cannot see a stationary person dressed in bright red. Both pasture and person appear to be grey.
Saving Lives with Color
In 2018, British researchers demonstrated horses’ inability to see the bright red-orange poles used to mark the base of steeplechase jumps that they sail over at high speed. Against green grass, white poles were much more visible to the horses. Jockeys could see both colors equally well, so the choice didn’t matter to them. The British Racing Association now plans to change the color of steeplechase poles throughout the United Kingdom, increasing safety for thousands of animals and their riders. That’s brain science saving lives: in 2018 alone, 201 horses died on British steeplechase racecourses. Invisible ground lines likely played a role in some of those deaths.
Next time you need to mark off an area to keep horses from entering a reseeded portion of their pasture or a weekend construction job, throw away the fuschia survey tape. It’s salient to you, but invisible to your horse. Tie bright yellow tape around your temporary fence instead.
Seeing Equine Vision for Yourself
By exploring just a few important features of the brain, we can already see that equine vision differs dramatically from our own. An excellent 2016 video clip pulls a few of these differences together. (It’s online—see the Source Notes on p. 270 for the link.) In the video, a horse-and-human team is simply walking through a grassy meadow and forest to a barn. The view seen by each species is shown simultaneously at the top and bottom of the screen, so they’re easy to compare. In two minutes, you’ll get an idea of the vast changes caused by differences in range of vision, acuity, dark and light adaptation, and color vision.
All of us who work with horses need to think about the senseless difficulties we impose on our horses by assuming they see like we do. Let’s try seeing through their eyes for a change.
Time to Dark Adapt
Eyes adapt to light by contracting their pupils, and they adapt to darkness by expanding their pupils. Human eyes require about 25 minutes to adjust from bright sunlight to complete darkness. But equine eyes need 45 minutes, almost twice as long. So, upon entering a dim building from daylight, your horse will struggle with darkness long after your eyes have adapted. After adaptation, his eyes are 25,000 times more sensitive than they were at the beginning. Now he can see. Unfortunately, his 45-minute training session is about done by then.
Chapter Five
“Did You Hear That?”
Reno, a blood bay off-the-track Thoroughbred, stood in the cross-ties one afternoon while his owner’s toddler went inside for a nap. A nursery monitor in the barn allowed the mother to hear the two-year-old in his crib while she worked. The horse was accustomed to baby and monitor, so he paid no attention to the noises of adult speech and the movement of blankets and pillows as the baby was put to bed. But a few minutes later, when the baby began babbling, Reno came to military attention. He lifted his head and peered around the corner, ears nailed to the monitor.
“Ba-ba-ba-ba-ba, na-na-na-na, muh-muh-muh-muh,” the baby repeated one syllable after another. Reno pushed his nose forward and jiggled the cross-ties; he seemed to bob his head in time with the repetition. Out of curiosity, I led him to the baby monitor to see what he would do. He approached eagerly, showing no fear, and sniffed every corner of it. He cocked his head from one side to another, listening. He did not want to turn away, even though the baby’s babbling continued for perhaps 15 minutes. Reno was spellbound.
Loudness
To explain equine hearing, it helps to distinguish between loudness, pitch, and localization. The easiest to describe is loudness. The volume of a sound is measured in decibels (dB). By convention, 0 dB represents the softest sound that human ears are capable of sensing. It’s the tiniest tick in a soundproof room of dead silence—a tick that can be detected only by that rare 18-year-old who’s never gone to a rock concert or cranked up her earbuds. Even the most sensitive horse will not hear that sound.
According to the best available data, the softest sound an average horse can hear occurs at 7 dB. That’s the volume of a person breathing quietly. In general, then, the same noise seems a little quieter to a horse than to a person. Horses hear us speak to them but at slightly lower volumes than we believe we have used. On one hand, we might be surprised that horses do not perceive hushed noises as well as humans do…after all, spooky horses certainly seem to have bionic hearing out on the trail! On the other hand, we have to give them credit for hearing something as quiet as an easy breath.
Pitch
Within the volumes that people and horses can detect, a second aspect of sound is pitch. Pitch refers to the range of low to high frequencies that corresponds to musical notes from low bass to high soprano. Horses can hear pitches from about 55 to 33,500 Hertz (Hz). That’s similar to the 10-octave range (20 to 20,000 Hz) humans can sense. By comparison, Indian elephants hear lower pitches, and wild mice sense tones up in super-squeak territory (fig. 5.1).
5.1 Pitch ranges of various animals in young adulthood, with a piano range included for comparison. Horses hear pitches slightly higher than human range, but fail to notice pitches that we experience as very low.
Equine ears miss about an octave and a half of the lowest bass notes we can hear, those between 20 and 55 Hz. However, horses can pick up many of these low frequency sounds through vibrations in their teeth and jawbones while grazing. Their hooves also transmit low vibrations. Ears aren’t everything!
At the high end of the sound spectrum, equine ears surpass ours by half an octave (from 20,000 to 33,500 Hz). Horses can hear the ultrasonic squeal of a bat or dolphin, the silent dog whistle, and the noise that insect and rodent repelling machines produce—all inaudible to humans. Although researchers have found no difference in hearing sensitivity between male and female horses, males do pay more attention to sound. This heightened reaction probably occurs because it is the male horse’s job to warn mares and youngsters of danger.
Hearing Over Time
Equine hearing declines with age just as human hearing does. By the time a horse is 20 years old, he’s usually experiencing mild to moderate hearing loss. Humans decline much faster in proportion to our life span, with higher frequencies generally inaudible by age 30. A controversial anti-loitering system called the “Mosquito” is used in many countries to disperse young people from public areas that are prone to vandalism. The frequency of the noise it makes (17,400 Hz) is too high for older adults to perceive, but young people won’t hang around in such an annoying environment. Nor will horses!
You know how your mount sometimes stops dead and goes taut to listen to something that you keep telling him “isn’t there?” I’m only the messenger, but uh…if it’s high frequency, it probably is there. Our ears just can’t hear it. An older rider on a younger horse is at a particular disadvantage in terms of being unable to hear high frequencies as well as the horse does. Often the horse freezes in place to reduce ambient noise—a squeaking saddle, hooves striking the ground, the soft jangle of a bit. Grazing horses stop chewing when they hear a quiet noise—that’s because the grinding of their jaws interferes with their ears’ ability to transmit sound signals to the brain.
Aside from their physical range of hearing, horses have evolved to pay much greater attention to small noises than humans do. Although the two of you are often capable of hearing the same sounds, you might heed the voice inside your head—your plans and thoughts and worries—more than the crackling of a dry twig.
Interpreting Sounds
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