Page 114 - August 2022

EQUINE HEALTH
SOUNDS BETTER THAN OTHERS
“We all depend on our eyes. The first thing we do when we hear a sound is turn and look in that direction. Horses are already looking everywhere, so their ears don’t have to be that accurate in localizing a sound to tell them where to look. What they hear just tells them the general direction. If they just have the sound (and no visual contact) to go on, they are not very good at knowing where that sound came from,” Rickye says. This may make them nervous until they can actually see what’s making the sound.
“If they hear something and can’t see it, they get worried. This may be one reason horses are so jumpy. Their ears are not very accurate at telling them where a sound is coming from, and if they also can’t see it (hidden in the grass, for instance) they fear it’s a predator. The horse can’t hide, and his only survival response is
to run—so he wants to know which way to
run. Since horses are not sound localizers, they depend more on vision. Their ears tell them where to focus their visual attention. If their ears tell them something is over to their right, and they look in that direction and don’t see it, they get worried and just run the other direction.”
By contrast, cats are excellent localizers. They can hear exactly where a squeaking mouse is hiding in the grass and pounce on it. “If they were to pounce 3 degrees off, the mouse would get away. The horse doesn’t need this precision; however, all he has to do is run the opposite direction from a frightening sound,” she says.
Timney says the detection of sound depends on intensity and frequency of incoming sound waves. “In all species that have been tested, we’ve found that animals are maximally sensitive to some middle range of frequency and less sensitive to high frequencies or low frequencies. The range or frequencies and the level of absolute sensitivity varies greatly between species,” says Timney.
Also, there is an inverse relationship between
the distance between an animal’s ears and high frequency hearing. “Animals with smaller heads (less distance between their ears) generally detect higher frequencies. Animals that do not hear
the higher frequencies often have better low frequency sensitivity. In horses, the distance between the ears - the distance that sound must travel around the head from the opening of one ear canal to the other - is approximately
20 centimeters (about 6.9 inches), compared with approximately 30 centimeters (about 11.8 inches) in humans. Therefore, horses have greater sensitivity in the high frequency range and poorer sensitivity at lower frequencies than humans,” explains Timney.
How an animal localizes sound regarding the direction from which it comes depends on two characteristics of the auditory signal. One is the delay in the time at which the signal arrives at each ear—letting the animal know which ear is closest to the source of the sound. If the sound source is directly ahead or behind the head, the sound waves arrive simultaneously at both ears.
“If the sound is off to the side, the head would be in the way of this happening. It casts a sound shadow and one ear hears it slightly louder coming in from that side, and also it takes slightly longer to get from one to the other. These cues are used in detecting direction,” says Timney.
The ability of the animal’s nervous system to use this cue depends on ability to resolve the small difference in time of arrival. “Animals with larger spaces between their ears may have better sound localization capabilities. Their larger heads would generate a longer range of time delays,” he explains.
The second characteristic is the frequency- intensity spectrum of sounds that reach the ear. As mentioned, the head may create a sound shadow that decreases intensity of sound arriving at the ear on the side of the head opposite
the sound source. “This effect is frequency- dependent. Sound localization depends on
the extent to which an animal can integrate information from both cues,” says Timney.
The horse is good at judging direction
of low frequency sounds, based on time differences, and better at judging location of high frequencies based on the sound shadow.
It also helps that the horse has mobile ears and can swivel them independently in any direction. “A lot of animals that have mobile ears have this advantage—like a direction finder. They can move their ears to point toward wherever they hear the sound coming from,” says Timney.
HEARING LOSS IN HORSES
Horses, like other animals, occasionally lose hearing ability. It’s hard to know how common this problem is. Deafness should be suspected in horses that are unresponsive to external noises. These horses must rely on their sense of sight and touch.
Timney says there’s not much data on incidence of deafness in horses because it’s hard to systematically test large numbers of horses. “In humans, the most common causes of sensory- neural impairment are heredity, and in later
life, noise-induced hearing loss. There probably are not many horses that are congenitally deaf, except for some Paints—in which this can be
a hereditary defect. Noise-induced deafness is probably less common, and less of a concern, than in humans. Some older horses do become hard of hearing, but serious deafness is not common in horses,” says Timney.
Dr. Gary Magdesian (School of Veterinary Medicine, University of California, Davis) says that reported causes of deafness in horses include head trauma, middle or inner ear infection, and a syndrome called temporal hyoid osteoarthropathy (THO). THO is a degenerative process in
the joint between the hyoid apparatus, that supports the tongue, and the skull. “During this degenerative process excessive bone builds up, which can cause inflammation and compression of the eighth cranial nerve that is responsible for
“There probably are not many horses that are congenitally deaf, except for some Paints—in which this can be a hereditary defect... Some older horses do become hard of hearing, but serious deafness is not common in horses,” - Brian Timney.
112 SPEEDHORSE August 2022