Horse Information

The Horse, common name for hoofed, herbivorous mammals that include the domestic horse and three groups of undomesticated species. One group comprises the zebra, native to Africa; another consists of the ass, including the kiang and onager of Asia and the wild ass of Africa. The third group contains Przewalski's wild horse, which is now found only in captivity. The only extant true wild horse, it produces fertile offspring when crossed with the domestic horse. Other so-called wild horses in various parts of the world, such as the mustang in North America, are descendants of domestic horses that have reverted to a wild state.

The evolution of the horse can be traced through fossil remains to the Hyracotherium, originally identified as the Eohippus: a small, leaf-browsing mammal of the Eocene Epoch, 55 million to 38 million years ago. Hyracotherium, about the size of a fox, had four toes on its forefeet and three on its hind feet. Several species and related genera appeared in North America and Eurasia during the Eocene. Then, apparently, the Eurasian species died out, but the American species gave rise in the Oligocene Epoch, 38 million to 24 million years ago, to the genus Mesohippus.

During the Pleistocene Epoch, which began 1.6 million years ago, the genus Equus apparently spread from North America to Eurasia, Africa, and South America. Subsequently, the native American horses died out, possibly as a result of disease. Cave dwellings in Europe indicate that horses were plentiful on that continent during the early Stone Age (about 2 million years ago to about 4000 BC). Dismembered skeletons of horses have been found in sufficient numbers in and near Stone Age dwellings to show that horses were frequently killed and eaten.

The most marked anatomical characteristic of the modern horse is the possession of a single toe on each of its four feet, which makes it a perissodactyl, or odd-toed ungulate, along with the rhinoceros and tapir. The horse's toe, which corresponds to the middle digit of the human hand, is much enlarged and protected by a horny hoof that surrounds the front and sides of the toe. Vestigial splints corresponding to the second and fourth toes are situated on either side of the foot above the hoof.

The skull of the horse is long; the facial bones are twice the length of the cranium. The mandible, or lower jaw, is long and has a broad, flat plate at its lower hind end. The spine is composed of 7 cervical, 18 dorsal, 6 lumbar, 5 sacral, and 15 caudal vertebrae. Horses have 44 teeth: 3 incisors, 1 canine, 4 premolars, and 3 molars on each side of each jaw. The incisors, which are used for cropping grass and other herbage, grow in the form of a semicircle. A pronounced gap exists between the canine teeth and the premolars; the metal bit used for controlling a horse is placed in this gap when the animal is ridden or driven. All the teeth have long crowns and comparatively short roots. The horse has a simple stomach, and fermentation of fibrous food takes place in a blind pocket, or cecum, analogous to the appendix in humans. It is located at the juncture of the small and large intestines. In a large horse the cecum may have a capacity of about 38 liters (about 10 gallons).

Both male and female horses are sexually mature by the age of two. They are seldom used for breeding purposes, however, before they are three years old. The gestation period is about 11 months, and single births are the rule. Twins are a genuine rarity, and only a few births of three or more foals have ever been recorded.