Sometime during my earliest dealings with horses, I was told by a now-forgotten authority that horses see only in black and white. I never questioned this version of equine reality, and over the years I've encountered others who shared the same view that most animals--certainly dogs and also horses--inhabit a colorless world.
But how to explain those plentiful barn anecdotes that fly in the face of the black-and-white theory? There's the horse who shies away from orange cones but doesn't take a second look at similar objects in other colors. There's the barrel racer who's startled by red barrels but not blue-and-white ones or the jumper who spooks only at blue jumps. Observant owners sometimes recognize color as the recurring factor in their horses' behavioral quirks. With little true science to go on, these apparent expressions of color perception have been explained away as reactions to the shade, the shape or the placement of the object rather than the color itself, if not purely random outbursts of flightiness.
Yet the anatomy of the equine eye suggests that some color perception is possible, and in the last 25 years, a few behavioral studies have attempted to test color recognition in horses. Using color as the distinguishing characteristic to mark the rewarded choice, some studies determined that shades of red are visible to horses, while others found that blue, not red, is a recognizable color. The inconsistent results may have arisen from flaws in the studies' designs, causing the horses to respond to the darkness or brightness of the color, rather than to the color itself.
More recent research has examined equine vision in a new and more objective light by monitoring horses' physiological reactions to the range of colors. In addition, more carefully designed behavioral tests have produced convincing support for the physiological findings that suggest horses do possess color vision.
Eyeballs vary in shape and size throughout the animal kingdom, but the color-sensing process is the same among all mammals. Two types of photoreceptors operate in the eye: rods, which are responsible for seeing in darkness or dimly lit conditions, and cones, which are sensitive to color. The well-studied human eye is known to contain millions of cones grouped into three classes that react in different ways according to wavelengths in the light.
"Light is made up of a lot of different wavelengths, just as sound is made up of a lot of frequencies," explains color-vision researcher Jay Neitz, PhD, a professor in the department of cell biology, neurobiology and anatomy with the Medical College of Wisconsin. "We recognize different frequencies when we hear different pitches. Light frequencies--what we call wavelengths--work the same way."
When light passes through the pupil, it is directed toward the retina, which consists of several layers of nerve cells--including rods and cones--lining the back of the eyeball. Light stimulates the pigments in the photoreceptors, which encode the information about each wavelength and send a message to the brain. Although each cone class responds best to a small range of wavelengths, they all respond in some way.
"With each wavelength of light, each of the receptors reacts to a different degree, and certain receptors prefer one wavelength," says Brian Timney, PhD, a researcher of mammalian vision who is dean of social science at the University of Western Ontario. In the human eye, the cones register short wavelengths as blue, medium wavelengths as green and long wavelengths as red. Horses' eyes have just two types of cones, and until recently, the visual effect was not known.
The Equine Palette
To evaluate horse color vision, Neitz tested six anesthetized ponies by exposing their eyes to individual colors and measuring the neurological responses using an electroretinogram. The instrument, which has also been used to examine cone pigments in cattle, goats and sheep, shines a narrow band of light into each eye. "It's like taking all the colors of the rainbow and showing each of them, one at a time," says Neitz.
When a photoreceptor responds to a wavelength, it sends out a nerve signal, which the testing equipment senses and records. "[The electrode] is a very thin thread that sits on the cornea and picks up electrical signals like a little antenna," says Neitz. "Those signals are processed by the computer. Basically, we're measuring the amplitude of the signal in response to different colors of light."
With only two types of cones in their retinas, horses have more limited color perception than people. Neitz found that the ponies' eyes responded to blue and green but not to red. Using the computer data, he constructed an equine color wheel showing that the horse's version of green is different from ours. "They have cones like our blue-sensitive ones," says Neitz, "and they have a cone [class] that's similar but not identical to our green-sensitive ones. Those cones perceive more of a yellow color."
When viewing red, horses see an earthy color with a faint yellow and blue hue. Magenta and its blue-green complementary color are seen as gray. "Basically, there are certain colors that the horse can't tell from gray," Neitz explains, "and there are certain colors that are not like gray but that can't be distinguished from one another."
Although horses can see blue and yellow as separate colors, when presented with blue-yellow, the image is perceived as gray or white. "When both types of cones are stimulated equally, you don't get an intermediate color, you get no color," says Neitz, "and they don't see its complementary color. It's the same for people. If you stimulate red and blue, you get purple. But put in green as well, and you get white."
Neitz's findings indicate that horses probably see the world similarly to people who suffer from red-green color blindness. Color-vision deficiencies vary greatly in people, but even those with severe abnormalities probably see more color variations than horses do. "Since horses have just two color receptors [to begin with], there will be several combinations of wavelength and light intensity that will induce equal response ratios in the receptors," says Timney. "As a consequence, various colors will appear similar to one another."
Color in Action
Timney has conducted two behavioral studies confirming that horses are able to discriminate among colors. In his first study Timney trained two horses to press on a trapdoor to access a feed treat. With two trapdoors set side by side, Timney projected a colored square on one door and a gray square on the other. The horses had to access the colored door to get the treat. To reduce the chance that the horses were responding to shading or brightness rather than color, Timney matched the color hues with the gray.
"The horses behaved more or less like red-green color-deficient people," says Timney. "A person who's red-green deficient doesn't have a problem with blue and yellow, and some red and green are OK. The horses responded similarly."
Timney found that the horses were able to distinguish red from gray, but the ability to differentiate between them doesn't mean that horses perceive the color red as we do. "We didn't have the horses judge between red and green," he says, "so we don't know if red looks distinct to them."
In a second study, Timney tested how different levels of brightness affected the horse's vision. "We measured the lowest intensity of light that a horse could see," he says. Again, the task was to locate the lighted trapdoor concealing the food reward, but this time the light became gradually dimmer. As the light dimmed, the rate of correct responses fell from 100 percent to only 50 percent. "In this study, the horses were most sensitive to green and yellow in the middle range of light," he says. "It doesn't necessarily tell you what they see. It just means that they respond better to those colors.
Color vision is not required for either successful foraging or reproduction, so it's not an essential survival tool for horses as it is for some other species. "Old-world monkeys have color vision similar to people, and you could say that monkeys need to find bright red fruit from green trees," says Timney. "As grazing animals, horses don't have the pressure to be very selective between the colors. They see what they need."
Yet the fact that equine vision has evolved with a degree of color capability indicated some survival advantage to seeing beyond black and white. More than likely, it's a function of their niche as a prey animal.
"[Color] breaks up the world, separating things on earth and things in the sky," Neitz says. "Blue is distinctly different. Even though they can't distinguish between brown and green, horses watching for predators can see them especially well against that background. If a lion suddenly appears against a blue background, that's a very salient thing for a horse."
This article originally appeared in the October 2003 issue of EQUUS magazine.