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Hearing

Last revised April 19, 2017.

Ears in different directions. This donkey demonstrates how to listen to two things at once.1

 

Many nights I sit out with the raccoons who live near my house. All come for the dry dog food I put out for them, but some also come for the broken cashew bits that I offer from my hand, and a very few seem to come for the companionship. You can learn a lot talking to a raccoon. For one, I’ve learned that you don’t need to speak very loudly to call one. My call, softly spoken, can be heard from a good distance (I know this, because often I can see them lurch to attention and begin walking toward me, and I know how far away they were when I called.). But their hearing is even more instructive when they are on the alert for danger. A raccoon might be sitting on my bare foot, staring into the ferns, eating a nut. All of a sudden, she turns her head, and focuses her ears on some distant point. I can hear nothing. I can’t see a fern moving. Minutes later, after prolonged attention, a fox or raccoon might emerge from the direction the raccoon was monitoring.

Horses are much like raccoons. Their hearing is much better than ours, and they’ll turn ears and head to focus on a sound, revealing to us the direction of the sound of interest.

Horses show a Preyer’s reflex,2 immediately pointing one or both ears in the direction of a new sound. If they are not much disturbed by the sound, they will monitor it with their ears, but not bother to turn their heads. More alarming sounds are targeted with ears, and then the head turns toward it, so that the eyes can look for the source of the sound. As additional sound comes in, he may adjust his ears slightly to better focus on the sound.

Volume: The external ears of a horse or mule — the pinnae (singular: pinna) — are much larger than ours. And unlike ours, they can be turned to focus on the source of a sound. When your horse spooks on the trail and freezes, he will turn his ears toward the source of the alarming sound he just heard. Watch those ears, and you will know where the sound came from, even if you didn’t hear it yourself.

Horses differ in what they then do with the information in what they hear. Some “bombproof” horses may not change their behavior at all. Other more reactive horses may stop in the tracks, raise their heads, freeze their body position, and wait. On receipt of further alarming information, a reactive horse may turn and move in the opposite direction from the sound.

Of course, a horse that seems to freeze for no reason on the trail actually has a reason. Because you didn’t hear what he heard, you did not know the reason. There are several reasonable responses to this situation:

  • Walk a mile in his shoes: use a hearing aid for yourself, turned up so that you can hear as well as your horse. There are many low-cost hearing amplifiers at Amazon.
  • Hear no evil: Use a deafening aid for your horse (cotton in the ears, for instance), so that he can’t hear any better than you. There are many such products available. I plan to try Panama’s Never Lost Ear Plugs or Cashel Equine Ear Plugs with String. Both include a string you can use to attach them to the browband, so you won’t lose them or forget to take them out. (See Product Recommendations at the end).
  • Identify the sound that is triggering his alarm, capture it with a tape recorder or your cell phone, and desensitize him to it by playing it again and again, while you offer him milk and cookies or whatever he likes.

Humans may hear better than horses when the sound is below 8 kHz in frequency, but horses excel at higher frequencies. If your horse balks on the trail, and you have heard nothing, it is likely that the sound heard was of a higher frequency than you can hear. High-pitched sounds include squeaks, squeals, breaking glass, mosquitoes, birds making an alarm call, chalk scratching on a blackboard. Come to think of it, if we could hear all that, we might spook too.

Most human speech is between 300 Hz and 3400 Hz (between .3 kHz and 3.4 kHz). If we are to speak at a normal frequency, how loud must we be for our horse to hear us? According to the graph, a horse can hear a 10 or 20 decibel sound if it is at the frequency of human sound. Twenty decibels is that of a whisper, or of rustling leaves.3 So a horse whisperer would be heard just fine, as would a rustler. Speaking louder than a whisper may not improve your communication.

The audiogram for horses is represented by the solid lines, while the audiogram for humans, shown for comparison, is represented by the dashed line. The dots show the average lowest intensity at which a particular frequency could be heard. Horses have better high-frequency hearing, while humans have better low and mid-frequency hearing. From Heffner & Heffner, 19834

Localization

The horse is able to determine the source of a sound by comparing the arrival time of that sound to each ear. Sound travels at 1,126 feet per second5. So if there are only six inches separating his left and right ear, a sound from the right would reach your horse’s right ear at time 0, and would reach his left ear (6″ farther) just 0.00044 seconds later. But your horse can do better than that because he can identify a sound coming in from any angle. With our eyes closed, we can also tell where a sound is coming from, if we can hear it.

Horses and humans are not very good at localizing those high frequency sounds that they hear well. Birds seem to have learned this 70 million years ago, and have shifted alarm calls up and up and up. So stand in the woods, and listen: you may hear a bird warning everyone about your presence, but you’ll have trouble finding it. It is not known why humans and horses can localize low frequencies well, but localize high frequencies so poorly.6

To assist in localization, horses will turn their ears, then their head, toward the source of a distant sound. With the head turned, their localization can be assisted by their vision. On an open plain, it is not likely that a mountain lion will be able to avoid detection by both a horse’s ears and eyes.

Two Ears, Two Brains. You may have noticed that your horse’s ears usually point in the same direction, focusing on the same sound source, but that they sometimes point in different directions.

