Although there are variations, most females reach maximum size by the seventh year, and males reach adult size by the twelfth year. Although only 20 percent longer, the average adult male weighs over twice as much as the average adult female. They mate with females which give birth, then come into estrus in their territory. Females give birth to a single pup anytime from mid-May through July. They breed shortly after giving birth, but the fertilized egg does not implant in the uterus and begin growing until October.
Some females first breed in their third year, producing young in their fourth year.
By their sixth year, nearly all females are breeding and producing pups. Males are physiologically able to breed at 3 to 6 years, but they probably do not participate until after 8 to 10 years because of strong territorial competition among the largest males. At birth the sex ratio seems to favor males slightly. Twins occur rarely. In the 1970s, the worldwide population of Steller's sea lions was about 281,800, with the Alaska population estimated at 242,000, including pups.