Tide line versus internal pools: mating system and breeding success of South American sea lion males

The spatio-temporal distribution of breeding males can help understand the existence of alternative reproductive tactics and their breeding success in colonially breeding species. We quantified the breeding success of South American sea lion (SASL) males in Uruguay according to their breeding location (tide line vs. internal pool) in a rookery and assessed the dynamics of territory use and the fine-spatial kin structure during the breeding season. We sampled 73 % of reproductive males and determined paternity for 80 % of the pups born the following year. We found that male breeding success differed only between breeding locations, being higher at the tide line. We found that close kin males were only present at the tide line, suggesting a fine scale kin structure in the study area. Because of the shorter distances between territorial males at the tide line and the high energetic cost of agonistic interactions, kinship may be a mechanism for decreasing agonistic interaction at the tide line. Even though SASL has a polygynous mating system across its geographic range, each rookery has a single or homogeneous mating system. This is the first study showing the coexistence of two distinct mating systems in an otariid rookery related to breeding location. While males of tide line have a female-defence polygyny into floating territories, males at the internal pools defended fixed territories establishing a resource-defence polygyny. We have shown the first evidence in otariids for the coexistence of two types of polygyny in a single breeding rookery.