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Several years of census at two colonies in the Gulf of California (Los Cantiles, situated in the northern portion and Los Islotes in the south) were used to define the reproductive period of the California sea lion (Zalophus californianus californianus) in this area. The pattern of births (the length of the breeding period and the mean date of birth) is described using two models: a direct model, based on cumulative counts, and an indirect model, which related the photoperiod and the implanting of the blastocyte. The results of both models show that births begin earlier at Los Cantiles and that synchronization among females was less pronounced at Los Islotes. The mean arrival time of females was similar at both colonies, but differences were observed among the males. This suggests the existence of distinct competitive tactics that may be related to geographic position and the size of the colony. When results are compared with those from San Nicolas (California), it is clear that at Los Cantiles, the reproductive period is more prolonged, begins earlier, and that the time between giving birth and copulation for Gulf sea lions (intervals >30 days) is greater than that estimated for California (21 days).

Palabras clave: california sea lion; Zalophus californianus; Breeding Seaton; mating system

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