Domestic horse
Conservation status
Domesticated
Scientific classification
Kingdom:	Animalia
Phylum:	Chordata
Class:	Mammalia
Subclass:	Theria
Infraclass:	Eutheria
Order:	Perissodactyla
Family:	Equidae
Genus:	Equus
Species:	E. ferus
Subspecies:	E. f. caballus
Trinomial name
Equus ferus caballus Linnaeus, 1758[1]
Synonyms
48[2]

The horse (Equus ferus caballus)^[2][3] is one of two extant subspecies of Equus ferus, or the wild horse. It is an odd-toed ungulate mammal belonging to the taxonomic family Equidae. The horse has evolved over the past 45 to 55 million years from a small multi-toed creature into the large, single-toed animal of today. Humans began to domesticate horses around 4000 BC, and their domestication is believed to have been widespread by 3000 BC. Horses in the subspecies caballus are domesticated, although some domesticated populations live in the wild as feral horses. These feral populations are not true wild horses, as this term is used to describe horses that have never been domesticated, such as the endangered Przewalski's Horse, a separate subspecies, and the only remaining true wild horse. There is an extensive, specialized vocabulary used to describe equine-related concepts, covering everything from anatomy to life stages, size, colors, markings, breeds, locomotion, and behavior.

Horses' anatomy enables them to make use of speed to escape predators and they have a well-developed sense of balance and a strong fight-or-flight instinct. Related to this need to flee from predators in the wild is an unusual trait: horses are able to sleep both standing up and lying down. Female horses, called mares, carry their young for approximately 11 months, and a young horse, called a foal, can stand and run shortly following birth. Most domesticated horses begin training under saddle or in harness between the ages of two and four. They reach full adult development by age five, and have an average lifespan of between 25 and 30 years.

Horse breeds are loosely divided into three categories based on general temperament: spirited "hot bloods" with speed and endurance; "cold bloods", such as draft horses and some ponies, suitable for slow, heavy work; and "warmbloods", developed from crosses between hot bloods and cold bloods, often focusing on creating breeds for specific riding purposes, particularly in Europe. There are more than 300 breeds of horses in the world today, developed for many different uses.

Horses and humans interact in a wide variety of sport competitions and non-competitive recreational pursuits, as well as in working activities such as police work, agriculture, entertainment, and therapy. Horses were historically used in warfare, from which a wide variety of riding and driving techniques developed, using many different styles of equipment and methods of control. Many products are derived from horses, including meat, milk, hide, hair, bone, and pharmaceuticals extracted from the urine of pregnant mares. Humans provide domesticated horses with food, water and shelter, as well as attention from specialists such as veterinarians and farriers.

Biology[link]

Points of a horse^[4][5]

Specific terms and specialized language are used to describe equine anatomy, different life stages, colors and breeds.

Lifespan and life stages[link]

Depending on breed, management and environment, the modern domestic horse has a life expectancy of 25 to 30 years.^[6] Uncommonly, a few animals live into their 40s and, occasionally, beyond.^[7] The oldest verifiable record was "Old Billy", a 19th-century horse that lived to the age of 62.^[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 at age 56.^[8]

Regardless of a horse or pony's actual birth date, for most competition purposes a year is added to its age each January 1 of each year in the Northern Hemisphere^[6][9] and each August 1 in the Southern Hemisphere.^[10] The exception is in endurance riding, where the minimum age to compete is based on the animal's actual calendar age.^[11]

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.^[12] Most domesticated foals are weaned at five to seven months of age, although foals can be weaned at four months with no adverse physical effects.^[13]
• Yearling: a horse of either sex that is between one and two years old.^[14]
• Colt: a male horse under the age of four.^[15] A common terminology error is to call any young horse a "colt", when the term actually only refers to young male horses.^[16]
• Filly: a female horse under the age of four.^[12]
• Mare: a female horse four years old and older.^[17]
• Stallion: a non-castrated male horse four years old and older.^[18] The term "horse" is sometimes used colloquially to refer specifically to a stallion.^[19]
• Gelding: a castrated male horse of any age.^[12]

In horse racing, these definitions may differ: For example, in the British Isles, Thoroughbred horse racing defines colts and fillies as less than five years old.^[20] However, Australian Thoroughbred racing defines colts and fillies as less than four years old.^[21]

Size and measurement[link]

The height of horses is measured at the highest point of the withers, where the neck meets the back.^[22] This point is used because it is a stable point of the anatomy, unlike the head or neck, which move up and down in relation to the body of the horse.

