The role of social structure and kin associations in the mating system of the polygynous New Zealand sea lion (Phocarctos hookeri)

Mating structure describes the number of mates individuals of each sex are able to acquire, as well as the variance in reproductive success between individuals of the same sex. Such structure has important evolutionary implications for populations. The social structure of a population can have large influences on the mating structure, by determining who interacts with whom in the population and consequently each individual’s reproductive success. Kin clustering is a type of social structure that allows the interaction of related individuals in such a way as to increase both direct and indirect fitness, but can also have negative consequences on the population such as the negative effects of inbreeding. Accordingly, ecologists are becoming increasingly interested in how the molecular composition of populations, that is the distribution of genetically related individuals, influences a mating system. The New Zealand (NZ) sea lion (Phocarctos hookeri) is an otariid (eared seal) that displays a polygynous mating system. Females are highly gregarious and aggregate into dense clusters in the breeding colony, allowing males access to many females at once and faciliating the polygynous breeding system. This highly structured breeding system makes NZ sea lions an interesting species for studying social interactions and kin clustering within breeding systems. The overall aim of this research was to assess the role of social structure, in particular association between kin, in NZ sea lions, in order to increase our understanding of the fine-scale structure of polygynous otariid mating systems. Specifically, this research aimed to determine whether interactions between male relatives influence male reproductive success, as well as the role of female mate choice in determining the genetic relatedness of mating pairs, an important factor for subsequent offspring fitness. Genotypes at 17 pinniped microsatellites were used to assess genetic relatedness of individuals within two populations, the Sandy Bay breeding colony on Enderby Island in NZ’s subantarctic islands, and the recently founded Otago Peninsula population on NZ’s mainland. Social network analysis was used to determine social interactions between males in the Sandy Bay breeding colony and look for a correlation between genetic relatedness of associates and harem tenure (a proxy for reproductive success). Estimates of genetic relatedness were also used to assess how female mate choice influences the genetic relatedness of mating pairs. This information was used to compare the mating system between the two populations. Genetic relatedness of male associates did not appear to influence length of harem tenure. However, males that attained high harem tenure displayed increased relatedness compared to males of low harem tenure, suggesting increased reproductive success of certain genetic lineages. Comparison between the two populations (Enderby Island and Otago Peninsula) suggested a difference in the relative importance of male competition and female mate choice. Females in the Otago Peninsula population were observed to choose genetically unrelated mates, while females in the Enderby Island population were mating with males that were more related to themselves than expected by chance. This difference in female mate choice may explain how the colonising population on the Otago Peninsula is maintaining genetic diversity despite its small size. The present study provides an increased understanding of the fine-scale structure of an otariid breeding colony, including a deeper understanding of the social interactions between kin, and the mechanisms of sexual selection that influence the breeding system of NZ sea lions