Zebra species have two basic social structures. Plains and mountain zebras live in stable, closed family groups or harems consisting of one stallion, several mares, and their offspring. These groups have their own home ranges, which overlap, and they tend to be nomadic. Stallions form and expand their harems by recruiting young mares from their natal (birth) harems. The stability of the group remains even when the family stallion dies or is displaced. Plains zebra groups also live in a fission–fusion society. They gather into large herds and may create temporarily stable subgroups within a herd, allowing individuals to interact with those outside their group. Among harem-holding species, this behaviour has otherwise only been observed in primates such as the gelada and the hamadryas baboon.