There are some basic features of sociopolitical organization that are shared by most of the Ainu groups, although their finer workings vary from region to region. Among most Ainu groups, the nuclear family is the basic social unit, although some extended families are present. In most Ainu settlements, males related through a common male ancestor comprise the core members who collectively own a hunting ground or a river with good fish runs. Although some scholars emphasize that among the Ainu along the Saru River in Hokkaidō women related through females comprise a corporate group, the exact nature of the group is unclear. Among these Hokkaidō Ainu, an individual is prohibited from marrying a cousin on his or her mother's side. Among most Ainu groups, a few prominent males in the community practice polygyny.