Horses share many of the same physiologic characteristics of people and domestic pets, in that they have a circulatory system, a respiratory system, a nervous system, and so on. However, they also have many unique characteristics that differ from those of people and other companion animals. For example, horses have developed a large strong band of connective tissue, called the nuchal ligament, to provide support for their relatively long and heavily muscled neck.
Horses have a slower metabolism than other companion animals, with resting heart and respiratory rates that are slower than those of people. This slower metabolism is associated with a longer life span than that of many other companion animals. Actual life span depends on overall health, level of care, and size. For example, horses of miniature breeds routinely live 40 years or longer (see Table: Normal Equine Physiologic Values).
Horses generate considerable heat during exercise. They lose heat primarily by evaporative cooling (usually sweating), just like people do. However, because sweat does not readily evaporate during hot, humid weather, evaporative cooling becomes ineffective under these conditions. During hot, sticky summer days, exercise should be limited to avoid heat stress. Heat stress and continued exercise can lead to dehydration and eventually shock. Adequate water, proper diet (including vitamins and minerals), and limits on exercise are all needed to avoid heat stress.
Horses are large, bulky animals that are good at conserving heat during periods of colder weather. Regardless, they still need adequate shelter, diet, water, and routine veterinary health care to prevent wintertime problems (see Providing a Home for a Horse).
Horses have the same 5 senses as people do but to very different degrees. Some senses are less developed than in people, while others are more powerful.
The primary sensory input in horses is sight. The importance of vision is reflected in the size of the equine eye, which is the largest of any land mammal, and by the fact that the visual cortex of the equine brain handles one-third of all sensory input.
Horses’ eyes are set on the side of the head, rather than facing front as in people, dogs, and cats. This gives them extraordinary peripheral vision, which is useful for animals (for example, rabbits and most birds) that must constantly watch for predators. Horses can generally see over a 340° arc without moving their heads, with only small blind spots directly behind and in front of them. These blind spots are caused by the body of the horse (behind) and the large forehead and muzzle (in front) obstructing the horse’s vision. Horses step slightly to the side to see things behind them, and back up and lower their head to see directly in front.
Horses see a panoramic view as a form of monocular vision, which means that each eye is viewing images independently. These images are transferred to a band of retinal cells called cones within a “retinal streak.” A second group of cones provides binocular vision in an arc of 55 to 65° in front of the horse. The relatively large number of cones enables horses to see distinct images better than dogs do but not quite as well as people can. These cones also mean that horses have some color vision. Horses also have a large number of rods, which are the specific type of cells in the retina responsible for night vision, as well as the reflective tapetum lucidum, which is also found in both dogs and cats. For this reason, horses see considerably better in the dark than people, and even cats. Horses also share the protective third eyelid that is found in both dogs and cats (see Description and Physical Characteristics of Dogs : Sight).
Because horses rely on monocular vision, they have poor depth perception. They can misjudge the depth of a small puddle or the distance to a fence. Horses compensate for this by comparing the size of an object to their memory of what they have seen in the past. For example, if a person or fence appears smaller, then it must be farther away. A horse will lower its head to judge closer distances and raise it to judge objects farther away.
In general, horse vision is a little blurrier and a little less colorful than human vision. However, horses see movement very well throughout the 340° arc of their peripheral vision. This is why horses may “spook” when confronted with even minor changes around them—another useful survival skill for herd animals.
As horses age, their vision may worsen. Unfortunately, there are few medical treatments for poor sight in horses. However, watching for signs of poor vision can help prevent injury to both you and your horse.
Horses have large ears that are good at magnifying sound and noting its direction. Each ear can swivel independently up to 180°, allowing horses to locate multiple sounds at the same time. The ears also provide clues to a horse’s emotional state. For example, a horse with ears that are laid back may be indicating aggression, pain, or fear (such as in response to a loud or unfamiliar noise).
In general, horses hear slightly better than people do and are able to hear sounds at both higher and lower frequencies. Horses are good at hearing the high-pitched squeaks or crackles associated with the stealthy approach of a predator.