The English-speaking world measures the height of horses in hands and inches: one hand is equal to 4 inches (101.6 mm). The height is expressed as the number of full hands, followed by a point, then the number of additional inches, and ending with the abbreviation "h" or "hh" (for "hands high"). Thus, a horse described as "15.2 h" is 15 hands plus 2 inches, for a total of 62 inches (157.5 cm) in height.^[23]

Size varies greatly among horse breeds, as with this full-sized horse and a miniature horse.

The size of horses varies by breed, but also is influenced by nutrition. Light riding 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).^[24] Larger riding horses 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).^[25] Heavy or draft horses are usually at least 16 hands (64 inches, 163 cm) high and can be as tall as 18 hands (72 inches, 183 cm) high. They can weigh from about 700 to 1,000 kilograms (1,500 to 2,200 lb).^[26]

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).^[27] 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 57 pounds (26 kg).^[28]

Ponies[link]

Ponies are taxonomically the same animals as horses. The distinction between a horse and pony is commonly drawn on the basis of height, especially for competition purposes. However, height alone is not dispositive; the difference between horses and ponies may also include aspects of phenotype, including conformation and temperament.

The traditional standard for height of a horse or 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,^[29] but there are many exceptions to the traditional standard. In Australia, ponies are considered to be those under 14 hands (56 inches, 142 cm),^[30] For competition in the Western division of the United States Equestrian Federation, the cutoff is 14.1 hands (57 inches, 145 cm)^[31] The International Federation for Equestrian Sports, the world governing body for horse sport, uses metric measurements and defines a pony as being any horse measuring less than 148 centimetres (58.27 in) at the withers without shoes, which is just over 14.2 h, and 149 centimetres (58.66 in), or just over 14.2½ h, with shoes.^[32]

Height is not the sole criterion for distinguishing horses from ponies. Breed registries for horses that typically produce individuals both under and over 14.2 h consider all animals of that breed to be horses regardless of their height.^[33] 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.^[34]

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 may 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.^[29] Small size, by itself, is not an exclusive determinant. For example, the Shetland pony which averages 10 hands (40 inches, 102 cm), is considered a pony.^[35] Conversely, breeds such as the Falabella and other miniature horses, which can be no taller than 30 inches (76 cm), are classified by their registries as very small horses, not ponies.^[36]

Colors and markings[link]

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 by a specialized vocabulary. Often, a horse is classified first by its coat color, before breed or sex.^[37] Horses of the same color may be distinguished from one another by white markings,^[38] which, along with various spotting patterns, are inherited separately from coat color.^[39]

Many genes that create horse coat colors and patterns have been identified. Current genetic tests can identify at least 13 different alleles influencing coat color,^[40] and research continues to discover new genes linked to specific traits. The basic coat colors of chestnut and black are determined by the gene controlled by the Melanocortin 1 receptor,^[41] also known as the "extension gene" or "red factor,"^[40] as its recessive form is "red" (chestnut) and its dominant form is black.^[42] Additional genes control suppression of black color to point coloration that results in a bay, spotting patterns such as pinto or leopard, dilution genes such as palomino or dun, as well as graying, and all the other factors that create the many possible coat colors found in horses.^[40]

Horses which have a white coat color are often mislabeled; a horse that looks "white" is usually a middle-aged or older gray. Grays are born a darker shade, get lighter as they age, but usually keep black skin underneath their white hair coat (with the exception of pink skin under white markings). The only horses properly called white are born with a predominantly white hair coat and pink skin, a fairly rare occurrence.^[42] Different and unrelated genetic factors can produce white coat colors in horses, including several different alleles of dominant white and the sabino-1 gene.^[43] However, there are no "albino" horses, defined as having both pink skin and red eyes.^[44]

Reproduction and development[link]

Gestation lasts approximately 340 days, with an average range 320–370 days,^[45] and usually results in one foal; twins are rare.^[46] Horses are a precocial species, and foals are capable of standing and running within a short time following birth.^[47] Foals are usually born in the spring. The estrous cycle of a mare occurs roughly every 19–22 days and occurs from early spring into autumn. Most mares enter an anestrus period during the winter and thus do not cycle in this period.^[48] Foals are generally weaned from their mothers between four and six months of age.^[49]

