and larger it is than a human pupil? Large horizontal pupils take in more light across a wider range of visual angle. Having entered the eye, this luminosity filters toward the back of the eye, shining onto that patch of rods and cones that changes particles of light into neural impulses. Those impulses are then routed to areas of the brain that interpret their meaning.
Horses also have iridescent collagen fibers in an upper area of the eyeball called the tapetum (“TAP-uh-tum”). These fibers reflect light from the ground into the eyeball, allowing the horse to gather small amounts of illumination while moving in darkness. Human eyes have a similar mechanism, the choroid coat, but it is not as large or strong as the tapetum and cannot reflect light to an equivalent degree. In both species, the collagen fibers become visible when reflected in a camera flash or headlight. Ours appear to be red, but in horses, the eye-shine varies among green, yellow, or blue depending on a horse’s coat color and age.
With large pupils and a reflective tapetum, horses have a reasonable degree of night vision—enough to wander from hay bin to water trough in the dark and notice movements in the shrubs. It’s more acute than human night vision, which is about as effective as underwater breathing, but still not sharp enough to identify details, hop a cross-rail, or load into a trailer comfortably. The fact that a horse will try to complete these tasks in the dark is evidence of a willingness to obey, not proof of good nocturnal eyesight.
Dark Adaptation
The real rub with night vision comes when we realize that horses can make out shapes in dark conditions only after a long period of pupillary adaptation. This makes sense because equine vision developed when horses stood outside as dusk settled very slowly on the earth. But today, we expect them to enter dark barns and indoor arenas from bright sunlight.
Show riders often expect horses to move from sunlight into an indoor arena during performance. Top facilities use intense lights for illumination, but lesser locations tend to skimp. And at home, most winter riders work horses indoors, where footing is good but lighting is not. How do these changes affect equine performance?
Although all disciplines are affected, let’s consider jumping because it is so dependent on acute equine eyesight. Jumping horses have to judge the height and width of fences very quickly, often scanning distances on a short approach, adjusting stride length for takeoff, carrying riders who transmit countless cues, and coordinating their bodies to clear obstacles by only an inch or two. These are meticulous feats that would cause many human athletes to stumble.
Riders all over the world warm up hunters and jumpers in bright sun prior to competition. After a typical warm up, Twinkletoes’ pupils are constricted to the maximum degree to admit as little light as possible. The chemicals that transduce light into neural impulses are at their ebb. This combination of pupillary contraction and chemical paucity allows the horse to jump in bright sun without being blinded by the glare. High elevation, central latitudes, low humidity, pale arena sand, and white fences demand even greater adaptation.
Twinkletoes is jumping well in the warm-up when the gate steward calls him to the indoor show arena. His rider removes her sunglasses at the in-gate, instantly improving her indoor vision. Too bad Twink can’t shove his face into some military night goggles. Through the gate they go, galloping into the murk to leap over 8 or 12, sometimes 15, jumps that the horse can barely see. And oh, by the way, like all of us, Twink’s rider expects top-quality performance. But he hasn’t come close to the 45 minutes he needs to accommodate to indoor lighting. Typically, he’s had one minute, if that.
The scenario described here is common. We allow it because we are not aware of the visual hardship for the horse. Most of us reprove mounts who balk at the in-gate, weave between fences, skirt the flower box, or refuse to jump. We assume that if we can see the course, they can, too. But imagine moving from a sunny parking lot into a darkened movie theater, then dashing around under a heavy backpack—hurdling seats, slipping on buttered popcorn, and dodging angry patrons. The fact that horses try to perform under such circumstances is a testament to their generosity.
Like humans, horses differ in individual visual abilities. One horse might require a little less than 45 minutes to adapt to dim light; another will require even more. Age plays a role, too. Older horses with healthy eyes take in less light than younger horses do. This age difference occurs in humans as well—in fact, the average person’s eyes take in 66% less light at age 60 than at age 20. No wonder the world can seem a little glum as we get older.
How can we help our horse’s eyes adapt before performing? No suggestion is ideal, but some will help. Allow your horse to walk in the shade before entering an indoor arena. Stand at the gate for a few minutes while the preceding team competes. When possible, schedule a jumping performance right after a flat class, giving your horse 15 minutes of adaptation time prior to starting over fences. Remember that a jumping round, no matter how easy it looks, is a complicated and unnatural coup for a horse. He can’t just phone it in.
The best solution is to require show organizers and facilities managers to brighten indoor arenas with strong artificial lighting. Exhibitors, trainers, boarders, and owners have the power to make such requests as a group, especially when safety is at stake. Veterinary schools and organizations can help the cause by publicizing the need for strong indoor lighting. Professional associations like the United States Equestrian Federation (USEF) and Fédération Equestre Internationale (FEI) should revoke horse show ratings when a venue’s lighting is not up to snuff.
As a show participant, suggest that officials cordon off a small strip of the indoor as a wait-and-walk area for horses about to perform. Ask that they open doors to illuminate the area inside. Be sure they have turned on all the lights—this sounds so elementary, but many show riders and trainers have experienced events at which the managers powered up only half of the indoor arena’s lights. When all else fails, scratch—and tell the event organizers why. Your horse’s long-term welfare and your own are more important than three minutes spent hopping around in Aladdin’s cave.
Horses need to see their surroundings, whether leaping 7-foot walls or just chillin’ in the barn. Those who can’t see well compensate by using other senses to a greater degree. This can yield new kerfuffles, as when the horse begins to listen to a dark indoor arena so closely that he spooks at the sound of two air molecules colliding.
Depth Perception
Eyes admit physical views, but it takes a brain to compute visual distance. When staring straight ahead, humans take in two views of a given sight—one from each eye. To see this for yourself, hold your finger in front of your nose at arm’s length. Close one eye and line your finger up with something vertical in the distance—a door frame or a fence post, whatever. Now open that eye and close the other. Your finger will appear to jump back and forth as you alternate eyes. Those are the two views that your right and left eyes send to your brain. The brain calculates the difference between them, and as if by magic, you become aware of depth. Using this automatic computation, you can look at a field of horses and note that the cute roan is farther from you than the pretty paint.
Human depth perception is extremely precise because our eyes are so close together. They are also yoked, moving in concert with each other for precise tracking. With this design, the average person can distinguish ¹⁄8 of an inch in depth from a distance of 16½ feet. In other words, if you were standing one long stride away from the takeoff to a double-rail vertical, your brain could tell you whether one of the rails was set ¹⁄8-inch behind the other one. That’s depth perception on steroids!
By contrast, the smallest amount of depth a horse can detect when standing the same distance away from something is 9 inches. Human stereoacuity is 72 times sharper than that (fig. 4.1).
Horses’ ability to see depth is limited because their eyes are set so far apart. From most angles, horses cannot get a left-eye and right-eye view of
