Horse

For other uses, see Horse (disambiguation).

The horse (Equus ferus caballus) is a domesticated, one-toed, hoofed mammal. It belongs to the animal family Equidae and is one of two extant subspecies of Equus ferus. Over the past 45 to 55 million years, the horse evolved from a small creature with several toes, Eohippus, into the large animal with a single toe that we know today. This change began in North America. Humans began domesticating horses around 4000 BCE in Central Asia, mainly linked to the Botai culture. Their domestication is believed to have spread widely by 3000 BCE. Newer genetic evidence suggests that modern domesticated horses do not come from this group. Instead, it points to the Western Eurasian steppes in the lower Volga-Don region as the homeland of modern domestic horses. From 2000 BCE, this modern horse spread quickly across Eurasia and replaced local horse populations.

Horses that belong to the subspecies caballus are domesticated, although some of these animals live in the wild as feral horses. Feral horses are not true wild horses, which have never been domesticated. Horses are adapted to run, which helps them escape predators quickly. They also have a good sense of balance and a strong fight-or-flight response. Horses can sleep both standing up and lying down, and younger horses sleep much more than adults. A female horse, called a mare, carries her young for about 11 months. A young horse, called a foal, can stand and run soon after birth. Most domesticated horses begin training under a saddle or in a harness between the ages of two and four. They reach full adult development by age five and usually live between 25 and 30 years.

Horse breeds are loosely grouped into three categories based on general temperament. Spirited "hot bloods" are known for speed and endurance. "Cold bloods," such as draft horses and some ponies, are suited to slow, heavy work. "Warmbloods" were developed from crosses between hot bloods and cold bloods, and they often focus on creating breeds for specific kinds of riding, particularly in Europe. Today, more than 300 horse breeds exist, developed for many different uses.

Horses and humans take part in many sports, competitions, and recreational activities. Horses also work with people in jobs such as police work, agriculture, entertainment, and therapy. Horses were historically used in warfare, and this led to a wide variety of riding and driving techniques. These techniques used many different styles of equipment and methods of control. Many products come from horses, including meat, milk, hide, hair, bone, and pharmaceuticals extracted from the urine of pregnant mares.

Lifespan and life stages

Depending on breed, management and environment, the modern domestic horse can live 25 to 30 years, and a few live into their 40s or occasionally longer. The oldest verified record was "Old Billy", a 19th-century horse that lived to 62. More recently, Sugar Puff, listed in Guinness World Records as the world's oldest living pony, died in 2007 at age 56. For most competitions, a year is added to a horse's age each January 1 in the Northern Hemisphere and each August 1 in the Southern Hemisphere, no matter its real birth date. The exception is endurance riding, where the minimum age is based on the animal's actual calendar age.

Special words describe horses of different ages. A foal is a horse under one year old, and a foal that is still nursing may be called a suckling. A yearling is one to two years old. A colt is a young male under four, and a filly is a young female under four. Calling any young horse a "colt" is a common mistake. A mare is a female four years or older, a stallion is a non-castrated male four years or older, and a gelding is a castrated male of any age. In horse racing, these definitions can differ. For example, in the British Isles, Thoroughbred racing treats colts and fillies as under five, but Australian Thoroughbred racing treats them as under four.

Size and measurement

Horse height is measured at the withers, the point where the neck meets the back. This spot is used because it stays steady, while the head and neck move up and down.

In English-speaking countries, height is often given in hands and inches. One hand equals 4 inches (101.6 mm). A horse described as "15.2 h" is 15 hands plus 2 inches, or 62 inches (157.5 cm) tall. Size depends on the breed and is also affected by nutrition. Light riding horses are usually 14 to 16 hands tall, while draft horses are usually at least 16 hands. The largest horse on record was probably a Shire horse named Mammoth, born in 1848, who was estimated to weigh 1,524 kilograms (3,360 lb). The smallest horse on record is Thumbelina, a miniature horse affected by dwarfism, who was 43 centimetres (17 in) tall.

Main article: Pony

Ponies are taxonomically the same animals as horses. People usually tell them apart by height, but phenotype, such as body shape and temperament, can also matter. The traditional line is 14.2 hands (58 inches, 147 cm), with smaller animals called ponies, though many exceptions exist. Ponies often have thicker manes, tails, and coats, shorter legs, and calmer temperaments. Height alone is not the only sign: the Shetland pony is a pony even though it averages about 10 hands, while Falabella and other miniature horses are classed as very small horses, not ponies.

