Group breeding in sea lions: pups survive better in colonies

Abstract Otariids are sexually dimorphic, polygynous and highly gregarious marine mammals. Females give birth and mate in large colonies during the annual breeding season. The effect of female grouping on reproductive success was studied by comparing female southern sea lions, Otaria byronia , raising pups in rookeries of hundreds of individuals, and in solitary mating pairs of one male and one female, at a distance from traditional breeding places. Only 1 of 143 pups born to gregarious females at Punta Norte, Peninsula Valdes, Argentina, died before the end of the breeding season, as opposed to 60% (34) of 57 pups born from solitary mating parirs. The main causes of pup death were infanticide by conspecific young males, and starvation, following mother-pup separation and failure to reunite. Pups in colonies were more protected from harassment by subordinate males and were readily found by mothers returning from feeding trips Females benefit from group breeding through increased survival of their pups.

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