How Come?: Cats' diamond-shaped pupils

How come a cat's pupil is a diamond shape, while a human's is a circle? asks reader Tim Youngquist.

If eyes are the windows to the soul, then pupils are the panes that the light floods through. A pupil is a hole in the eye, the opening where photons of light enter, allowing us to see the world around us.

Our pupils are black, like the entrance to a cave, because light that enters the eye is absorbed, rather than reflected back out. While pupils are featureless and dark, they're also jumpily alive. Grab a small mirror and look into your own eyes as you walk from your dim house into bright sunshine. You should see the small black circles shrink to tiny pencil points, then widen again as you duck back inside.

When your cat wanders through the cat door and onto the sunny lawn, his pupils also quickly react. However, instead of our circles-collapsing-to-dots, a cat's elongated pupils narrow to vertical slits. Among animals, it turns out, both pupil size and shape vary widely. Biologists say that eyes evolved independently at least 40 times during the long history of life on our planet. So it's no wonder that the eyes staring back at us can look so different themselves.

In human beings (and probably in many other animals), pupils serve a social function, too. Our pupils get bigger when we see something we're interested in, as if we want to collect every photon of light coming from the person, object or scenery. Studies show we think others are more attractive if their own pupils are wider -- perhaps because it indicates interest in us.

The number-one pupil job, however, is controlling incoming light. Human eyes, however, aren't the best at the task. In order to close and open our pupils, muscles in the iris -- the colored part of the eye -- must spring into action. Since our iris muscles must scrunch up to close down our circular pupils, the pupils don't close very fast or completely.

Here's where cats have the advantage: Night vision equips them to hunt in very dim light.

Cats have a special layer of cells at the back of their retinas, called the tapetum lucidum (Latin for "bright carpet"). This shiny layer of cells, acting like a mirror, reflects light back to the retina's cells. But in animals with such enhanced night vision, bright sunlight can quickly overwhelm the eyes.

Pupil shape to the rescue! Domestic cats have vertical, marquis-shaped pupils, which slam shut like sliding doors in blinding light. And in dim moonlight, a cat's pupils can open wide, expanding to fill much of his eyes. Meanwhile, an animal's vision gets sharper at right angles to its vertical, slit-shaped pupils. So cats prowling along the ground may have the edge over other animals in noticing mice or other prey across a (horizontal) lawn.

Other animals with vertical slits range from crocodiles to giant pandas to rattlesnakes. And then there's the horizontal pupil, sported by animals from sheep to kangaroos. Goats also have horizontal slits in the center of their eyes, which, in dim light, widen into striking black rectangles.