Sea lions are now protected under the Marine Mammal Protection Act of 1972. Once a sea lion becomes sexually mature at around age 4 or 5 they are able to breed. Male sea lions, bulls, who live in Baja California and southern California establish breeding territories from May through July. The sea lions who congregate on the Galapagos Islands to mate extend their stay from May to January. The females, called cows, congregate in moderate to large groups which are densely spaced.
Bulls defend territories and fast during the breeding season.
Males will compete for dominance leaving the younger males without a chance to mate for the season. One distinct characteristic of a full grown breeding male is the presence of a sagittal crest, a raised forehead, that appears at about 10 years of age. The dominant male will corner about ten or more females to mate with at one time. Once the egg is fertilized inside the female, she can delay the implantation of the egg to stop the embryo from growing. The delayed implantation may last up to three months with an average gestation period lasting 9 months.