Introductionhorse, hoofed, herbivorous mammal now represented by a single extant genus, Equus. The term horse commonly refers only to the domestic Equus caballus and to the wild Przewalski's horse. (Other so-called wild horses are feral domestic horses or their descendants.) Adapted to plains environments, all Equus species, including the ass and the zebra, have lengthened foot bones ending in a single toe covered by a hoof, for fast running; teeth shaped for grinding grass; and intestinal protozoa for digesting cellulose. All species have tufts of hair on the tail, used against insects, and manes on the neck. Horses, zebras, and asses can interbreed, but the offspring are usually sterile. The offspring of a horse and a donkey (domestic ass) is called a mule.
A male horse is called a stallion, or if castrated, a gelding; a female is a mare; her offspring are foals—males are colts, females are fillies. A male parent is a sire, a female parent is a dam. A single foal is born after a gestation of about 11 months. Horses reach sexual maturity in about two years, but are not fully grown for about five years. The average life span is 18 years, but 30-year-old horses are common. The standard unit of height is a hand, equal to 4 in. (10 cm).
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