After the males have arrived to the beach, the females arrive to give birth. Females fast for five weeks and nurse their single pup for four weeks; in the last few days of lactation, females come into estrus and mate. In this polygynous society, a high-ranking bull can have a harem of 30–100 cows, depending on his size and strength. Males unable to establish harems will wait on the periphery, and will try to mount nearby females. Dominant bulls will disrupt copulations of lower-ranking bulls. They can mount females without interference, but commonly break off to chase off rivals. While fights are not usually to the death, they are brutal and often with significant bloodshed and injury; however, in many cases of mismatched opponents, the younger, less capable males are simply chased away, often to upland dunes. In a lifetime, a successful bull could easily sire over 500 pups. Most copulations in a breeding colony are done by only a small number of males and the rest may never be able to mate with a female. Pups are sometimes crushed during battles between bulls.