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.