Horses experience age-related hearing loss, which means that older animals may not hear your approach as well as they did when younger. Spooking can be avoided by making sure that your horse can see you or knows you are approaching. Hearing loss can also be caused by ear infection or by a mite or tick infestation, so you should ask your veterinarian to periodically check your horse’s ears.
In addition to providing information about the world in general, the sense of smell is the primary way that horses recognize each other as well as people. For example, horses exchange breath on meeting, and stallions assess the sexual status of a mare through scent. The equine nose has a large internal surface area that contains many chemical receptors within the mucous membrane. The surface area devoted to scent detection is many hundred times greater than in people, again highlighting the importance of the equine sense of smell.
Horses enjoy their food through the sense of taste. Taste buds are located on the tongue, the soft palate, and the back of the throat. It is not known whether horses have the 4 -basic types of taste (sweet, sour, bitter, and salty). However, it is known that they can at least distinguish salty and sweet. In fact, like people, horses are known to have a sweet tooth, appreciating such things as apples, carrots, and honey. Horses are also known to tolerate substances (including medications) that people generally find very bitter.
One of the unique features of horses is their way of movement, which is designed for maximum speed. This natural ability has been enhanced by selective breeding. For example, Thoroughbreds are designed for high-speed sprints, Arabians for endurance racing, Quarter Horses for agility and bursts of speed, and Standardbreds for trotting and pacing. Horses have 4 natural gaits of progressively increasing speed: walk, trot, canter, and gallop. All 4 hooves leave the ground simultaneously during the canter and gallop, a fact that was discovered only through early stop-action photography. Some horses are also taught to pace, which is like the trot, except that both feet on the same side move together.
The equine leg is designed for rapid movement over a variety of surfaces. The upper part of the leg is heavily muscled, while the lower part acts as a springboard to enhance the stride. The leg is supported by a suspensory apparatus of tendons and ligaments. The tendons, which can be felt along the back of the lower leg, run the length of the limb, while the many joints are held together and protected by ligaments and joint capsules. Horses also have a unique anatomical feature called the stay apparatus, which allows them to “rest” a rear leg while standing on the other 3 for prolonged periods. This is why horses can sleep standing up.
Horses walk and run on their hooves. The cannon and splint bones are in the lower leg, while the pastern bones are between the fetlock and the hoof.
The long, lean, flexible equine leg is excellent for its purpose, but it is also delicate and easily injured. The tremendous amount of weight that is balanced on the hooves and lower limbs make these areas particularly subject to injury.
The hoof consists of a wall of horny keratin (a protein) that grows down from a band called the coronet at the top of the hoof. This process is similar to the way our fingernails grow out from the cuticle. The sole of the equine foot is concave, with a resilient wedge of tissue called the frog that juts forward from the heel. Inside the hoof is the coffin bone (which is shaped like the hoof) and additional resilient tissue. The sole and the frog protect the underside of the foot, while the resilient tissues cushion each step like a spring (see Figure: Parts of the hoof).
Because the hoof is subjected to tremendous, repetitive impact, it can develop numerous problems that lead to lameness, including bruises, infections, hoof rot, and a serious inflammation in the area of the coffin bone and coronet called laminitis. Therefore, hoof care is an important part of equine husbandry and preventive medicine.
Equine skin is similar to canine and feline skin, although it is not quite as sensitive. The main functions of the hair coat are to protect the skin and to help regulate temperature. The hair coat changes with the seasons, with hair being longer and coarser in the winter than in the summer. Like dogs and cats, horses are able to fluff up their hair (using small muscles attached in the hair follicles) to increase the amount of trapped air for insulation. Additional oil (sebum) is also produced by the skin during winter, adding insulation. Proper nutrition, daily cleaning and brushing with a curry comb, and occasional baths are generally all that is needed to keep the skin and hair clean and healthy.
All horse owners should watch for skin problems, which can be caused by infections, scratches, insect bites, parasites, allergies, or irritating chemicals. Signs vary depending on the problem and the individual horse. The location of the problem is often a clue to the cause. For example, fly bites often appear on the neck and ears, while irritation or contact allergy from the saddle or blanket is located on the back.
