Kathy Stevens, age 12, of Dunlap, Illinois, for her question:
Why do cats have vertical eye pupils?
Cats are born night prowlers and Nature, as usual, pave them the most suitable eyes to go about their business. They are supposed to forage for their food after sunset, when mice and ratty rodents scurry about the world. When a cat has to make her own living, she catches a tummy full of these pesky creatures between dusk and dawn. Her vertical pupils adjust to catnaps during the day and readjust to enhance her night time vision.
The pupil of the human eye is a round hole in the colored iris. In the dusk, the muscles around it relax and the pupil window grows wider to let in all the available light. In bright sunshine, these muscles contract. This causes the pupil to shrink to a small round hole, shutting out some of the razzle dazzle brilliance. This is how human vision automatically adjusts itself to different degrees of light intensity.
A cat can do a better job because instead of round pupils, she has up arid down slits in her irises. In the bright light of day, automatic muscles pull the sides of her vertical pupils together. A round pupil can close only so far, leaving a small round hole, even in the most dazzling light. A cat's vertical pupil can close much tighter, leaving merely a pinpoint. At high noon she can enjoy a catnap while keeping a lazy watch on the scenery through the tiniest of windows.
A vertical pupil can close tighter and also open wider than a round pupil. In the darkness, a cat's muscles automatically relax and her pupils open to form round windows, big enough to fill most of her eyes. This lets in all the available light, carrying dim dusky pictures of the surrounding scenery.
Some people say that she can see in the dark, but this is not so. Her night vision is sharper than ours, mainly because she can open her vertical pupils to form huge round windows. But she cannot see anything, at all in total darkness, any more than we can.
Her remarkable eyes have another feature that enhances her night vision. Behind her vertical pupils she has layers of special tissue that reflect the light. This gives her a double look at the scenery, making the dark patches twice as dark and the lighter patches twice as bright. What she sees is more vivid. But those reflectors also add the eye shine that sometimes warn the mice away.
Those slit pupils reduce the midday glare to a minimum and work with the cat's eyeshine reflectors to enhance her night vision. But tae would not want to trade our eyes for hers. We see a world of colors too varied and wonderful to describe. She sees only various tones of grey, accented with black and white.