Understanding your horse’s eyes and ears

0
239
Understanding your horse’s eyes and ears

This in-depth guide will help you gain a better understanding of how your horse’s eyes and ears work, and what you can do to keep them healthy.

It’s a lovely day, perfect for a trail ride. You and a friend load up your horses and tack, and head to your favorite trailhead. You start quietly making your way down the trail, when suddenly your horse freezes. Her head rises, ears erect, and her gaze is directed into the trees on your left. You scan the thicket, don’t see any cause for concern, and encourage her to go forward. She refuses, feet firmly planted. Your friend’s horse does the same. Again you try to see what your mare is seeing, but can’t make out anything. Then you hear some rustling, and a black bear emerges from the forest up ahead. You and your friend are stunned, but the horses knew what was coming. How were they able to detect that you weren’t alone on the trail, when you couldn’t see or hear anything?

Your horse’s ability to identify a potential threat among the trees is due to a finely tuned sensory system that has evolved over millennia to help her survive. Among all the senses, a horse’s vision is one of the most developed and complex. Playing a supporting role to the horse’s vision is his hearing. In this article, we will explore the anatomy of your horse’s eyes and ears, learn how they see and hear, and discuss how to keep them in tiptop shape.

The eyes

Anatomy

Let’s begin with some basic anatomy of the eye. A horse’s eye is laterally placed on his head, meaning the eyes are on the sides of the head rather than on the front, as they are with us. The cornea is the outer surface of the eye, the first structure that light passes through. While physically strong, the cornea is quite thin – the middle portion is only 1 mm to 1.5 mm thick (this is why corneal scratches/ulcers are so common in our equine friends!). A fibrous layer called the sclera coats the entire eye (except over the cornea), serving as an attachment point for ocular muscles; the sclera can be seen as the “whites of the eye.”

The colored part of the eye is known as the iris; while it gives our horse’s eyes their beautiful appearance, it also serves to control the amount of light entering the eye. Muscles within the iris dilate or constrict the pupil, depending on the amount of light present. The ciliary body is the structure behind the iris that produces aqueous humor (clear fluid that supports eye health) and helps focus the lens. Speaking of the lens, this structure sits behind the iris and acts to further focus light rays on the retina.

At the back of the eye are several key structures. One is the horse’s optic nerve, a bundle of nerve fibers responsible for carrying visual messages to the brain. Another is the retina, which is responsible for converting light energy into chemical energy, creating the electrical signal that gets sent from the eye to the brain in order for your horse to see.   Between the retina and the sclera is the choroid, which serves as the primary blood supply to the eye. The iris, ciliary body, and choroid are collectively called the uvea; together, these structures function to produce and drain aqueous humor, provide nutrition to the eye, and create the immune response within the eye. Uveitis is a disease in which this structure is affected/inflamed.

How your horse sees

What are the steps involved in seeing? First, light from the outside world enters the eye and is focused on the retina, with the help of the cornea and lens. Then, the retina interacts with particles of light (light photons), changing light energy into chemical energy and then into electrical energy. Billions of photons interact with more than 100 million photoreceptors (structures responsive to light) in each eye every second! The electrical energy becomes electrical signals. These electrical signals are divided into several categories – brightness of an object, motion, location, etc. – before being transmitted to the brain. This categorization of information prevents a sensory information overload.

Once they reach the brain, the signals are sent to specific areas of the brain and then processed into useful information for the horse. Relevant portions of the newly formed image are selected for further attention and action. The horse’s brain also compares this new sensory input with previous images, looking especially for any changes to the image, and comparing this new information with input from the other eye and senses.  Once this entire process is completed, only the information pertinent to the horse’s current situation – such as navigating a jump course or loading in the trailer – is privileged enough to make it to the level of conscious attention. So, unlike a photo from a camera which captures every detail of a scene, the horse’s brain picks out only the most important or relevant details. Why? Because as a prey animal, those are the details beneficial to his survival.

This sounds like quite the process, right? A ton of information travels along the vision “highways” and must be sorted through for only the most pertinent information. While each aspect of vision is critical to the horse, the most important among these is his ability to identify an object (say, a mountain lion) from its surroundings (dense woodland) – this ability is key to survival for this prey animal.

