Bos (from Latin bōs: cow, ox, bull) is the genus of wild and domestic cattle. Bos is often divided into four subgenera: Bos, Bibos, Novibos, and Poephagus, but including these last three divisions within the genus Bos without including Bison in the genus is believed to be polyphyletic by many workers on the classification of the genus since the 1980s. The genus as traditionally defined has five extant species but this rises to eight when the domesticated varieties are counted as separate species, and 10 when the closely related genus Bison is also included. Most but not all modern breeds of domesticated cattle are believed to have originated from the extinct aurochs. Many ancient breeds are thought to have originated from other species. Zebus and taurine cattle are thought to descend from ancient Indian and Middle Eastern aurochs, respectively. Description The species are grazers, with large teeth to break up the plant material they ingest. They are ruminants, having a four-chambered stomach that allows them to break down plant material. Distribution There are about 1.3 billion domestic cattle alive today, making them one of the world's most numerous mammals. Members of this genus are currently found in Africa, Asia, Europe, parts of North America, South America and also in Oceania. Their habitats vary greatly depending on the particular species; they can be found in prairies, rain forests, wetlands, savannah and temperate forests. Ecology Most Bos species have a lifespan of 18–25 years in the wild, with up to 36 being recorded in captivity. They have a 9–11 month gestation, depending on the species and birth one or, rarely, two young in the spring. Most species travel in small herds ranging in size from ten to thirty members. Within most herds, there is one bull (male) for all the cows (female). Dominance is important in the herds; calves will usually inherit their mother's position in the hierarchy. They are generally diurnal, resting in the hot part of the day and being active morning and afternoon. In areas where humans have encroached on the territory of a herd, they may turn nocturnal. Some species are also migratory, moving with food and water availability. Taxonomy In 2003, the International Commission on Zoological Nomenclature resolved a long-standing dispute about the naming of those species (or pairs of species) of Bos that contain both wild and domesticated forms. The commission "conserved the usage of 17