Horses, particularly colts, sometimes are physically capable of reproduction at about 18 months, but domesticated horses are rarely allowed to breed before the age of three, especially females.^[50] Horses four years old are considered mature, although the skeleton normally continues to develop until the age of six; maturation also depends on the horse's size, breed, sex, and quality of care. Larger horses have larger bones; therefore, not only do the bones take longer to form bone tissue, but the epiphyseal plates are larger and take longer to convert from cartilage to bone. These plates convert after the other parts of the bones, and are crucial to development.^[51]

Depending on maturity, breed, and work expected, horses are usually put under saddle and trained to be ridden between the ages of two and four.^[52] Although Thoroughbred race horses are put on the track as young as the age of two in some countries,^[53] horses specifically bred for sports such as dressage are generally not put under saddle until they are three or four years old, because their bones and muscles are not solidly developed.^[54] For endurance riding competition, horses are not deemed mature enough to compete until they are a full 60 calendar months (five years) old.^[11]

Anatomy[link]

Skeletal system[link]

The skeletal system of a modern horse

The horse skeleton averages 205 bones.^[55] A significant difference between the horse skeleton and that of a human is the lack of a collarbone—the horse's forelimbs are 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 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.^[56]

Hooves[link]

The critical importance of the feet and legs is summed up by the traditional adage, "no foot, no horse".^[57] 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.^[58] The end result is that a horse, weighing on average 500 kilograms (1,100 lb),^[59] travels on the same bones as would a human on tiptoe.^[60] 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 in most domesticated horses needs to be trimmed (and horseshoes reset, if used) every five to eight weeks,^[61] though the hooves of horses in the wild wear down and regrow at a rate suitable for their terrain.

Teeth[link]

Horses are adapted to grazing. In an adult horse, there are 12 incisors at the front of the mouth, adapted to biting off the grass or other vegetation. 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 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 gums, or "bars" of the horse's mouth when the horse is bridled.^[62]

An estimate of a horse's age can be made from looking at its teeth. The teeth continue to erupt throughout life and are worn down by grazing. Therefore, the incisors show changes as the horse ages; they develop a distinct wear pattern, changes in tooth shape, and changes in the angle at which the chewing surfaces meet. This allows a very rough estimate of a horse's age, although diet and veterinary care can also affect the rate of tooth wear.^[6]

Digestion[link]

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 digest cellulose, a major component of grass. Cellulose digestion occurs in the cecum, or "water gut", which food goes through before reaching the large intestine. Horses cannot vomit, so digestion problems can quickly cause colic, a leading cause of death.^[63]

Senses[link]

A horse's eye

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.^[64] They have the largest eyes of any land mammal,^[65] and are lateral-eyed, meaning that their eyes are positioned on the sides of their heads.^[66] This means that horses have a range of vision of more than 350°, with approximately 65° of this being binocular vision and the remaining 285° monocular vision.^[65] Horses have excellent day and night vision, but they have two-color, or dichromatic vision; their color vision is somewhat like red-green color blindness in humans, where certain colors, especially red and related colors, appear as a shade of green.^[67]

A horse's hearing is good,^[64] and the pinna of each ear can rotate up to 180°, giving the potential for 360° hearing without having to move the head.^[68] Their sense of smell, while much better than that of humans, is not their strongest asset; they rely to a greater extent on vision.^[64]

Horses have a great sense of balance, due partly to their ability to feel their footing and partly to highly developed proprioception—the unconscious sense of where the body and limbs are at all times.^[69] A horse's sense of touch is well developed. The most sensitive areas are around the eyes, ears, and nose.^[70] Horses sense contact as subtle as an insect landing anywhere on the body.^[71]

Horses have an advanced sense of taste that allows them to sort through fodder to choose what they would most like to eat,^[72] 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.^[73]

Movement[link]

The gallop

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 at 13 to 19 kilometres per hour (8.1 to 12 mph) (faster for harness racing horses); the canter or lope, a three-beat gait that is 19 to 24 kilometres per hour (12 to 15 mph); and the gallop.^[74] The gallop averages 40 to 48 kilometres per hour (25 to 30 mph),^[75] but the world record for a horse galloping over a short, sprint distance is 88 kilometres per hour (55 mph).^[76] Besides these basic gaits, some horses perform a two-beat pace, instead of the trot.^[77] There also are several four-beat "ambling" gaits that are approximately the speed of a trot or pace, though smoother to ride. These include the lateral rack, running walk, and tölt as well as the diagonal fox trot.^[78] Ambling gaits are often genetic in some breeds, known collectively as gaited horses.^[79] Often, gaited horses replace the trot with one of the ambling gaits.^[80]