Unlike dogs and cats, horses are herbivores designed to eat grains and grasses. Incisor teeth in the front of the mouth grasp and cut grasses, while the rearward molars and premolars are designed for grinding. Equine teeth grow continuously throughout life—as the crowns are ground down, new replacement material arises from the jaw.
Horses have 24 deciduous (baby or milk) teeth; these are replaced by 40 to 42 permanent (adult) teeth that erupt between the ages of 6 months and 5 years (see Table: Equine Dentition). All male and some female horses have small canine teeth between the incisors and premolars. Horses may also have up to 4 wolf teeth, which are vestigial and nonfunctional premolars. The wolf teeth are typically extracted when yearlings are being broken in the late fall or early in their 2-year-old year.
Because equine incisor teeth typically grow and wear predictably, dental wear can be used to estimate age. In adult horses, age is typically estimated by the shape and wear pattern of the incisor teeth (see Table: Estimation of Age of Adult Horses by Examination of Teeth).
The mouth also contains the salivary glands, which lubricate food and begin digestion. The tongue helps guide food to the back of the throat and is important for grasping grasses, drinking water, and taste.
Estimation of Age of Adult Horses by Examination of Teeth
People, dogs, and cats have simple stomachs that are good at breaking down meat, fruits, and vegetables. Most animals that eat grass (including cows and sheep) have a more complex system consisting of several stomachs, including a large fermentation vat called a rumen. Grasses (such as hay) in the rumen are digested by billions of bacteria that break down roughage into volatile fatty acids. These fatty acids are absorbed for energy further down in the digestive tract. The equine digestive system combines features of both the simple stomach and the multiple-stomach digestive systems.
At the beginning of the digestive tract, horses have a simple stomach that leads to a small intestine. At the end of the digestive tract, they also have a fermentation vat called the cecum that leads to the great colon. The cecum and great colon make it possible for horses to eat grasses for energy, but because these organs are located at the end of the digestive tract, problems tend to develop. Before they can be fully digested, grasses must pass through almost the full length of the digestive tract, with its many bends and narrowings. This increases the risk of dense, fibrous material becoming impacted and blocking the digestive tract, leading to a condition called colic. Colic can be a medical emergency, so all horse owners should be familiar with the signs of this very serious, potentially life-threatening condition.
Digestive problems can also show up as changes in the number or character of bowel movements. Normal horse feces are apple-sized lumps that are well formed but somewhat moist. Digestive problems can result in feces that are too soft or too hard. Hard, dry feces can predispose horses to colic. The number of bowel movements per day, the color of the feces, and any tell-tale signs of blood should be reported to your veterinarian.
Equine kidneys are very efficient, constantly filtering waste from the 10 gallons of blood within the horse’s body. Very few horses ever develop kidney problems. Most cases go undetected until the later stages, when signs appear as waste products build up in the blood. These signs are caused by the body’s attempt to maximize the amount of remaining kidney function.
The kidneys can be injured by a variety of problems, including dehydration; heat stroke; ingestion of poisonous plants; treatment with certain antibiotics, vitamin supplements, or pain medications; massive blood loss; shock; colic; bacterial infection; or obstruction of urine flow (for example, from stones). Your veterinarian may need to perform urine and blood tests to confirm kidney disease and decide on appropriate treatment.
The respiratory system of horses is very similar to that of dogs, cats, and people, although it is much larger. Horses are tremendous athletes, and they need a great deal of oxygen to perform their functions. Performance suffers noticeably when a horse’s respiratory system is compromised, which is why respiratory problems in horses are usually noticed more rapidly than in dogs and cats.
The equine respiratory system can be affected by bacterial infections (such as strangles), viral infections, allergies, asthma, mechanical obstruction (for example, “roaring”), and general irritation and inflammation associated with a dirty, dusty environment. Keeping your horse up to date on vaccinations and keeping his or her environment dry and clean are both important for respiratory health.