When reproduction occurs seasonally, males are expected to concentrate their reproductive effort when the abundance of females in oestrus is highest. In colonial species, large body size and prior residency correlate with the ability to establish dominance and ultimately to achieve matings. However, in addition to dominance, attendance in proximity to females may also enhance male mating success. If females come into oestrus highly asynchronously, males are unable to attend females for the entire breeding season. We expect the attendance of the most competitive males to increase with the availability of receptive females while less competitive males may primarily attend colonies when more competitive males are rare. In Galapagos sea lions, Zalophus wollebaeki, large males defend semiaquatic territories, but are unable to hold these over the entire reproductive season. They are sighted most often during the peak reproductive season. In this study, we found that nonterritorial males frequented the colony less during this time. However, the percentage of days intermediate-sized and resident males were sighted increased towards the peak of the reproductive season. Reproductive success was positively correlated with male attendance and therefore highest among territorial males. However, the correlation between attendance and reproductive success also existed for nonterritorial males. This suggests that male Galapagos sea lions adjust their attendance to the competitive situation in the colony. By increasing attendance during the peak of female oestrus, territorial males increase their chance of reproducing successfully. By adopting size-dependent strategies, nonterritorial males are able to achieve fitness while still growing.

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