more active during the day and spend most of their time feeding. Other activities include dust bathing, rubbing, drinking and intermittent resting which is very brief. At night, zebra activity is subdued except when threatened by predators. They may rest or sleep laying down, while one individual keeps guard. Social structure The plains zebra is highly social and usually forms small family groups called harems, which consist of a single stallion, several mares and their recent offspring. The adult membership of a harem is highly stable, typically remaining together for months to years. Groups of all-male "bachelors" also exist. These are stable groups of up to 15 males with an age-based hierarchy, led by a young male. These males stay in their groups until they are ready to start a harem. The bachelors prepare for their adult roles with play fights and greeting/challenge rituals, which take up most of their activities. Multiple harems and bachelor groups come together to form larger herds of hundreds of animals, especially during migrations. Plains zebras are unusual among harem-holding species in forming these groups. In addition, pairs of harems may create temporarily stable subgroups within a herd, allowing individuals to interact with those outside their group. Among harem-holding species, this has only been observed in primates such as the gelada and the hamadryas baboon. Bachelor groups tend to be at the periphery of herds and when the herd moves, the bachelors trail behind. Stallions form and expand their harems by abducting young mares from their natal harems. When a mare reaches sexual maturity and has her first oestrous cycle, she attracts the attention of nearby stallions, both bachelors and harem leaders. Her family stallion (likely her father) chases off or fights stallions attempting to abduct her. Even after a young mare is isolated from her natal harem, the fight over her continues until her oestrous cycle is over and it starts again with the next estrous cycle. It is rare that the mare's original abductor keeps her for long. When the mare finally ovulates, the male that impregnates her keeps her for good. Thus, the mare becomes a permanent member of a new harem. Oestrus in a female becomes less noticeable to outside males as she gets older, hence competition for older females is virtually nonexistent. Mares exist in a hierarchy, with the alpha female being the first to mate with the harem stallion