Genetics

Horses have 64 chromosomes. The horse genome was sequenced in 2007. It contains 2.7 billion DNA base pairs. This is more than the dog genome, but less than the human genome or the bovine genome. Researchers can use the map.

Colors and markings

Horses have many coat colors and distinctive markings, which are described with special words. People often describe a horse first by its coat color, before its breed or sex. Horses of the same color can look different because of their white markings and spotting patterns, which are inherited separately from coat color. Scientists have identified many genes that create horse coat colors and patterns. Current genetic tests can find at least 13 different alleles that affect coat color.

The basic colors chestnut and black are controlled by the gene for the Melanocortin 1 receptor, also called the "extension gene" or "red factor". Other genes can stop black from showing, creating a bay with point coloration. Some create spots such as pinto or leopard, some dilute colors such as palomino or dun, and others cause greying. A horse that looks white is usually an older gray horse, which is born darker and gets lighter with age, though it usually keeps black skin under its white hair. Truly white horses are born with white hair and pink skin, which is fairly rare. Different genetic factors, including dominant white and the sabino-1 gene, can produce white coats. There are no "albino" horses, meaning horses with both pink skin and red eyes.

Reproduction and development

A Gestation period lasts about 340 days, usually producing one foal, and twins are rare. Horses are precocial, so foals can stand and run soon after birth. Foals are usually born in spring. A mare's estrous cycle comes about every 19 to 22 days, from early spring into autumn, and most mares do not cycle during winter. Foals are generally weaned from their mothers at four to six months old.

Horses, especially colts, may be physically able to reproduce at about 18 months, but domesticated horses are rarely bred before age three, especially females. Horses are considered mature at four, although the skeleton keeps developing until about six. Bigger horses have bigger bones, so their bone tissue takes longer to form, and the epiphyseal plates, which change from cartilage to bone, take longer to convert. Horses are usually trained to be ridden between ages two and four, depending on maturity, breed, and the work expected. Thoroughbred race horses may start racing at two in some countries, but horses bred for dressage are usually not ridden until three or four. For endurance riding, horses are not considered mature enough to compete until they are five.

Main articles: Equine anatomy, Muscular system of the horse, Respiratory system of the horse, and Circulatory system of the horse

Main article: Skeletal system of the horse

Main articles: Equine digestive system and Equine nutrition

See also: Equine vision

A horse's skeleton has about 205 bones. Unlike humans, horses have no collarbone. Instead, the front legs are held to the body by strong muscles, tendons, and ligaments that join the shoulder blade to the spinal column. A horse's "knee" is really made of carpal bones, which match the human wrist. The hock holds bones like those in the human ankle and heel. The "ankle" is really the fetlock, which has sesamoid bones between the cannon bones, the metacarpal or metatarsal bones, and the proximal phalanges. Horses have no muscles below their knees and hocks. The hoof begins with the distal phalanges and includes cartilage and laminae. Its outer wall is made of keratin, the same material as a human fingernail. Horses travel on what would be the tips of a human's toes. Hooves keep growing and usually need trimming every five to eight weeks. Some horses wear horseshoes fitted by a farrier for protection. Horses are adapted to grazing and have 12 incisors at the front and 24 chewing teeth, the premolars and molars, at the back. Stallions and geldings also have canine teeth called "tushes", and some horses have small vestigial "wolf" teeth that are usually removed because they can interfere with the bit when a horse is bridled. Because teeth wear down over time, they can give a rough idea of a horse's age.

Horses are herbivores that eat forage steadily throughout the day and drink plenty of water. They are not ruminants, but they can digest cellulose through hindgut fermentation in the cecum and large intestine, with help from bacteria. Horses cannot vomit, so colic can quickly cause death. They have no gallbladder. As prey animals, horses have large eyes on the sides of their heads, giving them more than 350° of view. About 65° of this is binocular vision, and the rest is monocular vision. Horses see well in daylight and at night, but their color vision is dichromatic, similar to color blindness in humans. Their sense of smell is better than a human's but not as good as a dog's, and their vomeronasal organs help them detect pheromones. Each pinna can rotate up to 180°. A 2013 study in the UK found that stabled horses were calmest in quiet settings or with country or classical music, and that they seemed nervous with jazz or rock. Horses have a good sense of balance because of their proprioception, and their sense of touch is so sensitive that they can feel an insect landing on their body. They have a good sense of taste that helps them choose fodder, and their prehensile lips can sort even small grains.

