All Eyes and Ears
The horse in the wild depends on keen eyesight and acute hearing to detect danger and flee from predators before they get close enough to attack. The horse's vision is wide-range (taking in almost the entire horizon), and his hearing is well-developed to help him determine from which direction danger might be approaching. You'll see these sharp senses at work in the domesticated horse, too.
In this article we'll take a look at how the horse's eyes and ears function.
Horses don't see things the same way we do. Their eyes are a different size and shape than ours, and they have some different functions. Claire Latimer, DVM, MS, Dipl. ACVO, a veterinary ophthalmologist and consultant at Rood & Riddle Equine Hospital in Lexington, Ky., says the way a horse sees improves his chances of survival as a prey animal.
"Horses have better vision in dim light than we do, for instance," notes Latimer. "The eye is large, which lets in more light, and the size of the pupil when dilated in the dark is huge--compared to that of a human. The horse also has a reflective structure in the eye called the tapetum lucidum, which allows light to pass through the retina twice. This gives the horse two opportunities to capture visual information and process it."
Horses must also be able to see clearly in bright light. "They have some yellow coloration in the lens that filters out some of the shorter blue wavelengths of light, which diminishes glare," she explains. Horses also have the corpora negra--tiny appendages within the eye that serve as a sun visor along the top edge of the pupil.
The horse's eye is one of the largest of any land mammal. Because their eyes are at the sides of the head (and the pupil is a horizontal oval instead of a small circle, like ours), horses have a wider view of the horizon. Wendy Townsend, DVM, Dipl. ACVO, an ophthalmologist at Michigan State University, says horses essentially can see 350 degrees when standing still. "They have a few blind spots: the center of the forehead, as well as right under the muzzle and a small blind spot directly behind them about the width of the head," she says.
Their best vision tends to be down the nose rather than straight out in front, like ours. "They see very well when looking down the head a bit, and we think that's why horses coming up to a jump or trying intently to look at something tend to raise their heads. This lets them have better binocular vision (more depth perception) looking at it with both eyes," says Townsend.
The horse's peripheral vision is better than ours. "They can readily pick up motion and contrasts off to the side, even though they can't see a lot of details," explains Townsend. "This is why they tend to spook when they catch something out of the corner of the eye. They can tell something moved, even though they may not see it perfectly. They tend to jump away, then turn to look more closely.
"If we compare their vision to a person with 20/20 vision, horses may range from 20/30 to 20/60, depending on the way they are tested (since we can't get them to read an eye chart)," she says. Thus, they are fairly similar to the average human and not very near- or far-sighted.
Latimer says equine vision is well-adapted to detect motion and contrasts, which is important for eluding predators. "They must be able to discriminate certain things against a background, which is not just a matter of color, but also of movement and brightness," she says.
The horse is able to do all this while he's moving. "It's easy for a person sitting and looking out a window to pick up details, but to look for danger while walking or running would be more difficult for us," explains Latimer. "The horse is often moving, but he tends to keep his head fairly still in comparison to the movements of the rest of his body."
A number of things must happen for the horse to see properly. "Light must come into the eye and be focused properly on the retina and transmitted to the brain, where all this information is processed," says Latimer. "There is so much detail coming into the brain at once, with the horse's wide visual field, that the brain must assign priorities to what is coming in. If you work with horses a lot, you know that you can be standing well within their visual field, but if their interest and attention is drawn to something on the opposite side--focusing intently on it--you can walk into their visual field and scare them."
In such a case the brain is not taking in all the visual information, even though it's all available to the brain at that time.
Do Horses See Color?
Townsend says horses have color vision, but they see only combinations of blues and yellows, which would include green. "Everything else appears in shades of gray," he says. "They don't see reds and oranges like we do. We see the three primary colors (trichromatic color vision), whereas horses have dichromatic color vision."
The retina at the back of the eye contains rods and cones. The cones are the cells that facilitate color vision. "Horses don't have as many cones as we do, so the colors appear more pastel or sepia rather than bright," says Townsend.
A paper published in the Journal of Vision in 2001 included two different photos of a scene (at right) showing the way a person sees it, then presenting it digitally altered the way a horse would see it. The second photo has lost some focus and the colors are slightly different.
The funnel shape of the ear captures and conducts sound vibrations. The ears are independently mobile and can be turned in separate directions. This helps the horse focus on a sound and screen out background noise, as using an old- fashioned ear horn does.
The funnel shape gathers sound and keeps it from being diluted by other noises in the environment, much like cupping your hand around your ear to screen out wind noise or sounds coming from behind you.
Hearing serves three major functions: to detect sounds, to pinpoint the location of the sound source, and to help provide information that enables the animal to recognize the identity of those sources.
Brian Timney, PhD, professor of psychology and dean of Social Science at the University of Western Ontario, says there's very little available data on equine hearing except for a few studies in which scientists examined sound detection and localization to determine how well the horse can tell where sound is coming from.