When a horse looks out over the horizon, the visual fields of both eyes combine to give him a 350° view. This means that our horses have an almost complete visual sphere around themselves – save for a few small blind spots. These blind spots are located at the forehead, directly underneath the nose, and directly behind them.

Horses have slightly diminished visual acuity, or the ability to see detail. In people, visual acuity is described by the familiar 20/20 – meaning that if you have 20/20 vision, you can see objects normally and clearly at 20 feet. Horses have a visual acuity that ranges from 20/30 to 20/60, so a horse must be at 20 feet to see what a person with 20/20 vision can see at 60 feet. If only they made prescription glasses for horses!

Horses have a number of extraordinary ocular adaptations that have evolved over time.  They have one of the largest eyes of land-dwelling vertebrates, thus allowing more light to enter the eye. The equine pupil can dilate six times larger than a human’s, and three to three-and-a-half times larger than a cat or dog’s. Thanks to the location and horizontally-elongated shape of the pupil, the horse takes in a much broader view of the horizon than you or I do – like a panorama photograph. At the back of the eye, a tapetum lucidum allows your horse to see and function fairly well in dim lighting, while also giving his eye that strange green/yellow tint you sometimes catch in low light.

A study performed in 2009 showed that horses were capable of discriminating between shapes at light levels approximating those of moonlight, starlight on a moonless night, or under cloud cover at night. The study also demonstrated that even when horses lost their ability to make visual discriminations because conditions were so dark, they could still navigate well enough to locate a feed bowl in a stall without bumping into things. This is how wild horses can navigate rocky, uneven, and potentially hazardous terrain in the dead of night with relative ease and little injury.

You know those weird, brown, wavy-looking things in your horse’s eyes, near his pupil?  Those are the corpora nigra; they are located on both the upper and lower aspects of the pupil. Acting essentially as a hat brim or visor, these structures decrease the amount of light entering the eye in exceptionally bright circumstances, thus helping decrease glare and improve vision. Basically, built-in sunglasses!

A common question I get asked is: do horses see color? Yes, but not in the same way we do. Horses have dichromatic color vision, meaning they have two types of cones (light receptor cells in the eye). People have trichromatic color vision, (three types of cones). What this means is that horses see in only two hues – believed to be colors similar to blue and yellow. A horse’s perception of color is also much less vivid than a human’s – instead of seeing the bright, rich colors of that new blanket you just bought, your horse sees washed-out pastel or sepia tones. Because a horse’s food source is stationary – except while it’s being transported by you or her caretaker – and doesn’t need to be chased or captured, the ability to perceive a wide range of hues doesn’t offer much advantage in the way of survival for the horse.

The ears

Anatomy

The equine ear is divided into three different sections: outer ear, middle ear, and inner ear.

  • The outer ear is also known as the pinna (or pinnae, plural), and is what we externally see as the horse’s ear. The majority of the pinna is cartilage, covered by skin and hair. To help capture and direct soundwaves through the ear, the pinna is funnel-shaped, like a little satellite dish. Pinnae are mobile and can move independently of each other to locate or capture a sound. In fact, horses have ten different muscles surrounding their ears, while we humans only have three!  The outer ear is the start or opening of a long ear canal that ends in the ear drum, or tympanic membrane – this is a thin membrane that picks up soundwaves and begins the process of hearing. Because this ear canal is so long, and many horses are not fond of ear manipulation, veterinarians don’t routinely examine your horse’s ears with an otoscope, like your dog or cat’s vet might.
  • The middle ear begins on the inner side of the tympanic membrane. It is an air-filled cavity, and contains the three smallest bones in the body – the malleus, incus, and stapes, collectively called the ossicles. The eustachian tube is also found within the middle ear, and is a small tube connecting this area of the ear with the back of the nasal cavity, allowing air to enter the middle ear.
  • The inner ear is a complex maze of fluid-filled channels. These channels are lined with thousands of sensory cells, which are responsible for triggering the nerve involved in hearing and balance. Also within the inner ear is the cochlea, or “organ” of hearing.

How your horse hears

As mentioned above, the pinnae act as funnels, bringing sound waves to the tympanic membrane. Sound waves cause the tympanic membrane to vibrate. These vibrations are transmitted through the middle ear via movement of the ossicles. This movement of the ossicles in turn creates movement of the cochlea in the inner ear. As the cochlea moves, it creates back and forth movement of the hair-like cilia of the sensory cells in the inner ear.  Movement of the hair cells mediates the transformation of sound into action potentials which are then sent to the brain, and interpreted as a specific sound.

