Sea Lion | San Diego Zoo Animals & Plants

Life in a harem: Despite their adaptations for aquatic living, sea lions are still bound to the land for reproduction. Usually the males, called bulls, leave the water first in the spring to stake out territorial claims on ice, rocks, or the shore. The bulls gear up for this breeding season by gorging themselves on extra food to create an especially thick layer of blubber, allowing the individual to live for weeks without eating as he guards his territory and females. During the breeding season, bulls bark loudly and continuously to establish or defend their territories. Bulls also stare, shake their head, or lunge toward any opponent.

The bulls are several times larger that the adult females, called cows. During the breeding season, each adult bull tries to gather up as many cows as he can to form his "harem." Sea lion harems, or family groups, can number up to 15 cows and their young. The bull watches over his harem, protecting it from harm. But don’t think the harem has the place all to itself—popular resting spots can support dozens of harems. A large group of sea lions gathered together on land or floating ice is called a colony. During the birthing season these areas are known as rookeries.

The exception to this breeding behavior is the Australian sea lion bull; he does not stake out a territory or form a harem. Instead, the bulls fight for any available female.

Never timid about speaking what’s on their minds, sea lions make all sorts of barks, honks, trumpets, and roars. A baby sea lion, called a pup, can pick out its mother from among hundreds gathered on rocky shores just by the sound she makes. Between the bulls roaring, the mothers barking, and the pups bleating, the rookery is a noisy place indeed!

Several days or weeks after the bulls have established their territories on beaches and rocks, the breeding females come ashore to join them. Each male tries to herd as many breeding females as he can into a harem. Those females that conceived the year before are the last ones to arrive, gathering on land to deliver their pup. Sea lion females can delay implantation, which means that if a cow becomes pregnant, the fertilized egg only develops to a certain point before it temporarily stops growing until the time is right to continue. One hypothesis is that delayed implantation allows sea lions to combine the breeding season with the birthing season.

Female sea lions usually give birth to a single pup per year. Pups are born with their eyes open and their tummies ready for their mother's rich milk. The milk is high in fat, and this helps the pup grow that important layer of blubber to keep warm. Pups are born with a long, dense coat of hair called the lanugo that helps keep them warm until they develop that blubber. Mothers are very attentive to their pup during the first two to four days of life, nuzzling them and picking them up by the scruff of the neck. The pups are capable of awkward swimming at birth, but can walk at just 30 minutes old!

At just a few weeks of age, sea lion pups are ready for their first swimming and fishing lessons. They also must learn how to stay away from sharks and how to survive storms at sea! Nursing continues up to six months of age. After weaning, pups may stay with their mother at sea for up to one year.