In the horse, the corpus callosum — the primary neural connection between the two sides of the brain — is much smaller than in a human. Horses have found much value in this “weak” connection. While it is likely that most skills need to be learned by each side of the brain, it is also likely that one side of a horse’s brain can sleep while the other side watches for danger or goes about its business. And it is likely that each side controls its own ear, and so can focus on what that ear is hearing. A horse can be thorough in analyzing a sound by simply pointing both ears at the same sound, but can track two sounds from different sources quite independently. Us humans (especially us old men) are not so good at listening to two things at once, and so one sound either distracts us from listening to another, or escapes our attention. Our selective attention focuses on one thing, a horse’s can focus on two things.

While the corpus callosum connects both sides of the brain, each ear’s signals are processed largely by the hemisphere on the opposite side — contralateral dominance. His left ear appears to be most important in processing strange sounds and the sounds of strangers, and he prefers his right ear for listening to his neighbors.7

So if you are going to whisper sweet nothings in your horse’s ear, choose his right ear, because his processing of what we care about is done mostly on the left side of his brain. As I said in “The Mind of your Horse”, the left side is specialized for processing positive emotions, for approach, for vocalizing, and for recognizing species-specific vocalizations.

Tone and Changing Tone. Horses seem to be quite sensitive to pitch, and to changing pitch. So to get a lounged horse to pick up the speed, you might say “tr… ott!”, with the “ott” pronounced at a higher pitch than the “tr”. To get him to slow down, you might say “wa… alk!”, with the “alk” pronounced at a lower pitch than the “wa”.

The human voice not only carries denotative meaning (of the sort we understand when we read), but connotative meaning — emotions. As our arousal level rises because we are angry, excited, or afraid, the tone and volume of our voice also rises. The other day I was standing in the round pen with Bud, both of us relaxed. Bud was learning to touch the big ball, in preparation for an eventual soccer match with G. Someone yelled that I must not give Bud carrots, because Bud would learn to bite, and if this person were ever bitten by Bud, Bud would get punched in the face. I spoke on behalf of Bud, and told the speaker that he’d better never hit Bud. But I must have yelled in response. Bud took off in the round pen, and did three laps before I was able to settle him down and get him to walk back to me. At first, I felt funny, because I wasn’t talking to Bud. I wanted the jerk who complained to run around the round pen. But then I realized that Bud didn’t actually understand what was going on.

Horses and mules are sensitive to tone of voice, and read the emotion of our voice far better than they will ever read the words we are speaking. I suppose they are also sensitive to being punched in the face. Your job is to control the emotion in your voice — either insert emotion if you don’t feel it, or remove it if you do, until your voice conveys what you want your horse to understand.

Duration. If a high frequency sound is prolonged, such as a whinny, horses are able to isolate the source by turning their ears. But if a high frequency sound is brief, horses can’t pinpoint it using an interaural intensity difference that the sound shadow of our heads produces. So brief high frequency sounds are more likely to arouse panic in a horse than they are in a human. A snapping twig can trigger a flight reaction.

In contrast, lower frequency sounds can be localized well in both humans and horses, using the interaural time difference: the ear closest to the sound hears it first. Sounds below about 2,000 Hz are not likely to cause panic in your horse.

Birds are talkative. They issue warning calls, sing love songs, whisper to their babies. Pet birds are usually so noisy that they may interfere with your telephone conversations. In contrast, most mammals are normally silent except for warning calls. Sit quietly at your bird feeder until the birds and squirrels relax and return. Now say something to them. The birds will generally hold their position, but the squirrels may panic and run. How much an animal vocalizes provides us with a good hint on how important “verbal” communication is to them.

Horses are more like squirrels.

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1 From “Saturday Encore — The Secret Language of Donkeys” at the 7msnranch.com

2 Sankey, Carol, Marie-Annick Richard-Yris, Helene Leroy, Severine Henry, and Martine Hausberger. “Positive interactions lead to lasting positive memories in horses, Equus caballus.” Animal Behaviour 79, no. 4 (2010): 869-875.

3 See “Comparative Examples of Noise Levels. “ IAC Acoustics. http://www.industrialnoisecontrol.com/comparative-noise-examples.htm

4 Heffner, H. E., and Heffner, R. S. “The Hearing Ability of Horses”. Equine Practice March 1983 Vol 5, No. 3, p. 32

5 in dry air at 68° F.

6 Birds, in contrast, seem able to localize high frequency sounds quite well. But I’ve found no papers comparing mammals and birds for sound localization of high frequency sounds. See Nelson, Brian S., and Philip K. Stoddard. “Accuracy of auditory distance and azimuth perception by a passerine bird in natural habitat.” Animal Behaviour 56, no. 2 (1998): 467-477.; Schnyder, Hans A., Dieter Vanderelst, Sophia Bartenstein, Uwe Firzlaff, and Harald Luksch. “The avian head induces cues for sound localization in elevation.” PloS one 9, no. 11 (2014): e112178.

7 Basile, Muriel, Sarah Boivin, Anaïs Boutin, Catherine Blois-Heulin, Martine Hausberger, and Alban Lemasson. “Socially dependent auditory laterality in domestic horses (Equus caballus).” Animal cognition 12, no. 4 (2009): 611-619.

 

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