Behavior[link]

Horses are prey animals with a strong fight-or-flight response. Their first reaction to threat is to startle and usually flee, although they will stand their ground and defend themselves when flight is impossible or if 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. Most light horse riding breeds were developed for speed, agility, alertness and endurance; natural qualities that extend from their wild ancestors. However, through selective breeding, some breeds of horses are quite docile, particularly certain draft horses.^[81] Horses are herd animals, with a clear hierarchy of rank, led by a dominant individual, usually a mare. They are also social creatures that 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, but with training, horses can learn to accept a human as a companion, and thus be comfortable away from other horses.^[82] However, when confined with insufficient companionship, exercise, or stimulation, individuals may develop stable vices, an assortment of bad habits, mostly stereotypies of psychological origin, that include wood chewing, wall kicking, "weaving" (rocking back and forth), and other problems.^[83]

Intelligence and learning[link]

In the past, horses were considered unintelligent, with no abstract thinking ability, unable to generalize, and driven primarily by a herd mentality. However, modern studies show that they perform a number of cognitive tasks on a daily basis, meeting mental challenges that include food procurement and social system identification. They also have good spatial discrimination abilities.^[84] 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 learn from habituation, desensitization, Pavlovian conditioning, and operant conditioning. They respond to and learn from both positive and negative reinforcement.^[84] Recent studies even suggest horses are able to count if the quantity involved is less than four.^[85]

Domesticated horses tend to face greater mental challenges than wild horses, because they live in artificial environments that stifle instinctive behavior while learning tasks that are not natural.^[84] 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.^[86]

Temperament[link]

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,^[87] while the "cold-bloods", such as most draft breeds, are quieter and calmer.^[88] Sometimes "hot-bloods" are classified as "light horses" or "riding horses",^[89] with the "cold-bloods" classified as "draft horses" or "work horses".^[90]

Illustration of hotbloods, warmbloods and coldblood breeds

"Hot blooded" breeds include "oriental horses" such as the Akhal-Teke, Arabian horse, Barb and now-extinct Turkoman horse, as well as the Thoroughbred, a breed developed in England from the older oriental breeds.^[87] Hot bloods tend to be spirited, bold, and learn quickly. They are bred for agility and speed.^[91] They tend to be physically refined—thin-skinned, slim, and long-legged.^[92] 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.^[93][94]

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.^[88] They are sometimes nicknamed "gentle giants".^[95] Well-known draft breeds include the Belgian and the Clydesdale.^[95] Some, like the Percheron, are lighter and livelier, developed to pull carriages or to plow large fields in drier climates.^[96] Others, such as the Shire, are slower and more powerful, bred to plow fields with heavy, clay-based soils.^[97] The cold-blooded group also includes some pony breeds.^[98]

"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 milder temperament than a lighter breed.^[99] Certain pony breeds with warmblood characteristics have been developed for smaller riders.^[100] Warmbloods are considered a "light horse" or "riding horse".^[89]

Today, the term "Warmblood" refers to a specific subset of sport horse breeds that are used for competition in dressage and show jumping.^[101] Strictly speaking, the term "warm blood" refers to any cross between cold-blooded and hot-blooded breeds.^[102] Examples include breeds such as the Irish Draught or the Cleveland Bay. The term was once used to refer to breeds of light riding horse other than Thoroughbreds or Arabians, such as the Morgan horse.^[91]

Sleep patterns[link]

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.^[103] 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.^[104]

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,^[104] mostly in short intervals of about 15 minutes each.^[105] The average sleep time of a domestic horse is said to be 2.9 hours per day.^[106]

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.^[104] 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.^[107] This condition differs from narcolepsy, although horses may also suffer from that disorder.^[108]

Taxonomy and evolution[link]

From left to right: Size development, biometrical changes in the cranium, reduction of toes (left forefoot)

The horse adapted to survive in areas of wide-open terrain with sparse vegetation, surviving in an ecosystem where other large grazing animals, especially ruminants, could not.^[109] Horses and other equids are odd-toed ungulates of the order Perissodactyla, a group of mammals that was dominant during the Tertiary period. In the past, this order contained 14 families, but only three—Equidae (the horse and related species), the tapir, and the rhinoceros—have survived to the present day.^[110]

