Segregation of diets by sex and individual in South American fur seals

Abstract

Trophic niche width and individual specialization among marine predators are often subjected to seasonal constraints. Differences are expected to arise for sexually dimorphic species exposed to distinct ecological opportunities, as well as intrinsic differences in physiological abilities or energetic requirements. We assess seasonal and sexual differences in isotopic niche overlap and width and analyze inter-individual foraging variation throughout the year of the South American fur seal (Arctocephalus australis). We obtained chronologically ordered δ13C and δ15N data from vibrissae of 25 males and 24 females from southern Brazil and Uruguay. There were significant differences between sexes for both isotopes with males showing higher values (δ13C = − 14.5 ± 0.5‰; δ15N = 18.9 ± 1.2‰) than females (δ13C = − 15.2 ± 0.5‰; δ15N = 17.8 ± 1.2‰), but not among seasons or years. A very small isotopic niche overlap found between sexes (1.1–9.5%), with limited seasonal variation, likely resulted from differences in prey consumption and foraging habitats. Compared to other seasons, females had a wider isotopic niche in spring. While males showed seasonal stability in the foraging niche, females showed some small variation, which is probably influenced by central place foraging. Both sexes had considerable interindividual variation in estimated dietary composition and had different proportions of potential prey contribution at the population level. These findings enlighten the knowledge of South American fur seal trophic ecology, and how they may seasonally and spatially shift their foraging strategies according to their distinct life histories. It ultimately reflects in a small niche partitioning and possibly low intraspecific competition at the species northernmost area of occurrence in the western South Atlantic.