Horse anatomy is described by a large number of specific terms, as illustrated by the chart to the right. Specific terms also describe various ages, colors and breeds.
Age
Depending on breed, management and environment, the domestic horse today has a life expectancy of 25 to 30 years.[4] It is uncommon, but a few animals live into their 40s and, occasionally, beyond.[5] The oldest verifiable record was "Old Billy", a 19th-century horse that lived to the age of 62.[4][6] In modern times, Sugar Puff, who had been listed in the Guinness Book of World Records as the world's oldest living pony, died in 2007, aged 56.[7]
Regardless of a horse's actual birth date, for most competition purposes an animal is considered a year older on January 1 of each year in the northern hemisphere[4][8] and August 1 in the southern hemisphere.[9] The exception is in endurance riding, where the minimum age to compete is based on the animal's calendar age.[10] A very rough estimate of a horse's age can be made from looking at its teeth.[4]
The following terminology is used to describe horses of various ages:
- Foal: a horse of either sex less than one year old. A nursing foal is sometimes called a suckling and a foal that has been weaned is called a weanling.[11] Most domesticated foals are weaned at 5 to 7 months of age, although foals can be weaned at 4 months with no adverse effects.[12]
- Yearling: a horse of either sex that is between one and two years old.[13]
- Colt: a male horse under the age of four.[14] A common terminology error is to call any young horse a "colt", when the term actually only refers to young male horses.[citation needed]
- Filly: a female horse under the age of four.[11]
- Mare: a female horse four years old and older.[15]
- Stallion: a non-castrated male horse four years old and older.[16] Some people, particularly in the UK, refer to a stallion as a "horse".[17]
- Gelding: a castrated male horse of any age,[11] though for convenience sake, many people also refer to a young gelding under the age of four as a "colt".
In horse racing, the definitions of colt, filly, mare, and stallion may differ from those given above. In the UK, Thoroughbred horse racing defines a colt as a male less than five years old, and a filly as a female less than five years old.[18] In the United States, both Thoroughbred racing and harness racing define colts and fillies as four years old and younger.[19]
Size and measurement
See also: Hand (length)
The height of horses is measured at the highest point of an animal's withers, where the neck meets the back. This point was chosen as it is a stable point of the anatomy, unlike the head or neck, which move up and down.
The English-speaking world measures the height of horses in hands (abbreviated "h" or "hh", for "hands high") and inches. One hand is equal to 101.6 millimeters (4 inches), and one inch is equal to 25.4 millimeters. The height is expressed as the number of full hands, followed by a decimal point and the number of additional inches. Thus a horse described as "15.2 h", is 15 hands, 2 inches (160 centimeters, or 62 inches) in height.[20]
Size varies greatly among horse breeds, as with this full-sized horse and a miniature horse.
The size of horses varies by breed, but can also be influenced by nutrition. Light riding horses such as Arabians, Morgans, or Quarter Horses usually range in height from 14 to 16 hands (56 to 64 inches, 142 to 163 cm) and can weigh from 380 to 550 kilograms (840 to 1,200 lb).[21] Larger riding horses such as Thoroughbreds, American Saddlebreds, or Warmbloods usually start at about 15.2 hands (62 inches, 157 cm) and often are as tall as 17 hands (68 inches, 173 cm), weighing from 500 to 600 kilograms (1,100 to 1,300 lb).[22] Heavy or draft horses, such as the Clydesdale, Belgian, or Shire, are usually at least 16 tp 18 hands (64 to 72 inches, 163 to 183 cm) high and can weigh from about 700 to 1,000 kilograms (1,500 to 2,200 lb).[23]
The largest horse in recorded history was probably a Shire horse named Mammoth, who was born in 1848. He stood 21.2½ hands high (86.5 in/220 cm), and his peak weight was estimated at 1,500 kilograms (3,300 lb).[24] The current record holder for the world's smallest horse is Thumbelina, a fully mature miniature horse affected by dwarfism. She is 17 inches (43 cm) tall and weighs 27 kilograms (60 lb).[25]
Ponies
Main article: Pony
The general rule for cutoff in height between what is considered a horse and a pony at maturity is 14.2 hands (58 inches, 147 cm). An animal 14.2 h or over is usually considered to be a horse and one less than 14.2 h a pony.[26] However, there are a number of exceptions to the general rule. In Australia, ponies measure under 14 hands (56 inches, 142 cm).[27][page needed] The International Federation for Equestrian Sports, which uses metric measurements, defines the cutoff between horses and ponies at 148 centimetres (58.27 in) (just over 14.2 h) without shoes and 149 centimetres (58.66 in) (just over 14.2-1/2 h) with shoes.[28] Some breeds which typically produce individuals both under and over 14.2 h considered all animals of that breed to be horses regardless of their height.[29] Conversely, some pony breeds may have features in common with horses, and individual animals may occasionally mature at over 14.2 h, but are still considered to be ponies.[30]