The earliest known member of the Equidae family was the Hyracotherium, which lived between 45 and 55 million years ago, during the Eocene period. It had 4 toes on each front foot, and 3 toes on each back foot.^[111] The extra toe on the front feet soon disappeared with the Mesohippus, which lived 32 to 37 million years ago.^[112] Over time, the extra side toes shrank in size until they vanished. All that remains of them in modern horses is a set of small vestigial bones on the leg below the knee,^[113] known informally as splint bones.^[114] Their legs also lengthened as their toes disappeared until they were a hooved animal capable of running at great speed.^[113] By about 5 million years ago, the modern Equus had evolved.^[115] Equid teeth also evolved from browsing on soft, tropical plants to adapt to browsing of drier plant material, then to grazing of tougher plains grasses. Thus proto-horses changed from leaf-eating forest-dwellers to grass-eating inhabitants of semi-arid regions worldwide, including the steppes of Eurasia and the Great Plains of North America.

By about 15,000 years ago, Equus ferus was a widespread holarctic species. Horse bones from this time period, the late Pleistocene, are found in Europe, Eurasia, Beringia, and North America.^[116] Yet between 10,000 and 7,600 years ago, the horse became extinct in North America and rare elsewhere.^{[117][118][119]} The reasons for this extinction are not fully known, but one theory notes that extinction in North America paralleled human arrival.^[120] Another theory points to climate change, noting that approximately 12,500 years ago, the grasses characteristic of a steppe ecosystem gave way to shrub tundra, which was covered with unpalatable plants.^[121]

Wild species surviving into modern times[link]

A small herd of Przewalski's Horses

A truly wild horse is a species or subspecies with no ancestors that were ever domesticated. Therefore, most "wild" horses today are actually feral horses, animals that escaped or were turned loose from domestic herds and the descendants of those animals.^[122] Only two never-domesticated subspecies, the Tarpan and the Przewalski's Horse, survived into recorded history and only the latter survives today.

The Przewalski's Horse (Equus ferus przewalskii), named after the Russian explorer Nikolai Przhevalsky, is a rare Asian animal. It is also known as the Mongolian Wild Horse; Mongolian people know it as the taki, and the Kyrgyz people call it a kirtag. The subspecies was presumed extinct in the wild between 1969 and 1992, while a small breeding population survived in zoos around the world. In 1992, it was reestablished in the wild due to the conservation efforts of numerous zoos.^[123] Today, a small wild breeding population exists in Mongolia.^[124][125] There are additional animals still maintained at zoos throughout the world.

The Tarpan or European Wild Horse (Equus ferus ferus) was found in Europe and much of Asia. It survived into the historical era, but became extinct in 1909, when the last captive died in a Russian zoo.^[126] Thus, the genetic line was lost. Attempts have been made to recreate the Tarpan,^{[126][127][128]} which resulted in horses with outward physical similarities, but nonetheless descended from domesticated ancestors and not true wild horses.

Periodically, populations of horses in isolated areas are speculated to be relict populations of wild horses, but generally have been proven to be feral or domestic. For example, the Riwoche horse of Tibet was proposed as such,^[125] but testing did not reveal genetic differences with domesticated horses.^[129] Similarly, the Sorraia of Portugal was proposed as a direct descendant of the Tarpan based on shared characteristics,^[130][131] but genetic studies have shown that the Sorraia is more closely related to other horse breeds and that the outward similarity is an unreliable measure of relatedness.^[130][132]

Other modern equids[link]

Besides the horse, there are seven other species of genus Equus in the Equidae family. These are the ass or donkey, Equus asinus; the mountain zebra, Equus zebra; plains zebra, Equus quagga; Grévy's zebra, Equus grevyi; the kiang, Equus kiang; and the onager, Equus hemionus.^[133]

Horses can crossbreed with other members of their genus. The most common hybrid is the mule, a cross between a "jack" (male donkey) and a mare. A related hybrid, a hinny, is a cross between a stallion and a jenny (female donkey).^[134] Other hybrids include the zorse, a cross between a zebra and a horse.^[135] With rare exceptions, most hybrids are sterile and cannot reproduce.^[136]

Domestication[link]

Bhimbetka rock painting showing man riding on horse, India

Domestication of the horse most likely took place in central Asia prior to 3500 BC. Two major sources of information are used to determine where and when the horse was first domesticated and how the domesticated horse spread around the world. The first source is based on palaeological and archaeological discoveries, the second source is a comparison of DNA obtained from modern horses to that from bones and teeth of ancient horse remains.

