The breeding season for A. tropicalus begins in September when the males haul out on shore at the breeding grounds, or rookeries, to establish territories by displays, sparring, or actual battle. The females come ashore in October and November and choose male territories within which to bear their young. They later mate with the male possessing that territory. It is also at this time that immature animals begin avoiding the rookeries.
Pups are born beginning in late November through January. Most pups are born in early December. Cows give birth to a single pup and begin lactation almost immediately. Lactation is almost continuous for the next 8-12 days, after which the cow mates and begins returning to the sea. She will spend increasing amounts of time at sea over the next year, returning only to nurse her pup. Lactation continues throughout the pup's first year or until the cow's next pup is born. Copulation occurs between 8 and 12 days after the cow gives birth, but there is a delay of about 4.3 months in the implantation of the blastocyst (Riedman, 1990: 250). The pups are approximately 4.5 kilograms and 60 centimeters at birth, the males being slightly larger than the females. Females reach sexual maturity between 4 and 6 years of age while males reach sexual maturity earlier, between 3 and 4 years of age (Riedman, 1990: 237).
Average birth mass: 4420 g.
Average gestation period: 357 days.
Average number of offspring: 1.