Movement

All horses move naturally with four basic gaits. The walk has four beats and averages 6.4 kilometres per hour (4.0 mph). The trot or jog has two beats and runs at 13 to 19 kilometres per hour (8.1 to 11.8 mph), though harness racing horses can go faster. The canter or lope has three beats and runs at 19 to 24 kilometres per hour (12 to 15 mph). The gallop averages 40 to 48 kilometres per hour (25 to 30 mph), and the world record for a horse running a short sprint is 70.76 kilometres per hour (43.97 mph).

Some horses use a two-beat pace instead of the trot. Several four-beat 'ambling' gaits are about as fast as a trot or pace but smoother to ride. These include the lateral rack, the running walk, the tölt, and the diagonal fox trot. Ambling gaits are often inherited in some breeds, which are known together as gaited horses. These horses use an ambling gait in place of the trot.

Behavior

Horses are prey animals with a strong fight-or-flight response. When they sense danger, they usually startle and run away, but they will stand their ground and defend themselves when they cannot escape or when their young are threatened. They are also curious and often pause to find out what caused their fright. Most light riding breeds were developed for speed, agility, alertness, and endurance, qualities that come from their wild ancestors. However, through selective breeding, some breeds are quite gentle, especially certain draft horses.

Horses are herd animals with a clear rank order, usually led by a dominant mare. They can form bonds with other horses and animals, including humans, and they communicate through sounds like nickering and whinnying, mutual grooming, and body language. Horses kept alone can be hard to manage, but training can help them accept a person as a companion. Without enough companionship, exercise, or stimulation, they may develop stable vices, bad habits that are mostly stereotypies of psychological origin, such as wood chewing, wall kicking, and "weaving." Studies show that horses use cognitive skills daily, including food procurement, recognizing others in a social system, spatial discrimination, problem solving, and memory. They learn simple tasks easily and can also manage categorization and concept learning through habituation, desensitization, classical conditioning, operant conditioning, and positive and negative reinforcement. Domesticated horses may face greater mental challenges than wild horses, because they live in artificial environments that prevent instinctive behavior. Horses are mammals and are warm-blooded, or endothermic, unlike poikilothermic animals, but in horse terms the words describe temperament rather than body temperature. "Hot-bloods," such as many race horses, are spirited and quick to learn, including "oriental horses" like the Akhal-Teke, Arabian horse, Barb, Turkoman horse, and Thoroughbred. "Cold-bloods," such as draft horses like the Belgian, Clydesdale, Percheron, and Shire, are calm and patient and were bred for strength. "Warmblood" breeds such as the Trakehner and Hanoverian fall between these groups, and the term today refers to sport horse breeds used in dressage and show jumping. Horses sleep standing or lying down, using a "stay apparatus" in their legs to doze without collapsing. They take many short rests and must lie down to reach REM sleep. A horse that is never allowed to lie down may become sleep-deprived and, rarely, collapse as it slips into REM sleep while standing. This differs from narcolepsy, though horses may also suffer from that disorder.

Taxonomy and evolution

Main articles: Evolution of the horse, Equus (genus), and Equidae

The horse adapted to survive in wide, open land with few plants, where other large grazing animals, especially ruminants, could not live. Horses and other equids are odd-toed ungulates of the order Perissodactyla, a group of mammals that was common during the Tertiary. This order once had 14 families, but only three have survived to today: Equidae (the horse and related animals), Tapiridae (the tapir), and Rhinocerotidae (the rhinoceroses). The earliest known member of Equidae was Hyracotherium, which lived 45 to 55 million years ago in the Eocene period. Its toes slowly changed over time, and the extra side toes shrank until they vanished. Their legs grew longer, and by about 5 million years ago the modern Equus had evolved. It spread from North America to the Old World during the Pleistocene. Horse teeth also changed from eating soft plants to eating tougher grasses, so these animals moved from forests to open grasslands, such as the steppes of Eurasia and the Great Plains of North America.

