These groups in turn are composed of units consisting of a female and her young of the previous two birth seasons. A herd also contains several bachelor groups of up to 12 males each and a separate group of juvenile animals. Some males, especially very old ones, form small units that live apart from the rest of the herd. Dominance hierarchies exist within the various social units of a herd and are most evident in the bachelor groups. Although several mature males may be present in a mixed group, they compete for the estrous females.
Males usually are dominant to females but have only a limited leadership role.
Actual leadership of an entire herd seems to be shared by a number of ranking individuals of both sexes that alternate at the forefront during movements. Vocalizations resemble those of domestic cattle but tend to be lower in frequency. In the deep forests of West and Central Africa large herds are not evident, and groups seem to consist usually of 8-20 closely related individuals. Reproduction occurs throughout the year in some areas, but there are seasonal peaks associated with rainfall. In the Serengeti, the heavy rains extend from about February to July.