The domestic donkey is descended from the African wild ass and has been used by people for thousands of years as a pack, draft, riding, and guard animal. Standard donkeys vary widely in size, but mature jacks often weigh about 400–600 pounds, while jennies usually weigh about 350–500 pounds. Common colors include gray-dun, brown, black, sorrel, spotted, and white, often with a dark dorsal stripe and shoulder cross. Donkeys are hardy, intelligent, cautious, and well adapted to dry environments. They were introduced to the Americas by Spanish explorers and settlers in the 1500s and became widely established in the United States through ranching, mining, farming, and transportation.