The distinction between a horse and pony is not simply a difference in height, but takes account of other aspects of phenotype or appearance, such as conformation and temperament. Ponies often exhibit thicker manes, tails, and overall coat. They also have proportionally shorter legs, wider barrels, heavier bone, shorter and thicker necks, and short heads with broad foreheads. They often have calmer temperaments than horses and also a high level of equine intelligence that may or may not be used to cooperate with human handlers.[26] In fact, small size, by itself, is sometimes not a factor at all. While the Shetland pony stands on average 10 hands (40 inches, 102 cm),[31] the Falabella and other miniature horses, which can be no taller than 30 inches (76 cm), the size of a medium-sized dog, are classified by their respective registries as very small horses rather than as ponies.[32]
Colors and markings
Main articles: Equine coat color, Equine coat color genetics, and Horse markings
Bay (left) and chestnut (sometimes called "sorrel") are two of the most common coat colors, seen in almost all breeds.
Horses exhibit a diverse array of coat colors and distinctive markings, described with a specialized vocabulary. Often, a horse is classified first by its coat color, before breed or sex.[33] Flashy or unusual colors are sometimes very popular, as are horses with particularly attractive markings. Horses of the same color may be distinguished from one another by their markings.[34]
The genetics that create many horse coat colors have been identified, although research continues on specific genes and mutations that result in specific color traits. Essentially, all horse colors begin with a genetic base of "red" (chestnut) or "black", with the addition of alleles for spotting, graying, suppression or dilution of color, or other effects acting upon the base colors to create the dozens of possible coat colors found in horses.[35]
Horses which are light in color are often misnamed as being "white" horses. A horse that looks pure white is, in most cases, actually a middle-aged or older gray. Grays have black skin underneath their white hair coat (with the exception of small amounts of pink skin under white markings). The only horses properly called white are those with pink skin under a white hair coat, a fairly rare occurrence.[36] There are no truly albino horses, with pink skin and red eyes, as albinism is a lethal condition in horses.[37]
Reproduction and development
Main article: Horse breeding
Gestation lasts for approximately 335–340 days[38] and usually results in one foal. Twins are very rare.[39] Colts are carried on average about 4 days longer than fillies.[40] Horses are a precocial species, and foals are capable of standing and running within a short time following birth.[41]
Horses, particularly colts, may sometimes be physically capable of reproduction at about 18 months. In practice, individuals are rarely allowed to breed before the age of three, especially females.[38] Horses four years old are considered mature, although the skeleton normally continues to develop until the age of six; the precise time of completion of development also depends on the horse's size, breed, gender, and the quality of care provided by its owner. Also, if the horse is larger, its bones are larger; therefore, not only do the bones take longer to actually form bone tissue, but the epiphyseal plates are also larger and take longer to convert from cartilage to bone. These plates convert after the other parts of the bones, but are crucial to development.[42]
Depending on maturity, breed, and the tasks expected, young horses are usually put under saddle and trained to be ridden between the ages of two and four.[43] Although Thoroughbred race horses are put on the track at as young as two years old in some countries,[44] horses specifically bred for sports such as dressage are generally not entered into top-level competition until they are a minimum of four years old, because their bones and muscles are not solidly developed, nor is their advanced training complete.[45] For endurance riding competition, horses are not deemed mature enough to compete until they are a full 60 calendar months (5 years) old.[10]
Anatomy
Main article: Equine anatomy
See also: Muscular system of the horse, Respiratory system of the horse, and Circulatory system of the horse
Skeletal system
Main article: Skeletal system of the horse
The skeletal system of a modern horse
