During this time, the males establish a harem and mate with many females on their territories, demonstrating their polygamous nature. Mating occurs soon after the birth of the previous year's pups. The pups are fed on their mother's milk, and they enter the water at four to six weeks. Some pups will nurse from one to three years, but most are believed to wean before their first birthday. Females give birth to one pup only, and may not give birth every year.
Pups are able to crawl and swim soon after birth.
Females accept only their pups, recognizing their pup's vocal and olfactory cues. Pups will approach other females, but are often bitten or thrown by females who have their own pups. Males defend territories for an average of two years. There is a high incidence of aborted Steller sea lion fetuses in the wild. Steller sea lions are mammals, so they need to come to the surface to breathe air.