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.
After giving birth, they become aggressive toward each other and usually to pups other than their own, but some adoptions have been noted. The period from initial landing to the birth of the young conceived during the previous breeding season is 6-7 days. Toward the end of the lactation period, females enter estrus for 3-5 days. Most mating occurs in the third week of February. Gestation lasts for 11 months and includes a delayed implantation period of about 4 months.