Most bovids are polygynous, and in some of these species males exhibit delayed maturation. For example, male blue gnus do not reach sexual maturity until 4 years of age, while females become reproductively active between 1.5 to 2.5 years of age. Sexual dimorphism is more prevalent in medium to large bovid species, particularly in members of the subfamily Reduncinae. In general, males of sexually dimorphic artiodactyls become sexually active later in life than females, which is probably due to male-male competition for mates. In some species, males may fight for and defend territory, which gives them breeding rights to females residing within each territory. It is not uncommon for territorial males to try and prevent resident females from leaving (e.g., impalas). Alternatively, males of other species fight for and defend small groups of females known as harems. Adult males that successfully defend their harem often breed with each member of the group, therefore increasing there reproductive fitness. Some bovid species also form leks, a small collection of males that compete for territory or mating rights. Successful males win occupation rights to high quality habitats and thus are able to mate with a greater number of high quality females. Once an individual gains territorial rights, individuals guard their territory and the females within. For example, waterbuck males defend areas of less than 0.5 km2, puka maintain areas of less than 0.1 km2, and lechwe and Uganda kob guard areas of about 15 to 30 m^2. Some species live in large groups consisting of both males and females in which males compete for mating opportunities (e.g., water buffalo). This behavior is somewhat common among members of the subfamily Hippotraginae.