … It makes the pupil small horizontally and tall vertically. Predators also use this blurry, shallow depth of field to estimate distance.īut to use both measuring tricks at once, a cat would need a small and a large pupil at the same time. ![]() Set a wide aperture on your camera and while your subject will remain in focus, the foreground and background will blur. And for that, a wide open pupil is best – as any photographer could tell you. Paradoxically, an alternative way to judge distance is to blur parts of the image. Tiny pupils deliver the sharpest image and perform best for stereopsis. We do this too – try walking down stairs with one eye shut.Why might being an ambush predator dictate the shape of your pupil? Because to pounce on a mouse, a cat must be superb at judging distance – and that’s where a slit-shaped pupil can help, says Banks. Their brain compares the slightly different images relayed from the left and right eye to help estimate distance – a process called stereopsis. Like most predators, a cat’s eyes face forward. Those that chase down their prey, such as cheetahs and wolves, tend to have circular pupils. Banks’ study shows the slit-shaped pupil is most often found in animals that hunt by day and night, and especially among predators that ambush their prey – including cats, snakes and crocodiles. The slit-shaped pupil, with its remarkable light control, is how a cat can hunt in near-darkness, and also in bright daylight, says Ron Douglas, an animal vision biologist at City University London. By contrast human pupils expand by a factor of 15. Overall a cat’s pupils can expand by 135-fold and can perform like built-in night vision goggles. Like opening or closing theatre curtains, muscles on either side of the cat’s pupil open the slit wide or cause it to narrow. A cat’s pupilĭepending on the light, the shape of a domestic cat’s pupil changes from vertical slit to alluring almond to almost fully round. Their findings were published in Science Advances in August. ![]() Martin Banks from the University of California in Berkeley and colleagues examined the eyes of more than 200 land animals and found their status as predator or prey was correlated to the shape of their pupils. Ever wondered why cat eyes have vertical slit-shaped pupils, while sheep have horizontal bars? A recent study suggests it has to do with their place in the food chain.
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