The donkey has been used as a working animal for at least 5000 years. Of the more than 40 million donkeys in the world, about 96% are in underdeveloped countries, where they are used principally as pack animals or for draught work in transport or agriculture. After human labour, the donkey is the cheapest form of agricultural power. They may also be ridden, or used for threshing, raising water, milling and other work. Working donkeys are often associated with those living at or below subsistence levels. Some cultures that prohibit women from working with oxen in agriculture do not extend this taboo to donkeys, allowing them to be used by both sexes.