During the breeding season, which is usually once (but sometimes twice) a year, males form 'leks'. Leks are small breeding areas where males gather and attempt to gain and defend a small patch of ground. Sexually receptive females leave their herd and travel to the lek. They stay for a day or two, choosing several males with which to mate. Gestation lasts about 8 months, after which a single calf is born. The calf can show one of two behaviours - a 'hider' or 'follower'. 'Followers' immediately join the herd for protection, whilst 'hiders' remain hidden in underbrush, being nursed by the mother, for several months. Weaning is complete at about 1 years old, when males calves are expelled from their mother's territory by the territorial male. Sexual maturity is reached at 16-18 months in females (full size is reached at around 2 years old), whilst males mature at about 3, although they are unlikely to breed before 4 years of age.