E. Everette Benfield III, O.D.
A question I am often asked goes something like this, “Does my dog see in color?” Most people believe that animals only see in black and white. So what do animals really see? Do they see color like you and me?
To answer this question we first have to know a little bit about how humans perceive color. Light is really just a wave of electromagnetic radiation generated by some source like the sun, or an x-ray machine, or a microwave. The difference in color is related to the wavelength of the wave. We can only see certain wavelengths. The range of wavelengths we can actually see is called visible light (Fig 1).
Fig. 1 Here you can see the entire electromagnetic spectrum. Notice that only a small portion, labeled “visible spectrum,” is perceived by the human eye.
Light from a light bulb is made of many different colors but appears white to us because all the colors are mixed together. When light hits an object, most of the light is absorbed by that object. However, some light is reflected back resulting in that object having a particular color. For example, light hits a lime and the lime absorbs most colors but reflects the color green.
The human eye has specialized nerve cells called cones that detect colored light. Humans typically have three types of these cone cells that detect the colors red, blue, and, green. Just like an artist mixes two different colors of paint to make a new color, the different types of cones are stimulated to form all the many colors we see.
Some humans have less than the average number of one type of these cones or are missing one type of cone all together. We refer to these people as color blind. A common misconception is that people who are color blind see in black and white. This is not true. A color blind individual sees color, however, the color blind individual does not perceive as many different colors as those people with all three types of cones. For example, a person without red cones would see red, yellow, and green as a similar color. This person would still be able to distinguish between green and blue because this person still possesses blue and green cones.
In reality, few people who are color blind are missing all of one type of cone. Most color blind individuals simply have fewer of one type making color discrimination more difficult. So now that we know how color vision works in humans, what about animals?
First, a question you may be asking is, if animals see in color how would we know? After all, we cannot enter inside an animal’s mind to see what the animal sees. There are actually at least three ways that we can find out what an animal sees.
First, we can study the cones of a particular animal’s eye to see if the animal has the different types of cones. Second, we can set up experiments where food is placed behind colored doors, for example a red door, and change the position of the doors and see if the animal learns to find the food. Third, there is a phenomenon called optokinetic nystagmus where the eyes will dart back and forth rapidly when staring at moving, alternating bands of color (Fig. 2). So if we move alternating bands of red and green in front of an animal and we observe the animal’s eyes moving back and forth, then we would know that red and green looked different to the animal.
Fig. 2 An optokinetic nystagmus drum. The patient is asked to look at the drum and then it is spun by the examiner. The examiner will see the patient’s eyes move back and forth rapidly, a phenomenon known as optokinetic nystagmus.
Let’s start with the most familiar animals, dogs and cats. It seems dogs and to a lesser extent cats can see color. However, their perception of color is not the same as a human’s perception of color. Dogs and cats have fewer cones and rely on their color vision less. They are better than humans in seeing in lower light conditions. Dogs can actually discriminate colors, but they are missing the red cone type and so perceive color similar to the color blind human in the above example. A dog would be able to tell the difference between a purple ball and a red ball but not between a lemon and a lime. Red and green would appear the same color to a dog and so they would have a hard time at a stoplight. So do not let your dog drive your car!
Cats do not see color as well as dogs. Cats are designed to see better at night and all the extra eye equipment for night vision crowds out their color vision so they have fewer cones. Still studies show that cats can see colors that are different enough like red and blue.
Most monkeys and all the great apes (gorillas, chimpanzees, and orangutans) have color vision like humans. They possess three types of cones and so can discriminate between as many colors as humans. Some monkeys such as lemurs only have two types of cones. Interestingly, the male spider monkey has two types of cones, while the female spider monkey has three types of cones (Fig 3). So if you are a male spider monkey you should absolutely have your wife pick out your necktie for you.
Fig 3. A black headed spider monkey.
Most all mammals that are not monkeys or apes have two types of cones. This would include dogs and cats but also pigs, rats, duck-billed platypuses, and deer. These animals see color, but not as many colors as humans. Two colors that look different to us would look like the same color to these animals. We can use this to our advantage. For example, when we go deer hunting we can wear bright orange hats because orange and green are very different to other hunters but look the same to a deer. In this case, we would stand out to other hunters but still remain fairly well hidden to deer.
Most birds and fish actually have excellent color vision. Fish actually have quite good color vision and most have three types of cones like humans do. Some fish that live very deep in the water do not see color that well, but then there is not very much light. With so little light no organism would see well regardless of how many types of cones it had. In fact a fish known as the skate, is the only confirmed animal that truly sees in black and white (Fig 4).
Fig 4. A skate, the only animal whose eye contains no cones and therefore only sees in black and white.
This means that different colored fishing lures probably do play a large role when trying to catch fish. For example, chartreuse is a common color of fishing lure, but it also lies in the middle of most fishes color range so there is good reason to use a chartreuse lure when fishing. Goldfish actually see a wider range of color than humans do. That is because goldfish have four different cone types, one of which can detect ultraviolet light invisible to humans.
Birds have a much more complex eye than humans that is not entirely well understood. We do know that birds experience color at least as well as humans. However, there is a great deal of variation between different species. Birds have many other specialized adaptations that help them see well, but that is a topic for another article.
The cone cells of birds are surrounded by oil droplets that may help them discriminate colors even better than humans. We still know very little about the function of these oil droplets. They likely act as filters, enhancing the differences between colors enabling better color discrimination. Some birds have more than three types of cones meaning they would see more colors than humans. Several species can see into the ultraviolet wavelengths. The wings of many songbirds reflect ultraviolet light and communicates gender and species to other birds.
Most reptiles like snakes have cones and see in color. In addition to perceiving color, snakes have something called a “pit organ” which detects infrared light (Fig. 5) This is another wavelength that humans cannot see. Snakes actually use both their eyes and their pit organs to see the world. So do not taunt a blindfolded rattlesnake, because it still knows exactly where you are. Lizards and turtles have oil droplets that surround their cones like birds. They both see well at night but can also detect a wide range of colors. In fact, geckos have been shown to see color in low light conditions better than humans.
Fig. 5 A copperhead, notice the pit organ just below the nostril and in front of the eye.
The complexity of the animal does not necessarily determine color vision. Most mammals, with the exception of humans and other primates (monkeys, gorillas, chimpanzees) have two types of cones. Other vertebrates (reptiles, birds, amphibians, and fish) have at least three types of cones. Many animals see things we do not. We have already mentioned that snakes can see infrared and many songbirds can see ultraviolet. Mice, gerbils, bees, and butterflies can see ultraviolet light as well as many other colors.
Conclusion: Myth Busted
Color vision is a fascinating subject. I hope you have learned a lot from this article. If you have any questions or suggestions for another topic you would like to learn more about, please email your suggestions to me at firstname.lastname@example.org.