The earliest archaeological evidence for the domestication of the horse comes from sites in Ukraine and Kazakhstan, dating to approximately 3500–4000 BC.^[137][138] By 3000 BC, the horse was completely domesticated and by 2000 BC there was a sharp increase in the number of horse bones found in human settlements in northwestern Europe, indicating the spread of domesticated horses throughout the continent.^[139] The most recent, but most irrefutable evidence of domestication comes from sites where horse remains were interred with chariots in graves of the Sintashta and Petrovka cultures c. 2100 BC.^[140]

Domestication is also studied by using the genetic material of present day horses and comparing it with the genetic material present in the bones and teeth of horse remains found in archaeological and palaeological excavations. The variation in the genetic material shows that very few wild stallions contributed to the domestic horse,^[141][142] while many mares were part of early domesticated herds.^{[132][143][144]} This is reflected in the difference in genetic variation between the DNA that is passed on along the paternal, or sire line (Y-chromosome) versus that passed on along the maternal, or dam line (mitochondrial DNA). There are very low levels of Y-chromosome variability,^[141][142] but a great deal of genetic variation in mitochondrial DNA.^{[132][143][144]} There is also regional variation in mitochondrial DNA due to the inclusion of wild mares in domestic herds.^{[132][143][144][145]} Another characteristic of domestication is an increase in coat color variation.^[146] In horses, this increased dramatically between 5000 and 3000 BC.^[147]

Before the availability of DNA techniques to resolve the questions related to the domestication of the horse, various hypothesis were proposed. One classification was based on body types and conformation, suggesting the presence of four basic prototypes that had adapted to their environment prior to domestication.^[98] Another hypothesis held that the four prototypes originated from a single wild species and that all different body types were entirely a result of selective breeding after domestication.^[148] However, the lack of a detectable substructure in the horse has resulted in a rejection of both hypotheses.

Feral populations[link]

Feral horses are born and live in the wild, but are descended from domesticated animals.^[122] Many populations of feral horses exist throughout the world.^[149][150] Studies of feral herds have provided useful insights into the behavior of prehistoric horses,^[151] as well as greater understanding of the instincts and behaviors that drive horses that live in domesticated conditions.^[152]

There are also semi-feral horses in many parts of the world, such as Dartmoor and the New Forest in the UK, where the animals are all privately owned but live for significant amounts of time in "wild" conditions on undeveloped, often public, lands. Owners of such animals often pay a fee for grazing rights.^[153][154]

Breeds[link]

The concept of purebred bloodstock and a controlled, written breed registry has come to be particularly significant and important in modern times. Sometimes purebred horses are incorrectly or inaccurately called "thoroughbreds". Thoroughbred is a specific breed of horse, while a "purebred" is a horse (or any other animal) with a defined pedigree recognized by a breed registry.^[155] Horse breeds are groups of horses with distinctive characteristics that are transmitted consistently to their offspring, such as conformation, color, performance ability, or disposition. These inherited traits result from a combination of natural crosses and artificial selection methods. Horses have been selectively bred since their domestication. An early example of people who practiced selective horse breeding were the Bedouin, who had a reputation for careful practices, keeping extensive pedigrees of their Arabian horses and placing great value upon pure bloodlines.^[156] These pedigrees were originally transmitted via an oral tradition.^[157] In the 14th century, Carthusian monks of southern Spain kept meticulous pedigrees of bloodstock lineages still found today in the Andalusian horse.^[158]

Breeds developed due to a need for "form to function", the necessity to develop certain characteristics in order to perform a particular type of work.^[159] Thus, thus a powerful but refined breed such as the Andalusian developed as riding horses with an aptitude for dressage.^[159] Heavy draft horses developed out of a need to perform demanding farm work and pull heavy wagons.^[160] Other horse breeds developed specifically for light agricultural work, carriage and road work, various sport disciplines, or simply as pets.^[161] Some breeds developed through centuries of crossing other breeds, while others descended from a single foundation sire, or other limited or restricted foundation bloodstock. One of the earliest formal registries was General Stud Book for Thoroughbreds, which began in 1791 and traced back to the foundation bloodstock for the breed.^[162] There are more than 300 horse breeds in the world today.^[163]