In newly established colonies, with relatively few animals, a dominant male may be able to defend a discrete group of females, but this pattern breaks down as more crowded conditions develop. Generally, a single male cannot control more than about 40-50 females. If a colony has a larger number of females, there will be several bulls present. The latter compete for access to the females and eventually form a dominance hierarchy. A successful male in this colony mates with about 100 females during the season.
Males establish dominance through stares, gestures, vocalizations, and physical combat.
If a fight develops, the opponents confront one another chest to chest and rock back and forth seeking an opportunity to deliver a downward blow with the canine teeth. Serious wounds occur frequently on the chest and proboscis but rarely lead to death. The roaring vocalization is rhythmic, resonant, and metallic and carries for nearly a kilometer. Distinctions have been detected between the sounds produced by the males of different islands. Females have a loud and prolonged threat vocalization.