Many readers have written to ask whether dogs are capable of seeing colors. Whether an animal has color vision sounds like a simple question, but answering it is challenging. Many books claim that dogs and cats have no color vision, but that statement is not accurate. Because my job is to inform people of scientific facts, I was delighted to find a recent report of a careful study. Scientists discovered they had much to learn in order to discover the correct answer to the question of dogs' color vision. We usually think of colors as lined up in order, as in a rainbow. That special order is not one that anyone invented. Rather, it is the order in which colors occur when white light is broken into its colors, as in a rainbow or in light coming out of a prism. That whole band of colors is called a spectrum. Color depends on the wavelength of light, which we measure in nanometers (nm). A nanometer is a billionth of a meter. Our color spectrum runs from 400 nm in the blue to about 700 nm in the red.

To help us understand how dogs see, we might consider how our own eyes perceive light and color. At the back of the eye is a filmy layer called the retina, which contains millions of light-sensitive cells. Each of these cells is connected to the brain by a nerve pathway. Through these pathways, the eyes communicate an image to the brain.

Actually, our retinas contain two types of light-sensitive cells. One type, the rods, works only in dim light and doesn't assist us in seeing color. The other type, the cones, comes in three different kinds, each tuned to one of three colors: blue, yellow, or red. For this reason, blue, yellow, and red are known as the three primary colors. Any other color, including white, is created by adjusting the amounts of blue, yellow, and red light. In other words, you see color because of your blue-, yellow-, and red-sensitive cones.

Most people have eyes with these three kinds of cones, so they see a world of color made by mixtures of blue, yellow, and red. However, some people do not see all colors so well. About one person in 20 is partly color-blind, most frequently because their red-sensitive cones are missing.

So does a dog see colors in a similar way, or does it only receive impressions of different shades of gray? Unfortunately we cannot question a dog, so we must find another way to communicate. We can test how a dog responds to colors, but it is difficult to interpret its reaction. If the dog can see two colors, such as blue and yellow, it might choose yellow simply because the brighter color is attractive. Therefore, discovering whether a dog has color vision requires careful experimentation.

The most recent study on dogs employed a creative experiment that really hit the nail on the head. First, the dogs were taught a game that they could play repeatedly. A box with three small windows in one wall was lit from the outside by white light or by chosen colors, so that two windows were alike and one was different. For each dog, the game was to pick the window that looked different and then touch that one with its nose. If the dog made the correct choice, it received a delicious beef-and-cheese treat. After a wrong choice, there was no treat—only a jarring buzzer and a new combination of window lights for the next game. Once they were trained, the dogs repeated their performance several hundred times each day. A computer controlled the apparatus. This machine also kept track of results.

For the initial experiments, all three windows were lit equally by white light, while one window also received a weak colored light. If a dog correctly picked the window with added color, it got the reward. The process was repeated, each time with a smaller amount of added color. Then the series was repeated for 21 other colors that differed slightly from blue through red. The results helped determine how sensitive the dogs were to different colors.

The scientists discovered some surprising results, namely that a dog can tell the difference between colors such as blues and greens and yellows. However, a dog is not so capable of seeing reds. This suggests that dogs' eyes must be different from most humans' eyes. Dogs likely have only two kinds of receptors, one tuned to blue and one tuned to yellow. All the colors a dog can see can be made from some mixture of blue and yellow. As with some color-blind people, it appears that dogs' eyes are missing the red-sensitive cones. So the answer to the question of whether or not dogs can see in color is revealed: Dogs do have color vision, but they are color-blind to reds. So if you want to make stop signs for dogs, don't use red!

What is the main idea of the text?

Human eyes and a dog's eyes are more different than they are similar.

Light-sensitive cells in the eyes determine whether animals can see color.

Careful experiments were necessary to determine that dogs see most colors.

The scientific community has conflicting views on whether dogs can see color.