Ronna Dittman, age 10, of Gassaway, W. Va., for her question:
What causes a cat's eye pupils to change?
Her big, bright beautiful eyes may be heavenly blue or olive green, golden yellow or some other shade that blends or contrasts with her furry color scheme. But the black pupil in the center of a cat's eye is not always round, as yours is. Sometimes it is a tiny slit; sometimes it is shaped like an almond; sometimes it is round and almost big enough to hide all the color.
Some people say that a cat can see in the dark, which is not quite so. True, she can see when the light is very dim much better than you can. But neither you nor Miss Puss can see in total darkness. The cat is one animal that likes to stare steadily into your face. This gives you a chance to study her pupils, which is one reason why her eyes are different from yours.
Your pupils are those round black dots in the center of the colored irises. Actually they are small windows that let the light shine through onto the retinas in the back of your eyeballs. The light carries pictures of the scenery; the retinas are a sort of movie screen that flashes the pictures to your brain.
When the light is very dim, your eye muscles cause the pupils to open wider to let in as much as possible. When the light is dazzling bright, your pupils shrink to smallish dots to shut out the glare. A cat's eye pupils are not round like yours. They are up and down slits. In brilliant sunshine, the sides of her slit pupils can squeeze together, leaving just tiny pinpoints. This means that she can shut out far more of the glare than your round pupils can.
Her pupils also can open up much wider than yours can. This is what happens when she sneaks out to go hunting after dark. The sides of her slit pupils open up into huge black circles that cover most of the color. This means that her eyes can capture more of the dim light. But it is not the only reason why she sees better at night than you do.
No doubt you have noticed that her eyes shine in the dark. This eye shine is caused by a layer of glassy cells behind her retinas, which act as mirrors to reflect the light so that it bounces back. In this way she gets a double picture of the scenery, making the dark patches darker and the light patches lighter. These contrasts help her to see better than you can in the dusky darkness.
Her eyes can shut out more glare and also see better in dim light than yours can. But you would not want to trade with her. She cannot admire the pretty flowers or appreciate color TV. For Miss Puss is colorblind. She sees the world only in black and white and shades of gray.