I volunteer with a group helping kids with simple science concepts, and I was wondering if it has been sorted out whether horses can see in color? This question comes up over and over.

Back in Pony Club, we were taught the traditional view that horses see in black and white. Then, somewhere along the line I read that there was some debate about that. Other than what we all have experienced that would suggest some horses are able to tell color, how do you know for sure what they really see?

The current evidence indicates that horses do, in fact, experience color vision. It is not the same type of color vision we humans perceive. In humans and other primates, color vision is called trichromatic, meaning color vision is based on the ability to detect red, green, and blue wavelengths and their combinations. In other mammals studied, mostly the farm animal species and birds, evidence supports dichromatic color vision, meaning these animals have the apparatus and ability to detect only two of those three colors in the spectrum and their combinations.

For horses it is currently believed that their dichromatic color vision includes the ability to perceive green and blue, and probably their color vision is somewhat similar to what is known commonly in people as red-green color blindness (or more correctly called color deficiency). This means that certain colors, especially red and related colors, appear more green.

How Do You Know?

Scientific evidence of color vision in animals has come from research of several types, including anatomy, physiology, and behavior. Those folks who study the basic anatomy of vision among species look for particular structures in the eye that are necessary for color vision. The basic requirement for color vision is the cell type in the retina called a cone. There are subtypes of cones based on the type of photo pigments they contain. Each type of photo pigment is responsive to the light of specific wavelength ranges. Horses are known to have only two of the three types of cones found in humans.

Once the cone structures are identified in a species, basic physiologists then culture those cones in the laboratory to characterize the photo pigment's ability to react chemically to light stimulation. That way the range of light wavelength to which the photo pigment is sensitive can be measured in vitro (in a dish, outside the body). There is also a technique to stimulate an animal's eye in vivo (in the living animal) to measure the electrical response of the retina, indicating that not only the pigment is responding as expected to light of certain wavelengths, but that the retina, in turn, is generating and sending the electrical signal via the optic nerve to the brain.

These in vitro and in vivo studies have been done in horses, and the results are more or less consistent for dichromatic color function.

Another test is whether the whole animal can demonstrate that it is, in fact, perceiving colors and integrating that information at the level of the brain. In carefully designed color discrimination tests, horses have performed well in a manner consistent with the dichromatic vision suggested by the cones and the function tests.

When it comes to behavior indicating that horses recognize colors, I find that people who know horses are often very certain of color vision, and wonder why it is so hard to prove scientifically. "My horse knows the green bucket is his grain and the exact same bucket in black is for another horse." But that can be misleading evidence, since, in addition to hue (or color) of an object, there are brightness and intensity characteristics upon which the horse can discriminate.

Your question about what they really see is important. Discrimination tests can answer the question of whether--when brightness and intensity are controlled--the horse can discriminate one color from another. But what a horse or any other organism really "sees" is a deeper question.

There was a nice summary article in the Journal of the American Veterinary Medical Association a few years ago that discussed vision and hearing in horses, written by Drs. Timney and Macuda, a scientist team who have been doing the cutting-edge work in horse vision. The paper is available online or through veterinary libraries. The authors very nicely explain the challenges and progress in perception research with animals.

There are also great Web sites about color vision in humans and in animals. There is a free scientific paper available online (www.journalofvision.org/1/2/2) that gives the scientists' best approximation of what the horse's color view might be, compared to humans. h

Further Reading

  • Carroll, J.; Murphy, C.J.; Neitz, M.; Ver Hoeve, J.N.; Neitz, J. Photopigment basis for dichromatic color vision in the horse. Journal of Vision, Vol. 1, No. 2, Article 2, 80-87, http://journalofvision.org/1/2/2, doi:10.1167/1.2.2, 2001.
  • Hanggi, E.B.; Ingersoll, J.; Waggoner, T.L. Color vision in horses: Deficiencies identified using a pseudoisochromatic plate test. Journal of Comparative Psychology, 121 (1), 65-72, 2007.
  • Macuda, T.; Timney, B. Luminance and chromatic discrimination in the horse (Equus caballus). Behavioural Processes, Vol. 44, 301-307, 1999.
  • Macuda, T.; Timney, B. Wavelength discrimination in horses. Investigative Ophthalmology & Visual Science (Supplement), Vol. 41, S809, 2000.
  • Sandmann, D.; Boycott, B.B.; Peichl, L. Blue-cone horizontal cells in the retinae of horses and other Equidae. The Journal of Neuroscience, Vol. 16, 338, 1996.
  • Timney, B.; Macuda, T. Vision and hearing in horses. Journal of the American Veterinary Medical Association, Vol. 218, 1567-1574, 2001.

About the Author

Sue McDonnell, PhD, Certified AAB

Sue M. McDonnell, PhD, is a certified applied animal behaviorist and the founding head of the equine behavior program at the University of Pennsylvania's School of Veterinary Medicine. She is also the author of numerous books and articles about horse behavior and management.

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