hoof

hoof, hard, flexible covering that protects the toes of ungulate mammals. Because all hoofed animals essentially walk on the tips of their toes, hooves provide a strong, insensitive surface to bear the weight of their bodies. In animals such as the horse and antelope, hooves are an adaptation for fast running and lend the animal both speed and endurance. The sharp hooves of some animals are also used for defense.

Hooves have the same evolutionary origin as the nails and claws of other mammals, all of which are made of the fibrous protein keratin, occur in approximately the same place (at the tip of the leg, toe, or finger), and cover only the upper surface. Hooves grow from the living cells at the base, or attached end, and continue to grow throughout the animal’s life. Like nails and claws, their length is controlled by wear.

The hoof consists of two parts. The broad, hard, upper portion is called the unguis; it completely surrounds the end of the toe, extending down and forming a rim around the bottom of the hoof. A somewhat softer plate, called the subunguis, covers the bottom of the toe and is extensively developed in hoofed animals to form a tough pad. (In humans the subunguis is only a small ridge under the tip of the nail.) Horseshoes are nailed to the hard unguis to protect the horse’s hooves against breaking and splitting.

Mammals with hooves are called ungulates and form a diverse, non-taxonomic group. They usually are divided into two principal groups: those with an even number of toes and those with an odd number. The even-toed animals, known as artiodactyls, have an even number of toes on all their feet and include pigs, hippopotamuses, cattle, deer, camels, goats, antelopes, and sheep. Two-toed ungulates are often called cloven-hoofed because each toe is covered by a separate hoof, so that their hooves appear to be split, or cloven. Hippopotamuses have four toes on each foot, and the terminal phalanges have nail-like hooves. The toe bones of camels are completely enclosed in hardened, horny hooves, and the lateral toes are spread across a broad pad, which aids in walking on desert sands. Chevrotains have four hoofed toes on each foot.

The odd-toed ungulates—the perissodactyls—have an odd number of toes on each of their hind feet. Horses, asses, and zebras are known as equids and have a single solid hoof on each foot. They are the only mammals to walk on the tips of single digits, and their hooves are the most highly developed structure of this kind among mammals. Other perissodactyls, notably rhinoceroses, have three toes on each foot, each covered by a separate hoof. Tapirs have four hoofed toes in the forefoot and three in the hind.

A variety of diseases and conditions may afflict the hoof. By far the most familiar is hoof-and-mouth disease (also called foot-and-mouth disease and aftosa), a highly contagious and potentially fatal viral disease affecting practically all even-toed mammals, including domestic cattle, sheep, goats, pigs, and wild ungulates such as reindeer, bison, and giraffes. However, horses and possibly zebras are resistant to this infection. The disease is characterized by the formation of painful fluid-filled blisters on the tongue, lips, and other tissues of the mouth and on parts of the body where the skin is thin, such as the udder and teats and around the hoof.