Horses have a skeleton that averages 205 bones.[46] A significant difference between the horse skeleton, compared to that of a human, is the lack of a collarbone—the horse's front limb system is attached to the spinal column by a powerful set of muscles, tendons, and ligaments that attach the shoulder blade to the torso. The horse's legs and hooves are also unique structures. Their leg bones are proportioned differently from those of a human. For example, the body part that is called a horse's "knee" is actually made up of the carpal bones that correspond to the human wrist. Similarly, the hock, contains the bones equivalent to those in the human ankle and heel. The lower leg bones of a horse correspond to the bones of the human hand or foot, and the fetlock (incorrectly called the "ankle") is actually the proximal sesamoid bones between the cannon bones (a single equivalent to the human metacarpal or metatarsal bones) and the proximal phalanges, located where one finds the "knuckles" of a human. A horse also has no muscles in its legs below the knees and hocks, only skin, hair, bone, tendons, ligaments, cartilage, and the assorted specialized tissues that make up the hoof.[47]
Hooves
Main article: Horse hoof
See also: Horseshoe and Farrier
The critical importance of the feet and legs is summed up by the traditional adage, "no foot, no horse".[48] The horse hoof begins with the distal phalanges, the equivalent of the human fingertip or tip of the toe, surrounded by cartilage and other specialized, blood-rich soft tissues such as the laminae. The exterior hoof wall and horn of the sole is made of essentially the same material as a human fingernail.[49] The end result is that a horse, weighing on average 500 kilograms (1,100 lb),[50] travels on the same bones as a human on tiptoe.[51] For the protection of the hoof under certain conditions, some horses have horseshoes placed on their feet by a professional farrier. The hoof continually grows, and needs to be trimmed (and horseshoes reset, if used) every five to eight weeks.[52]
Teeth
Main article: Horse teeth
Horses are adapted to grazing. In an adult horse, there are 12 incisors, adapted to biting off the grass or other vegetation, at the front of the mouth. There are 24 teeth adapted for chewing, the premolars and molars, at the back of the mouth. Stallions and geldings have four additional teeth just behind the incisors, a type of canine teeth that are called "tushes". Some horses, both male and female, will also develop one to four very small vestigial teeth in front of the molars, known as "wolf" teeth, which are generally removed because they can interfere with the bit. There is an empty interdental space between the incisors and the molars where the bit rests directly on the bars (gums) of the horse's mouth when the horse is bridled.[53]
The incisors show a distinct wear and growth pattern as the horse ages, as well as change in the angle at which the chewing surfaces meet. The teeth continue to erupt throughout life as they are worn down by grazing, so a very rough estimate of a horse's age can be made by an examination of its teeth, although diet and veterinary care can affect the rate of tooth wear.[4]
Digestion
Main articles: Equine anatomy#Digestive system and Equine nutrition
Horses are herbivores with a digestive system adapted to a forage diet of grasses and other plant material, consumed steadily throughout the day. Therefore, compared to humans, they have a relatively small stomach but very long intestines to facilitate a steady flow of nutrients. A 450-kilogram (990 lb) horse will eat 7 to 11 kilograms (15 to 24 lb) of food per day and, under normal use, drink 38 litres (8.4 imp gal; 10 US gal) to 45 litres (9.9 imp gal; 12 US gal) of water. Horses are not ruminants, so they have only one stomach, like humans, but unlike humans, they can also digest cellulose from grasses due to the presence of a "hind gut" called the cecum, or "water gut", which food goes through before reaching the large intestine. Unlike humans, horses cannot vomit, so digestion problems can quickly cause colic, a leading cause of death.[54]
Senses
See also: Equine vision
The horse's senses are generally superior to those of a human. As prey animals, they must be aware of their surroundings at all times.[55] They have the largest eyes of any land mammal, and because their eyes are positioned on the sides of their heads, horses have a range of vision of more than 350°, with approximately 65° of this being binocular (seen with both eyes) and the remaining 285° monocular (seen with only one eye).[56] Horses have excellent day and night vision, but studies indicate that they have two-color, or dichromatic vision; their color vision is somewhat like red-green color blindness in humans. This means that certain colors, especially red and related colors, appear more green.[57]
Their hearing is good,[55] and the pinna of each ear can rotate up to 180°, giving the potential for 360° hearing without having to move the head.[58] Their sense of smell, while much better than that of humans, is not their strongest asset; they rely to a greater extent on vision.[55]
Horses have a great sense of balance, due partly to their ability to feel their footing and partly to highly developed proprioceptive abilities (the unconscious sense of where the body and limbs are at all times).[59] A horse's sense of touch is well developed. The most sensitive areas are around the eyes, ears, and nose. Via touch, horses perceive and respond immediately to changes in their environment,[60] sensing contact as subtle as an insect landing anywhere on the body.[61]
