In all species, excess males gather in bachelor groups. These are typically young males that are not yet ready to establish a harem or territory. With the plains zebra, the males in a bachelor group have strong bonds and have a linear dominance hierarchy. Bachelor groups tend to be at the periphery of herds and when the herd moves, the bachelors trail behind. Mountain zebra bachelor groups may also include young females that have recently left their natal group, as well as old males they have lost their harems. A territorial Grévy's zebra stallion may tolerate non-territorial bachelors who wander in their territory, however when a mare in oestrous is present the territorial stallion keeps other stallions at bay. Bachelors prepare for their adult roles with play fights and greeting/challenge rituals, which make up most of their activities.