Carl Wegener, age 11, of Newport News, Va., for his question:
HOW CAN THEY PROVE THAT ANIMALS ARE COLORBLIND?
Color vision in humans is hard to detect. For example, suppose you call a certain color red. Perhaps a friend sees this as the color you call green. Imagine how hard it is to test the color vision of more than a million animal species. Besides, colorblindness in the animal world is different from most human colorblindness.
Scientists who study animal vision usually start by examining the eyes. If certain color sensitive cells are not present, then the animal sees the world in black and white and tones of gray. A very small percentage of people have colorblindness of this kind. But most people who are colorblind merely confuse either the blues and yellows or the reds and greens. Cats, dogs, pigs, horses and cattle are totally colorblind. most animals have color vision, and some seem to see colors more or less as we do. Testing each species calls for a lot of patient experiments. As a rule, the tests are based on the animal's food preferences.
For example, we can test a bird by setting out several bowls of different colors. We put food in the red one and leave the others empty. After checking the whole setup a few times, he flies straight to the red bowl. He will do this even when the shapes and positions of the colored dishes are changed. After he proves he can recognize red, the food can be switched to test him with other colors.
Experiments of this sort show that the daytime birds can detect colors. So can certain reptiles and fishes. One of the favorite fish subjects is the cute little blenny who darts around tidal pools. His big round eyes soon learn to select the color that matches his favorite food.
Flowery shapes and colors and also various lights are used to check the colors seen by insects. For example, bees tend to ignore reds and select blues and yellows. They select yellow even when the model is merely a paper flower.
Under a red light, the white cabbage butterfly seems to be quite blind. Apparently red is one of the colors she cannot see. On the other hand, under a red light, the tortoise shell butterfly is able to select a red flower. Bees and several other animals seem able to detect subtle colors in shades of gray. The luna moth can detect colors in ultraviolet rays which we cannot do.
Dogs have been tested with a checkerboard of light and dark gray box tops. one square is colored, and this one hides a reward. But no dog can distinguish the colored square from the various shades of gray. Dogs and also cats are totally colorblind.