Horses hear sounds over a wider range of frequencies than humans do. The typical human range of frequencies is 20 to 20,000 Hertz. A horse can hear frequencies within the 55 to 33,500 Hertz range. While our equine companions might have superior hearing compared to us, they are beat out by our other four-legged companions, dogs. Dogs can hear frequencies as high as 45,000 Hertz, sometimes even higher.

Overall, a horse’s hearing is not nearly as well-developed as his vision. Because a horse can see so well, he really only uses his hearing to help pinpoint a sound in order to be able to turn and look at where it is coming from. The ears help direct their gaze.

Caring for the eyes and ears

What is the best way to care for such intricate and important structures?  Let’s start with the eyes.

EYE CARE

Use a fly mask

First, I recommend using a fly mask during the warmer months (which may be year round, depending on where you live). Fly masks serve a dual purpose in protecting your horse’s eyes from the sun and from insects.

Reach for the repellent

In addition to a fly mask, you can also use fly repellant around the horse’s eyes. Use either a roll-on repellent, or fly spray applied on a cloth, and apply under the eye only – products applied above the eye can mix with sweat and may drip down into the eye, causing irritation.

Check for hazards

Carefully analyze your horse’s environment – stall, run-in shed, pasture, etc. – and remove or modify any objects that he could potentially injure an eye on. Sharp objects, hardware, bucket clips, and more are all potential hazards. Due to the large size and lateral position on the head, horses’ eyes are more prone to corneal abrasions (which can lead to corneal ulcers) and ocular trauma. And, as we learned above, the cornea is not very thick – so it doesn’t take much to damage it!

Wipe them down

When grooming, if needed, gently use a soft cloth dampened with water to clean up any discharge, dirt, or debris near the eyes. Avoid using any chemicals or products around the eye, as they may cause harm if they get in the eye.

Leave the hair

I recommend leaving the hairs around your horse’s eyes alone. These hairs are whiskers that act as tactile sensors, aiding the horse in gathering more sensory information about his environment. While the whiskers themselves have no nerves, the follicles they grow from is very innervated, and send signals to the brain about the information they are receiving. In some countries, such as Germany, shaving the whiskers around the eyes, and muzzle, is now outlawed. Check in with any organizations/associations you show with for further rules/regulations around this practice.

Check with a vet

And lastly, do not put anything into your horse’s eyes without direct instruction from your veterinarian.

When thinking about equine eye health, I encourage you to look for the following signs, and if noted, contact your veterinarian. Any abnormality of the eye should be considered an emergency.

  • Excessive tearing
  • Squinting or holding one/both eyes closed
  • Obvious trauma to the eyelid — laceration, presence of foreign body, bleeding, etc.
  • Swollen eyelids (upper, lower, or both)
  • Yellow or green discharge
  • Sudden change in appearance of the eye — swollen, cloudy, sunken, etc.
  • Any growths or masses around the eye/on the eyelids
  • Redness of the eye or the tissues surrounding the eye

EAR CARE

Luckily, ear problems in horses are infrequent, but ears still require the same care and diligence you give the rest of your horse. Many of the same care recommendations for the eyes also apply to the ears. Fly masks with ears are excellent at keeping most biting insects out of your horse’s ears. Use fly spray, applied on a cloth and then wiped in/around the ear, or a lotion/cream with fly repellant in it. Avoid vigorously cleaning the ears (anything more than rubbing with a damp cloth) or applying any type of liquid into the ears (unless your veterinarian has directed you to do so). You do not want to accidentally introduce fluid down into the ear. Consider leaving the hairs on the inside of your horse’s ears alone – these hairs function as a barrier to keep dirt and insects from getting further down into the ear canal. Clipping the ears can also be a stressful procedure to perform if your horse is head-shy.

Keep an eye out for these signs, and if you notice any of them, contact your veterinarian for further direction:

  • Excessive headshaking
  • Discharge from the horse’s ears (blood or fluid)
  • Repeatedly rubbing his ears on something

The eyes and ears play a vital role in not only your horse’s day-to-day life, but in his ability to fulfil his part as your companion and riding partner. Now that you know more about the anatomy, function, and care of equine eyes and ears, you can include these structures in your holistic health plan.