| Summary: | Parental characteristics and environment can affect survival and future reproductive success of offspring. In pinnipeds, mothers provide maternal care by producing milk and protecting their young from threats on land, e.g., attacks from other females. Contest behaviour and dominance structure of females have been studied little in pinnipeds. I studied these subjects in breeding northern elephant seals (Mirounga angustirostris). The purpose of my research was to investigate inter-female contest behaviour in relation to social and physical environments, female social organization, and maternal investment, with emphasis on sexual-size dimorphism in pups. I analysed data collected by the Elephant Seal Research Group during the 2006-07 and 2007-08 breeding seasons at Islas San Benito, Mexico, near the southern limit of the species’ breeding range; almost all past research has been conducted in California. Through behavioural observations, I investigated relationships of aggressive behaviour to female morphological characteristics and their social and physical environments. Aggressive behaviour increased with female body size, presence of offspring, and density of females. I also provide the first description of dominance relationships and hierarchical structure in female pinnipeds using measures of orderliness and steepness, which describe the transitivity of dominance hierarchies and the strength of the difference between individuals’ probability to dominate others, respectively. I found weak to moderate dominance hierarchies and intermediate values of steepness. This new information contributes to our understanding of contest behaviour and the social environment, which may affect female reproductive success. Finally, my study broadens general knowledge of breeding biology of northern elephant seals throughout the breeding range. I provide the first description of body size at birth and weaning at this colony of the species. My finding of similar body mass between the sexes at weaning contrasts with the great ...
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