Pregnant females give birth soon after arriving on a rookery, and copulation generally occurs one to two weeks after giving birth, but the fertilized egg does not become implanted in the uterus until the fall. A fertilized egg may remain in embryonic diapause for up to three months before implanting and beginning to divide. Twins are rare. After a week or so of nursing without leaving the rookery, females begin to take progressively longer and more frequent foraging trips leaving their pups behind until at some point in late summer, when both the mother and pup leave the rookery together. This maternal attendance pattern is common in otariids. As pups get older the amount of time spent by females foraging out at sea increases.This continues until pups obtain the ideal body weight and energy reserves to eat on their own. A study conducted by the University of California, Santa Cruz found that on average male pups consume more milk than females. This may be due to the sexual dimorphism common to otariids. Reproductive males fast throughout the reproductive season, often without entering the water once from mid-May until August, when the structure of the reproductive rookeries begins to fall apart and most animals leave for the open seas and disperse throughout their range.