Horses have an advanced sense of taste that allows them to sort through grains and grasses to choose what they would most like to eat,[62] and their prehensile lips can easily sort even the smallest grains. Horses generally will not eat poisonous plants. However, there are exceptions and horses will occasionally eat toxic amounts of poisonous plants even when there is adequate healthy food.[63]
Movement
Main articles: Horse gait, Trot (horse gait), Canter, and Ambling
All horses move naturally with four basic gaits: the four-beat walk, which averages 6.4 kilometres per hour (4.0 mph); the two-beat trot or jog, which averages 13 to 19 kilometres per hour (8.1 to 12 mph) (faster for harness racing horses); and the leaping gaits known as the canter or lope (a three-beat gait that is 19 to 24 kilometres per hour (12 to 15 mph), and the gallop.[64] The gallop averages 40 to 48 kilometres per hour (25 to 30 mph).[65] The world record for a horse galloping over a short, sprint distance is 88 kilometres per hour (55 mph).[66] Besides these basic gaits, some horses perform a two-beat pace, instead of the trot.[67] In addition, there are several four-beat "ambling" gaits that are approximately the speed of a trot or pace, though smoother to ride. These include the lateral slow gait, rack, running walk, and tölt as well as the diagonal fox trot.[68] Ambling gaits are often genetic traits in specific breeds, known collectively as gaited horses.[69] In most cases, gaited horses replace the standard trot with one of the ambling gaits.[70]
Behavior
Main articles: Horse behavior and Stable vices
Horses are prey animals with a well-developed fight-or-flight instinct. Their first response to threat is to startle and usually flee, although they are known to stand their ground and defend themselves or their offspring in cases where flight is not possible, or when their young are threatened. They also tend to be curious; when startled, they will often hesitate an instant to ascertain the cause of their fright, and may not always flee from something that they perceive as non-threatening. Through selective breeding, some breeds of horses are quite docile, particularly certain large draft horses. Most light horse riding breeds were developed for speed, agility, alertness and endurance; natural qualities that extend from their wild ancestors.[71] Horses are herd animals, with a clear hierarchy of rank, led by a dominant animal (usually a mare). They are also social creatures who are able to form companionship attachments to their own species and to other animals, including humans. They communicate in various ways, including vocalizations such as nickering or whinnying, mutual grooming, and body language. Many horses will become difficult to manage if they are isolated. Through proper training, it is possible to teach any horse to accept a human as a type of companion, and thus be comfortable away from other horses.[72] When confined with insufficient companionship, exercise or stimulation, individuals may develop stable vices, an assortment of bad habits, mostly psychological in origin, that include wood chewing, wall kicking, "weaving" (rocking back and forth), and other problems.[73]
Intelligence and learning
In the past, horses were considered unintelligent, with no abstract thinking ability, unable to generalize, and driven primarily by a herd mentality. However, recent studies show that they perform a number of cognitive tasks on a daily basis, and frequently engage in mental challenges that include food procurement and social system identification. They have also been shown to have good spatial discrimination abilities.[74] Recent studies suggest horses are able to count if the quantity involved is less than four.[75]
Studies have assessed equine intelligence in the realms of problem solving, learning speed, and knowledge retention. Results show that horses excel at simple learning, but also are able to solve advanced cognitive challenges that involve categorization and concept learning. They have been shown to learn from habituation, desensitization, Pavlovian conditioning, and operant conditioning. They respond to and learn from both positive and negative reinforcement.[74]
Domesticated horses tend to face greater mental challenges than wild horses, due to living in artificial environments that stifle instinctual behavior while learning tasks that are not natural.[74] Horses are creatures of habit that respond and adapt well to regimentation, and respond best when the same routines and techniques are used consistently. Some trainers believe that "intelligent" horses are reflections of intelligent trainers who effectively use response conditioning techniques and positive reinforcement to train in the style that fits best with an individual animal's natural inclinations. Others who handle horses regularly note that personality also may play a role separate from intelligence in determining how a given animal responds to various experiences.[76]
Temperament
Main articles: Draft horse, Warmblood, and Oriental horse
Thoroughbred race horses are a "hot blooded" breed.
