Horse

I

Introduction

Horse, common name for the domestic horse and three groups of wild mammal 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 horse, now extinct in the wild (although it is gradually being reintroduced). The only surviving true wild horse, Przewalski's horse cross-breeds with the domestic horse and produces fertile progeny. Other so-called wild horses in various parts of the world are descendants of domestic horses that have reverted to the wild state.

II

Prehistoric Horses

The evolution of the horse can be traced through fossil remains to Hyracotherium, a small, leaf-browsing mammal of the Eocene epoch. 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, to the genus Mesohippus.

In the Miocene epoch, Mesohippus was succeeded by Hypohippus and Anchitherium, both of which are thought to have colonized Eurasia from North America. Other descendants of Mesohippus were Miohippus and Merychippus; the latter genus developed high-crowned teeth, permitting it to feed by grazing on grass rather than by browsing on leaves. Among the descendants of Merychippus in the Pliocene epoch were Hipparion, which apparently spread from North America to Eurasia, and Pliohippus, which appears to be the progenitor (ancestor) of the modern genus Equus.

During the Pleistocene epoch the genus Equus apparently spread from North America to Eurasia, Africa, and South America. Subsequently, the native American horses died out. Cave dwellings in Europe indicate that horses were plentiful on that continent during the early Stone Age. Dismembered skeletons of horses have been found in and near such dwellings in sufficient numbers to show that horses were frequently killed and eaten. In Neolithic times, when Europe was largely forested, the number of horses evidently declined. Remains of the Bronze Age include bits and other pieces of harness, and they clearly demonstrate that horses had become domestic animals in this period.

III

Modern Horses

The most marked anatomical characteristic of the modern horse is the possession of only 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 is 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 each 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. Male horses have 3 incisors, 1 canine, 4 premolars, and 3 molars on each side of each jaw. Females do not have canines. The incisors, which are used for cropping grass and other herbage, grow in the form of a semi-circle. 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 caecum, analogous to the appendix in humans. It is located at the juncture of the small and large intestines.

Both male and female horses (stallions and mares) 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.

IV

Gaits

Horses are capable of a wide range of gaits; a few of them are artificially developed by trainers. For convenience they may be divided into symmetrical gaits, in which equal time intervals separate the touching of the ground by the left and right front legs (and also by the left and right hind legs); and asymmetrical gaits, in which the time intervals are not equal. Familiar symmetrical gaits, in order of increasing speed, include the walk, pace, and trot. A horse walks by swinging its legs in the order of left front, right rear, right front, and left rear. It paces by swinging the left and then the right legs nearly in unison, and trots by swinging the diagonally opposite legs nearly in unison. Artificial symmetrical gaits include the slow gait, or stepping pace, and the faster rack, or single-foot, both of which are done with legs lifted high and no more than two legs on the ground at the same time. Asymmetrical gaits include the canter and the faster gallop, in which the left and then the right hind leg touches the ground, followed by the left and then the right front leg; the sequence may be reversed right to left.