Ear-ly Warning Systems By Erika Street /THE HORSE Because we rarely encounter problems with our horses ears, we often take them for granted. The equine ear, however, is an indispensable communication tool. A horses acute sense of hearing allows him to detect danger, communicate with other horses, and respond to his handlers vocal cues. Even the direction of a horses ears imparts a world of information. If you watch carefully, they will reveal the animals temperament and will even let you know where his attention is focused. Because the equine ear can convey so much information, learning how your horses ears work and how his hearing differs from yours will help you to better understand and predict his behavior. Structure of the Ear Horses ears, like yours, are finely tuned instruments designed to convert sound waves in the environment into action potentials in the auditory nerve. This nerve, which is located at the base of the skull, then sends the information to the brain to be translated and interpreted. To collect sound waves from the environment, a horse uses his pinna, the large, cup-like part of the ear that you can see. Made of cartilage, the pinna can rotate to capture sound waves from all directions. This useful ability is due to the fact that horses have 16 auricular muscles controlling their pinna. Humans, in contrast, only have three such muscles, all of which are vestigial (almost useless). After being trapped by the pinna, the collected sound waves are funneled through the external ear canal (commonly referred to as the auditory canal) to the middle ear, where they cause the eardrum, a thin membrane, to vibrate. These vibrations are then sent through the ossicles, a series of three tiny bones called the malleus, incus, and stapes. Finally, they reach the inner ear, where they cause vibrations in a snail-shaped structure called the cochlea. Running up and down the cochlea are extremely sensitive hair cells that act as transducers. When these hair cells bend, they generate electrical signals that stimulate the auditory nerve. This nerve then passes the impulses on to the brain. Ears in Communication When attempting to hear something, horses will automatically flick their ears toward the source of the sound. Most horse owners are familiar with this phenomenon; we often see horses prick their ears forward when they are concentrating on something directly in front of them. This easily observable honing in on a noise is called the Pryer reflex, and it allows a horse to instinctively focus his attention on sound sources in the environment. Knowing that a horse intuitively directs his ears toward whatever hes focusing on can come in very handy, especially when riding. The Pryer reflex can help you anticipate a spook or check where your horses attention is directed. In addition to showing us what a horse is concentrating on, the direction of the ears can tell us a lot about that particular animals temperament. Although lop ears that flop either forward or out to each side are not correct from a conformational standpoint, they are said to be signs of a kind and generous horse. Similarly, a horse whose ears are frequently laid back against his head is considered to have a bad temper. The reason horses often make this threatening gesture when they are angry or aggressive is thought to date back to prehistoric times, when horses adopted this posture to prevent their ears from being damaged when fighting. Hearing As they flick back and forth, a horses sensitive ears pick up a large range of sounds. According to Rickye Heffner, PhD, professor of psychology at the University of Toledo and a specialist in mammal hearing, horses can hear moderately loud sounds between 55 Hz and 33.5 kHz. This is in sharp contrast to a humans hearing; we cannot hear sounds higher than 20 kHz. Although horses have very sensitive hearing even at such high frequencies, their ability to locate the source of a sound is not very precise. Spooks very often arise from the fact that horses can only locate the general direction of a noise, not its exact origin. Because horses are prey animals, one might think this disability would make wild horses more susceptible to predators. A horses eyes, however, have evolved to make up for his lack of accurate sound localization. Horses have a well-rounded and long field of vision, allowing them to easily see oncoming predators from nearly any direction or distance. So what makes some horses spook more often than others? Because sounds can be associated with negative experiences, a spooky horse might connect a particular noise with the threat of danger. Some horses seem to jump at every little sound, regardless of its origin. While such animals might appear to have ultra- sensitive hearing, Sandra Edgar-Sargent, DVM, believes those horses are probably just more responsive to the sounds in their environment. No horse, however, reacts to every sound it hears. Although horses instinctively pay attention to the vocalizations of other equines, as well as sounds that are not part of their normal repertoire, they filter out much of what they hear. This helps them make sure that only relevant sounds are acted upon. Not all horses who fail to respond to a sound do so because they cant hear it, says Heffner. They may not be attending to the sound or they may have learned that the sound is not informative or important. We ignore most of the sounds around us even though we can certainly hear them. Hearing Loss Horses can, however, lose their hearing. Like any other animal, says Heffner, horses can have hearing loss due to age, some antibiotics, ear mites, and genetic disorders. According to Heffner, age-related hearing loss generally begins to be noticeable in a horses middle age (around 15 years), but can occur much earlier if a horse is exposed to loud sounds. Hearing loss almost always affects high frequencies the most, she says, and these are particularly useful for sound localization. However, horses are unusual in that they dont use high frequencies to locate sound sources along the horizon, so they probably would not be affected in this ability by a high-frequency hearing loss. They are very poor at localizing sound in the horizontal plane and cant get much worse. High frequencies are also useful, however, for using the pinna to tell whether a sound is in front of or behind the horse, and for localizing sounds in the vertical plane (up-down distinctions), and these abilities could be reduced. Horses that do lose their hearing generally compensate very well. However, if you suspect your horse is hard of hearing, Heffner suggests that you try making a very soft hissing sound with your back to the horse every time you give him grain for a few days. Then make the sound and see if the horse responds by associating the noise with getting grain. If he does, he probably isnt hard of hearing. If he does not react, Heffner stresses that this does not necessarily mean that your horse has a hearing