The horse (Equus caballus, sometimes seen as a subspecies of the Wild Horse, Equus ferus caballus) is a large odd-toed ungulate mammal, one of ten modern species of the genus Equus. Horses have long been among the most economically important domesticated animals; although their importance has declined with mechanization, they are still found worldwide, fitting into human lives in various ways. The horse is prominent in religion, mythology, and art; it has played an important role in transportation, agriculture, and war; it has additionally served as a source of food, fuel, and clothing.

Almost all breeds of horses can, at least in theory, carry humans on their backs or be harnessed to pull objects such as carts or plows. However, horse breeds were developed to allow horses to be specialized for certain task; lighter horses for racing or riding, heavier horses for farming and other tasks requiring pulling power. In some societies, horses are a source of food, both meat and milk; in others it is taboo to consume them. In industrialized countries horses are predominantly kept for leisure and sporting pursuits, while they are still used as working animals in many other parts of the world.

Biology

Anatomy of a horse from an Egyptian document (15th century)

Age

Depending on breed, management and environment, the domestic horse today has a life expectancy of 25 to 30 years. Some specific breeds of horse can live into their 40s, and, occasionally, beyond. The oldest verifiable record was "Old Billy," a horse that lived in the 19th century to the age of 62.^[1]

Size

The size of horses varies by breed, but can also be influenced by nutrition. The general rule for cutoff in height between what is considered a horse and a pony at maturity is 14.2 hands(h or hh) (147 cm, 58 inches) as measured at the withers. An animal 14.2h or over is usually considered a horse and one less than 14.2h is a pony.

However, there are exceptions to the general rule. Some smaller horse breeds who typically produce individual horses both under and over 14.2h are considered "horses" regardless of height. Likewise, some pony breeds, such as the Pony of the Americas or the Welsh cob, share some features of horses and individual animals may occasionally mature at over 14.2h, but are still considered ponies.

The difference between a horse and pony is not simply a height difference, but also a difference in phenotype or appearance. There are noticeable differences in conformation and temperament. Ponies often exhibit thicker manes, tails and overall coat. They also have proportionally shorter legs, wider barrels, heavy bone, thick necks, and short heads with broad foreheads.

Light horses such as Arabians, Morgans, Quarter Horses, Paints and Thoroughbreds usually range in height from 14.0 to 16.0 hands and can weigh from 850 lbs to about 1500 lbs. Heavy or draft horses such as the Clydesdale, Belgian, Percheron, and Shire are usually at least 16.0 to 18.0 hands high and can weigh from about 682 kg (1500 lb) up to about 900 kg (2000 lb). Ponies are less than 14.2h, but can be much smaller, down to the Shetland pony at around 10 hands, and the Falabella which can be the size of a medium-sized dog. The miniature horse is as small as or smaller than either of the aforementioned ponies but are classified as very small horses rather than ponies despite their size.

The largest horse in history was a Shire horse named Sampson, later renamed Mammoth, foaled in 1846 in Bedfordshire, England. He stood stood 21.2½ hands high (i.e. 7 ft 2½ in or 2.20 m ), and his peak weight was estimated at over 3,300 lb (approx 1.5 tonnes). The current record holder for the world's smallest horse is Thumbelina, a fully mature miniature horse affected by dwarfism. She is 17 inches tall and weighs 60 pounds.^[2]

Reproduction and development

Pregnancy lasts for approximately 335-340 days and usually results in one foal (male: colt, female: filly). Twins are rare. Colts are usually carried 2-7 days longer than fillies. Females 4 years and over are called mares and males are stallions. A castrated male is a gelding. Horses, particularly colts, may sometimes be physically capable of reproduction at approximately 18 months but in practice are rarely allowed to breed until a minimum age of 3 years, especially females. Horses four years old are considered mature, though the skeleton usually finishes developing at the age of six, and the precise time of completion of development also depends on the horse's size (therefore a connection to breed exists), 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 (bones are made of cartilage in earlier stages of bone formation), but the epiphyseal plates (plates that fuse a bone into one piece by connecting the bone shaft to the bone ends) are also larger and take longer to convert from cartilage to bone as well. These plates convert after the other parts of the bones do 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 and American Quarter Horse race horses are put on the track at as young as two years old in some countries (notably the United States), horses specifically bred for sports such as show jumping and dressage are generally not entered into top-level competition until a minimum age of four years old, because their bones and muscles are not solidly developed, nor is their advanced training complete.

Teeth

Horses are adapted to grazing, so their teeth continue to grow throughout life. There are 12 teeth (six upper and six lower), the incisors, adapted to biting off the grass or other vegetation, at the front of the mouth, and 24 teeth, the premolar and molars, adapted for chewing, 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.

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, and while the diet and veterinary care of the horse can affect the rate of tooth wear, a very rough estimate of the age of a horse can be made by looking at its teeth.

General anatomy

Horses have, on average, a skeleton of 205 bones. A significant difference in the bones contained in the horse skeleton, as compared to that of a human, is the lack of a collarbone--their 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, interesting 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 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 and hair, bone, tendons, ligaments, cartilage, and the assorted specialized tissues that make up the hoof (see section hooves, below).

A horse's digestive system is adapted to a forage diet of grasses and other plant material, consumed regularly throughout the day, and so they have a relatively small stomach but very long intestines to facilite a steady flow of nutrients. 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 cecum, or "water gut" that is part of their large intestine. Unlike humans, horses cannot vomit, so digestion problems can quickly spell trouble, with colic a leading cause of death.

As prey animals, they have very large eyes (only the whale has a larger eye), with excellent day and night vision, though they may have a limited range of color vision. While not color-blind, studies indicate that they have difficulty distinguishing greens, browns and grays. Their hearing is good, and their ears can rotate to pick up sound from any direction. Their sense of smell, while much better than that of humans, is not their strongest asset; they rely to a greater extent on vision.

The hoof

The critical importance of the feet and legs is summed up by the traditional adage, "no foot, no horse." 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, with the exterior hoof wall and horn of the sole made essentially of the same material as a human fingernail. The end result is that a horse, weighing on average 1,000 pounds, travels on the same bones as a human on tiptoe. For the protection of the hoof under certain conditions, some horses have horseshoes placed on their feet by a professional farrier.

Wikipedia

http://en.wikipedia.org/w/index.php?title=Horse&action=history

http://www.gnu.org/copyleft/fdl.html