"There's also some data on auditory thresholds--the minimum amount of sound the horse hears, as a function of sound frequency," he says.
Sound detection depends on the intensity and frequency of the incoming sound waves. "In all species tested, we've found that animals are maximally sensitive to some middle range of frequency and less sensitive to high or low frequencies, but the range or frequencies and level of absolute sensitivity varies greatly within species," says Timney.
"Animals with smaller heads (less distance between the ears) generally can detect higher frequencies," he notes. "Animals that do not hear the higher frequencies often have better low-frequency sensitivity. In horses, the distance between the ears, measured as the distance the sound must travel around the head from the opening of one ear canal to the other, is about 6.9 inches, compared with about 11.8 inches in humans. Thus, horses have relatively greater sensitivity in the high-frequency range and poorer sensitivity at lower frequencies than humans."
Rickye Heffner, PhD, professor of psychology at the University of Toledo in Ohio, specializes in mammalian hearing. "The basic sounds horses can detect are similar to those heard by humans, except horses can hear higher pitched sounds--but not as high as a dog or a cat can hear," she explains. "Healthy, young humans can hear up to about 18 or 20 kilohertz, while horses can hear up to 35 kilohertz, which is another two-thirds of an octave.
"The thing that's unusual about horses, compared with other hoofed mammals we've worked with, is that they don't localize high-pitched sounds very well," Heffner says. "A horse can hear a high-pitched whistle, for instance, but may not know where it's coming from."
The horse might prick or rotate his ears, searching for the direction of the sound.
"Low-pitched sounds they hear better, but they are not good localizers," she continues. "Animals like horses, cattle, and rabbits are adapted to living out on the plains with broad horizons and have eyes on the sides of their heads so they can see all around them."
But since horses use their vision so well to locate potential danger, their ears don't have to be as accurate; what they hear just helps them know the direction to look.
"If they hear something and can't see it, this makes them nervous, and may be one reason horses are so jumpy," says Heffner. If they can't see the source of the hidden sound, for instance, they might fear it to be a predator. "The horse's response is to run, so he wants to know which way to run."
By contrast, cats are excellent localizers. They can hear exactly where a squeaking mouse is hiding and pounce on it. "If they were to pounce three degrees off, the mouse would get away," says Heffner. "A horse doesn't need this precision, however. All he has to do is run in the opposite direction from a frightening sound."
An animal's senses must be accurate enough for the appropriate response. "In horses, there's no shame in being a poor localizer of sounds because they don't need to know where a mouse is squeaking in the grass," she says. "Cattle and horses are better at localizing low-pitched sounds. Most of the sounds in their natural environment that might indicate danger are low-pitched, like a bear stepping on a branch. By contrast, a mouse in the grass creates high-frequency rustling sounds. Little things that make high-pitched noises are not a threat to horses, whereas bigger things that step on bigger sticks that snap with lower frequency sound components are what they might have to contend with, and are the sounds they localize best."
Not Much Research on Hearing
In a study published in 1963, researchers said horses could respond to certain faraway sounds, but we still don't know how well they respond to soft sounds. One of the first systematic attempts to assess horses' hearing was done by F.O. Oldberg and published in 1978, using behavioral clues (ear twitching and flight reactions) as indicators that horses had heard sounds generated with an audio oscillator.
"The purpose of that study was to determine the range of frequencies to which horses would respond, and no attempt was made to obtain thresholds for detection of sound," says Timney. Much of the research involving what horses hear (high or low frequency), and the most complete data on sound localization was done by Heffner and her husband, Henry E. Heffner, during the 1980s and 1990s.
The Heffners did a number of experiments and studies to evaluate equine hearing. "Rats, cats, and other small animals are fairly easy to work with, but it's not as convenient to bring a horse into a sound-proof chamber," says Heffner.
While we know horses are sensitive to sounds and can see pretty well in the dark, there are not many scientific studies that give us exact data on how or how well they see or hear. Based on observation and some research, however, veterinarians have learned more about how horses take in their environment, and we can manage them better because of that.
About the Author
Heather Smith Thomas ranches with her husband near Salmon, Idaho, raising cattle and a few horses. She has a B.A. in English and history from University of Puget Sound (1966). She has raised and trained horses for 50 years, and has been writing freelance articles and books nearly that long, publishing 20 books and more than 9,000 articles for horse and livestock publications. Some of her books include Understanding Equine Hoof Care, The Horse Conformation Handbook, Care and Management of Horses, Storey's Guide to Raising Horses and Storey's Guide to Training Horses. Besides having her own blog, www.heathersmiththomas.blogspot.com, she writes a biweekly blog at http://insidestorey.blogspot.com that comes out on Tuesdays.
POLL: University Equine Hospitals