The horse is a hoofed mammal, a subspecies of one of seven extant species of the 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 BCE, and their domestication is believed to have been widespread by 3000 BCE; by 2000 BCE the use of domesticated horses had spread throughout the Eurasian continent. Although most horses today are domesticated, there are still endangered populations of the Przewalski's Horse, the only remaining true wild horse, as well as more common feral horses which live in the wild but are descended from domesticated ancestors.

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. Their 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.

Reproduction and Development

Horse breeding

Gestation lasts for approximately 335–340 days[38] and usually results in one foal. Twins are very rare. Colts are carried on average about 4 days longer than fillies. Horses are a precocial species, and foals are capable of standing and running within a short time following birth.

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. 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.



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. Although Thoroughbred race horses are put on the track at as young as two years old in some countries, 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. For endurance riding competition, horses are not deemed mature enough to compete until they are a full 60 calendar months (5 years) old.

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Colors and Markings

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. 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.

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.



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. There are no truly albino horses, with pink skin and red eyes, as albinism is a lethal condition in horses.

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Size and Measurement

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.

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). 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). 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).

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). 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).

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Age

Age

Depending on breed, management and environment, the domestic horse today has a life expectancy of 25 to 30 years. It is uncommon, but a few animals live into their 40s and, occasionally, beyond. The oldest verifiable record was "Old Billy", a 19th-century horse that lived to the age of 62. 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.

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 and August 1 in the southern hemisphere. The exception is in endurance riding, where the minimum age to compete is based on the animal's calendar age. A very rough estimate of a horse's age can be made from looking at its teeth.

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. Most domesticated foals are weaned at 5 to 7 months of age, although foals can be weaned at 4 months with no adverse effects.

* Yearling: a horse of either sex that is between one and two years old.

* Colt: a male horse under the age of four. 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.

* Mare: a female horse four years old and older.

* Stallion: a non-castrated male horse four years old and older. Some people, particularly in the UK, refer to a stallion as a "horse".

* Gelding: a castrated male horse of any age, 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. In the United States, both Thoroughbred racing and harness racing define colts and fillies as four years old and younger.

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