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Horses are members of the family Equidae, which includes the wild asses of Africa and Asia and the zebras of African plains and mountains. The origins of horse-like mammals have been traced back some 55 million years to a small dog-sized, plant-eating animal known as Hyracotherium. More recently, during the Pliocene and Miocene periods (which ended some 1.5-2 million years ago) horses and their relatives as we know them today were probably the most abundant medium-sized grazing animals in the world. Since then, every species has experienced a major reduction in population size.

One wild horse, the tarpan, a small, shy, grey species lived on the Russian steppes of Eurasia until some time in the eighteenth century, when it became extinct because of overhunting and cross-breeding with domesticated species. Almost nothing is known about this animal apart from scant information in a few museums. The only other true wild horse, the slightly larger Prze walski's horse (Equus przewalskii), is now also thought to have gone extinct in the wild as recently as the mid 1960s. Some members of this species were, however, preserved in captivity so at least some representatives of this ancient lineage remain.

Horses are grazing animals of wide open plains, where constant vigilance is necessary in order to avoid predators such as lions, tigers, leopards, and wild canids. Apart from their keen senses of vision, hearing, and smell, horses are well equipped to outrun most potential attackers. Wild horses also undergo extensive seasonal migrations in search of optimal feeding and watering habitat. The feet of these hoofed animals (perissodactyls) are modified for agility and rapid movement. Horses have light feet with just one toe and, when moving, the hoof is the only part of the foot to touch the ground. Horses are also characterized by their long, slender legs, capable of a steady, prolonged movement or a long, striding gait. A deep chest allows for their large lungs, as well as the animal's large stomach, which is important for digesting the great amounts of relatively bulky plant materials.

Grasses and herbs form a major part of the diet. While these materials are relatively abundant, they are often not very nutritious, being low in protein and difficult to digest. Horses eat large quantities of plant materials each day and must be able to transform this into energy and nutrition. Plant cells are composed of cellulose, which the digestive system of few mammals is capable of breaking down. To assist with this process horses and their relatives rely on microorganisms within the large intestine and colon to break down and ferment their bulky diet. In contrast to ruminating animals such as deer and cattle, horses have a small and relatively simple stomach in which proteins are digested and absorbed. The digestive system of horses is far less efficient than that of a cow, for example, which means that the former must eat considerably more of the same materials in order to acquire a similar amount of energy.

Przewalski's horse is closely related to the domestic species (Equus caballus), but is distinct in its appearance. Reaching more than 7 ft (2 m) at the shoulder, and with a length of almost 8 ft (2.5 m), these horses are a dark bay-dun color with a much lighter underside and muzzle patch. The dark mane narrows to a single, narrow dorsal stripe along the back, ending in a black tail. Early Stone Age cave paintings feature many illustrations of horses that closely resemble this species. It was formerly A Przewalski's horse (Equus przewalskii). Photograph by J. Gordon Miller. Reproduced by permission. widespread in steppe and semiarid habitats of Kazakhstan, Sinkiang, Mongolia, and parts of southern Siberia. The Przewalski's horse first became known to Western science in 1879, when it was discovered by a Polish explorer after whom the horse is named. Although there are no known estimates of the initial population size, by the early twentieth century it was already rare and found only in parts of southern China and Mongolia.

These animals were once highly prized by Mongolian people for their stamina. The wild herds once also provided semi-nomadic tribes with an essential supply of milk, meat, and hides, the latter being used for clothing as well as construction materials for their hut-like homes. Although the species is now extinct in its native habitat, sufficient animals are kept in zoological collections to enable a systematic program of captive breeding to take place. As a result of these efforts, there are now more than 1,000 individuals in captivity in many parts of the world. Apart from the hopes of conservationists to see this horse returned to its natural habitat, there is also a strong national desire amongst people in Mongolia to see these animals returned to the plains of its rightful heritage.

In their natural habitat, wild horses live in herds that consist of a number of mares, a single stallion, and foals and colts of a wide age span. The stallion is responsible for leading the herd to safe watering and feeding grounds and for protecting the females and young from predators. Stallions are extremely protective of their herds, and fights with other males who attempt to overthrow the stallion are common. Male horses fight with their hooves and teeth, especially the enlarged canines of the lower jaw—a prominent feature on mature males. A wide range of facial and other expressions are used to help avoid conflicts or to ensure that these are of short duration, as animals risk injury in such sparring events. Baring the teeth and curling the lips, while at the same time flattening the ears, is one of the most aggressive threats, while a number of vocalizations and stomping movements with the feet are also used to enhance the meaning of the gestures.

Almost everything we know about the social life of these animals is based on observations of semi-wild Przewalski's horses and feral populations of domestic horses. In the Przewalski's horse, young are born from April to June, following a gestation period of about 330 days. Mares usually bear a single foal which, shortly after birth, is able to stand up and follow its mother—an essential ability if the foal is not to fall prey to ever-vigilant predators. Foals remain close to their mothers for the first few weeks of life and do not become independent until they are almost two years old. Following this, they remain with the herd for several more years until they mature. In a natural situation, males are driven away from the herd as they reach sexual maturity. These solitary males usually join with other males to form small bachelor herds. Females, in contrast, may remain with the herd they were born into and will, in time, breed with the dominant male of the herd.

The precise origins of the domestic horse are not known but they likely arose from either the tarpan or Przewalski's horse. The earliest records of domestication are unclear and it is possible that this took place simultaneously in different parts of the world. Some reports suggest that it was attempted as early as 4000 B.C. in Mesopotamia and China, while evidence suggests that by 2000 B.C. domesticated horses were in use in China. Since then, horses have been bred for a number of purposes and there are now thought to be more than 180 different breeds. The powerful Shire horses were bred as draught animals in England, while most modern thoroughbreds, bred for their speed, stamina, and grace, are derived from breeding other species with primarily Arabian horses. The increasing spread of agriculture almost certainly played an important role in the use of domesticated species for draught purposes, but others were also bred and crossbred for their hardiness in extreme climates. Horses have also featured heavily in warfare, and many battles have been won and empires taken by mounted warriors.

Wild horses have suffered considerably since the arrival of humans on Earth. Horses and asses were once widely harvested for their meat and skins, particularly in parts of Asia. Elsewhere, the integrity of true wild species became diluted as domestic species interbred with wild animals. Natural changes may also have had some role to play in the demise of the wild horse, but it is more likely that human encroachment on the great plains of Asia, with spreading agriculture, has had the greatest and most long-term effect.

It is now too late to protect the last true wild horses, but considerable efforts are required to ensure that the last member of this ancient lineage, Przewalski's horse, and its natural habitat are protected in a manner that would enable this species to be reintroduced to its native habitat. Consideration should also be given to the preservation of wild stocks of domesticated varieties, such as the mustangs of North America, the Dartmoor and Exmoor ponies of Great Britain, and the brumbies of Australia, where these species have a role to play in maintaining the ecology of their respective habitats. In some countries, however, feral horses have caused considerable destruction to local plants and control programs are required to limit herd size so that they do not cause irreversible damage to fragile ecosystems. In other regions, feral horses play a useful role in cropping long coarse grasses, which helps keep the ecosystem open for other smaller, more fastidious grazing animals and plants. Some plants are known to germinate only when their seeds have passed through a horse's digestive system, as many of these plants may have evolved at a time when large herds of wild horses roamed the plains and acted as natural seed dispersers.

See also Livestock.

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