loss. He just might not be fooled by your trick. According to Sargent, the best way to truly evaluate a horses hearing is through a BAER (brainstem auditory evoked response) test. This hearing test uses small electrodes placed under the skin of the scalp to detect electrical activity in the cochlea and auditory pathways. If your horse does have a hearing loss, you should make sure your commands are loud and clear and given when there are not other competing noises around, cautions Heffner. You should not depend on the horse to know where sounds are coming from when out riding, and if the loss is severe, the horse may not detect oncoming vehicles or other animals. It might, therefore, be startled more than usual when something suddenly comes into view. Hearing loss can also occur because of conditions that do not involve the ear itself. Parotitis, for example, is the swelling and inflammation of the parotid salivary gland, just below the ear. That can cause hearing loss. Diseases of the guttural pouches can also cause swelling in this area and loss of hearing. The function of guttural pouches--sacs that open into the Eustachian tubes of the inner ears--is not known. However, because the sacs are located so close to the pharynx, bacterial upper respiratory infections can sometimes spread into them and cause the accumulation of pus. Fungi can also invade the guttural pouches, causing pain in the parotid area, nasal discharge, neck stiffness, and abnormal head posture. Potential Ear Problems Problems in the guttural pouches can unfortunately also migrate to the middle ear. A middle ear infection can be the result of bacterial or fungal infections that come from the bloodstream or from the guttural pouch, says Sargent. Unlike other species, when a horse gets a middle ear infection, it often migrates ventrally (downward). Then, instead of rupturing the eardrum and draining into the external ear canal, it causes inflammation of the tympanic bulla, which houses the middle ear, as well as the stylohyoid bone (part of the skull). Because this inflammation causes excessive bone formation, it results in fusion of the temporohyoid joint where the stylohyoid bone and temporal bone meet. Fusion of this joint can lead to stress fractures in the petrous temporal or stylohyoid bones of the skull, which can in turn cause neurological problems. If this occurs, Sargent says the horse might exhibit ear rubbing, head tossing, and chomping movements, and he might have pain when the base of the ear is palpated. A severely affected horse will be depressed, keep his head tilted, walk in circles, and appear dizzy. The nerves of the face might also be paralyzed, resulting in drooping ears and lips, drooling, and an inability to blink. Fortunately, Sargent says, ear problems are rare in the horse. Middle ear infections are uncommon, and horses rarely get infections of the external ear canal like dogs and cats do. Probably the most frequently encountered problem with a horses ears is external parasites. Ticks, chiggers, and Psoroptes mites can all sometimes get down in there, says Sargent. In the southwestern and western parts of the United States, there is a certain kind of tick called the spinose ear tick that can get in a horses ear, but it occurs uncommonly. According to Heffner, ear mites can be the cause of head rubbing, head shaking, and irritability. Some ear mites produce a waxy plug in the ear canal between them and the outside world, which prevents medication from reaching them and also affects hearing, she says. You cant always see the mites because they live in the ear canal near the eardrum, but if the horse is restrained, they might be visible using an otoscope. If you suspect that your horse has ear mites, even if you cant see them, it is best to consult your veterinarian so that the horse can be treated promptly. Your vet will likely need to use heavy sedation in order to examine the deep ear canal and might recommend a dewormer or special drops to get rid of the mites. Like mites, blackflies are also quite irritating to horses. Although these tiny, blood-feeding gnats are only 1-6 mm long, their minuscule, serrated mouthparts can inflict painful bites. In addition to being extremely aggravating to horses, blackfly bites are thought to spread the virus that can cause aural plaques--flat, gray-white papillomas that are sometimes found on horses ears. Aural plaques are scaly lesions that form on the inside of the pinna, says Sargent. They are caused by a virus and are typically asymptomatic, meaning that they really cause the horse no problems, so we typically recommend leaving them alone. General Ear Care Although ear problems arise infrequently in horses, Sargent suggests keeping an eye out for excessive head shaking and discharge from your horses ears. If your horse is shaking his head repeatedly and rubbing his ears on anything he can find to do the job, or if there is blood or fluid coming from his ears, you should call your veterinarian. Normally, however, if you are not seeing any of those things, I would not recommend doing anything to your horses ears, comments Sargent. Horses often resent their ears being handled, so if youre not having any problems, I would leave them alone. Because the hair in horses ears prevents dirt and insects from getting inside, Sargent also recommends that you refrain from clipping your horses ears. If theyre not having any problems, I wouldnt stress a horse out by clipping his ears, she says. The only time I would trim the ears is when the horse is real sensitive to gnats, and the bites are making his ears bleed. If his ears are scabby and the hair is matted up, then you might want to shave the ear hair, clean the area out, and put some kind of fly repellent on the ear. I would only clean a horses ears, however, upon the recommendation of a veterinarian, because you can do more harm than good if you get fluid down in the ear. Unfortunately, not all horses will let you clip the hair from their ears. Some horses will require sedation for you to be able to trim the hair out of their ears or even examine them very closely, says Sargent. If your horse is not head shy, however, Sargent recommends using small, quiet clippers like those used on dogs. If you have a horse who is excessively bothered by gnats, Sargent also suggests that you wipe the ears with fly repellent or use a fly mask that includes ear covers. There are also lotions and creams that have fly repellent in them, she says. If your horse will let you apply them, they have more of a residue action and last longer than just using fly spray. She recommends choosing the method that your horse will tolerate. Take-Home Message Taking precautions will help keep your horses ears comfortable and allow him to hear properly. Along with keeping him healthy, paying attention to your horses ears will give you valuable insight into his behavior.
Posted on: Mon, 05 May 2014 14:09:43 +0000
Trending Topics
Recently Viewed Topics
© 2015