By about 15,000 years ago, Equus ferus was a widespread holarctic species, and its bones from that time are found in Europe, Eurasia, Beringia, and North America. Between 10,000 and 7,600 years ago, horses died out in North America. Scientists are not fully sure why, but one idea is that this happened when people arrived, and another points to climate change, when grasslands turned into tundra covered with plants that animals could not eat. Today, most "wild" horses are actually feral horses, which are descendants of animals that escaped or were let go from domestic herds. Only the tarpan and the Przewalski's horse lived into recorded history, and only the Przewalski's horse survives today. It is named after the Russian explorer Nikolai Przhevalsky and is known as the Mongolian wild horse. The tarpan became extinct in 1909. Some isolated horse groups, such as the Riwoche horse and the Sorraia, were once thought to be relict wild populations, but tests showed they are not truly wild. Besides the horse, six other species belong to the genus Equus, including the donkey, several zebras, the kiang, and the onager. Horses can crossbreed with these relatives. The most common hybrid is the mule, a cross between a male donkey and a mare, while the hinny and the zorse are other hybrids. Most hybrids are sterile and cannot have young.

Main article: Wild horse

Domestication and history

Main articles: History of horse domestication theories and Domestication of the horse

Horses were most likely first domesticated in central Asia before 3500 BCE. Scientists learn about this from two kinds of clues. One comes from palaeological and archaeological discoveries. The other compares DNA from modern horses with DNA from the bones and teeth of ancient horses. The earliest signs of attempted domestication come from the Western Eurasian Steppes, especially the lower Volga-Don region, around 4000–3500 BCE. The horses kept at the Botai culture in Kazakhstan were Przewalski's horses, not ancestors of modern horses. By 3000 BCE, horses were fully domesticated. By 2000 BCE, many more horse bones were found in settlements in northwestern Europe. The clearest evidence comes from graves of the Sintashta and Petrovka cultures, around 2100 BCE, where horses were buried with chariots. DNA studies show that modern domestic horses carry the DOM2 gene, which spread quickly around 2200 BCE, at the same time as the Sintashta culture and spoked chariots. This suggests that certain traits were strongly favored because of riding, and the equestrian material culture that spread with the horse. DNA also shows that very few wild stallions contributed to domestic horses, while many mares joined early herds. Men's Y-chromosome variation is very low, but mitochondrial DNA from the mother's line varies a great deal. Coat color variety also increased sharply between 5000 and 3000 BCE. Earlier ideas, made before DNA tests, suggested four basic body types, or that all types came from one wild species through selective breeding. Both ideas were rejected because no clear divisions were found in horses.

Main article: Feral horse

Feral horses are born and live in the wild, but they are descended from domesticated animals. Many feral horses live around the world, and studying them helps scientists learn about the behavior of prehistoric horses. There are also semi-feral horses, such as those on Dartmoor and the New Forest in the UK. These horses are privately owned but spend much of their time in "wild" conditions on open, often public, land.

Today, purebred horses with a controlled, written breed registry are very important. A "purebred" is a horse with a defined pedigree recognized by a registry, while Thoroughbred is one specific breed. Breeds are groups of horses with traits, such as conformation, color, or ability, that are passed on to their young. These traits come from natural crosses and artificial selection. The Bedouin were an early example of people who carefully bred horses, keeping detailed pedigrees of their Arabian horses that were first passed on by oral tradition. Breeds developed to match their work. The Andalusian was developed for riding and dressage, and heavy draft horses were developed for demanding farm work. Some breeds came from many crosses, while others descended from a single foundation sire. One of the earliest formal registries, the General Stud Book for Thoroughbreds, began in 1791. There are more than 300 horse breeds in the world today.

Interaction with humans

Horses have been part of human life around the world for thousands of years. People ride them for fun, for sports, and for work. The Food and Agriculture Organization (FAO) estimates that there were almost 59,000,000 horses in the world in 2008. About 33,500,000 lived in the Americas, 13,800,000 in Asia, and 6,300,000 in Europe, with smaller numbers in Africa and Oceania. The American Horse Council estimates that horse-related activities have a direct impact of over $39 billion on the United States economy and support 1.4 million full-time jobs. When riding, a horse usually wears a saddle to help the rider keep balance, and a bridle or similar headgear to help the rider stay in control. Many horses are also driven, pulling a vehicle with a harness and bridle.

Many equestrian sports, such as dressage, eventing, and show jumping, began in military training, and rodeo grew out of skills used on working ranches and stations. Horses still do jobs that machines cannot fully replace, such as mounted police patrols and search and rescue work. About 100 million horses, donkeys, and mules are still used for farming and transport in less developed areas. Horses have been used in warfare for most of recorded history, and their use was widespread by the end of the Bronze Age. Horses also help people with disabilities at the Paralympic Games. Horses are raw material for products, such as mare's milk, which the Mongols fermented into kumis. Horses need good forage such as hay or pasture, clean water, shelter, regular farrier hoof care, and checkups from a veterinarian.