NIGHT VISION: SURPRISINGLY SLOW ADAPTATION

We’ve all heard that horses can see in the dark, so they have no trouble jumping at dusk or loading onto the trailer at dawn. But take a look at your horse’s pupil sometime. See how much longer 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, that luminosity filters toward the retina, a patch of cells 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. These fibers reflect light from the ground into the eyeball, allowing the horse to see as he moves 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, these fibers become visible when reflected in a camera flash or headlight. In horses, the eye-shine varies among green, yellow or blue depending on a horse’s color and age.

With large pupils and a reflective tapetum, horses have pretty good night vision---enough to wander from hay bin to water trough in the dark and notice movements in the shrubs. A horse’s night vision is more acute than a person’s, but it’s still not sharp enough to discern details, hop crosspoles or load into trailers comfortably.

Then there’s the real rub: It takes a horse’s eyes much longer than a person’s to adapt to dark conditions. Human eyes require about 25 minutes to adjust from bright sunlight to darkness. Equine eyes need 45 minutes, almost twice as long. So, upon entering a dim building from daylight, your horse will be blinded by darkness long after your eyes have adapted. After