The donkey or ass, Equus africanus asinus, is a domesticated member of the Equidae or horse family. The wild ancestor of the donkey is the African Wild Ass, E. africanus. In the western United States, a small donkey is sometimes called a burro (from the Spanish word for the animal). A male donkey or ass is called a jack, a female a jenny, and an offspring less than one year old a foal (male: colt, female: filly). While different species of the Equidae family can interbreed, offspring are almost always sterile. Nonetheless, horse/donkey hybrids are popular for their durability and vigor. A mule is the offspring of a jack (male donkey) and a mare (female horse). The much rarer successful mating of a male horse and a female donkey produces a hinny.