Horses are mammals, and as such are "warm-blooded" creatures, as opposed to cold-blooded reptiles. However, these words have developed a separate meaning in the context of equine terminology, used to describe temperament, not body temperature. For example, the "hot-bloods", such as many race horses, exhibit more sensitivity and energy,[77] while the "cold-bloods", such as most draft breeds, are quieter and calmer.[78] Sometimes "hot-bloods" are classified as "light horses" or "riding horses",[79] with the "cold-bloods" classified as "draft horses" or "work horses".[80]
The "cold blooded" draft breeds are powerful and heavily-muscled
"Hot blooded" breeds include "oriental horses" such as the Akhal-Teke, Barb, Arabian horse and now-extinct Turkoman horse, as well as the Thoroughbred, a breed developed in England from the older oriental breeds.[77] Hot bloods tend to be spirited, bold, and learn quickly. They are bred for agility and speed.[81] They tend to be physically refined—thin-skinned, slim, and long-legged.[82] The original oriental breeds were brought to Europe from the Middle East and North Africa when European breeders wished to infuse these traits into racing and light cavalry horses.[83][84]
Muscular, heavy draft horses are known as "cold bloods", as they are bred not only for strength, but also to have the calm, patient temperament needed to pull a plow or a heavy carriage full of people.[78] They are sometimes nicknamed "gentle giants".[85] Well-known draft breeds include the Belgian and the Clydesdale.[85] Some, like the Percheron are lighter and livelier, developed to pull carriages or to plow large fields in drier climates.[86] Others, such as the Shire, are slower and more powerful, bred to plow fields with heavy, clay-based soils.[87] The cold-blooded group also includes some pony breeds.[88]
A modern "Warmblood" horse is large, but agile and athletic.
"Warmblood" breeds, such as the Trakehner or Hanoverian, developed when European carriage and war horses were crossed with Arabians or Thoroughbreds, producing a riding horse with more refinement than a draft horse, but greater size and more phlegmatic temperament than a lighter breed.[89] Certain pony breeds with warmblood characteristics have been developed for smaller riders.[90] Warmbloods are considered a "light horse" or "riding horse".[79]
Today, the term "Warmblood" refers to a specific subset of sport horse breeds that are used for competition in dressage and show jumping.[91] Strictly speaking, the term "warm blood" refers to any cross between cold-blooded and hot-blooded breeds.[92] Examples included breeds such as the Irish Draught or the Cleveland Bay. Less often, the term was even used to refer to breeds of light riding horse other than Thoroughbreds or Arabians, such as the Morgan horse.[81]
Sleep patterns
See also: Horse sleep patterns and Sleep in non-humans
When horses lie down to sleep, others in the herd remain standing, awake or in a light doze, keeping watch.
Horses are able to sleep both standing up and lying down. In an adaptation from life in the wild, horses are able to enter light sleep by using a "stay apparatus" in their legs, allowing them to doze without collapsing.[93] Horses sleep better when in groups because some animals will sleep while others stand guard to watch for predators. A horse kept alone will not sleep well because its instincts are to keep a constant eye out for danger.[94]
Unlike humans, horses do not sleep in a solid, unbroken period of time, but take many short periods of rest. Horses spend four to fifteen hours a day in standing rest, and from a few minutes to several hours lying down. Total sleep time in a 24-hour period may range from several minutes to a couple of hours,[94] mostly in short intervals of about 15 minutes each.[95]
Horses must lie down to reach REM sleep. They only have to lie down for an hour or two every few days to meet their minimum REM sleep requirements.[94] However, if a horse is never allowed to lie down, after several days it will become sleep-deprived, and in rare cases may suddenly collapse as it involuntarily slips into REM sleep while still standing.[96] This condition differs from narcolepsy, although horses may also suffer from that disorder.
