Diurnal and nocturnal, though generally more active during the daytime. Zebras are gregarious, and are usually found in groups of up to six females accompanied by their offspring and a mature male who is the dominant member of the group. Young males leave their family group at between one and three years of age and join bachelor herds. These herds number from two to ten males (sometimes more), consisting entirely of young or older males, or a mixture of all age groups. Seniority in these bachelor herds is age-based. The younger males often chase one another and play-fight, testing themselves against age-mates, though they do not begin to try and form harems of their own until they are five years old. Members of a family herd often groom one another, facing in opposite directions so that each can reach the other's head, neck and shoulders. Individuals often rest in this position, enabling them to keep watch for predators. These activities help to reinforce bonds among family members. The largest herds are a feature of the dry season when the zebras migrate long distances in their tens of thousands in search of good grazing and water, though the individual families still remain intact.