Data from: Determinants of individual foraging specialisation in large marine vertebrates, the Antarctic and Subantarctic fur seals

1. The degree of individual specialisation in resource use differs widely among wild populations where individuals range from fully generalised to highly specialised. This inter-individual variation has profound implications in many ecological and evolutionary processes. A recent review proposed four main ecological causes of individual specialisation: inter- and intra-specific competition, ecological opportunity and predation. 2. Using the isotopic signature of sub-sampled whiskers, we investigated to what degree three of these factors (inter- and intra-specific competition and ecological opportunity) affect the population niche width and the level of individual foraging specialisation in two fur seal species, the Antarctic and subantarctic fur seals (Arctocephalus gazella and A. tropicalis), over several years. 3. Population niche width was greater when the two seal species bred in allopatry (low inter-specific competition) than in sympatry or when seals bred in high density stabilized colonies (high intra-specific competition). In agreement with the Niche Variation Hypothesis (NVH), higher population niche width was associated with higher inter-individual niche variation. However, in contrast with the NVH, all Antarctic females increased their niche width during the inter-breeding period when they had potentially access to a wider diversity of foraging grounds and associated prey (high ecological opportunities), suggesting they all dispersed to a similar productive area. 4. The degree of individual specialisation varied among populations and within the annual cycle. Highest levels of inter-individual variation were found in a context of lower inter- or higher intra-specific competition. Contrasted results were found concerning the effect of ecological opportunity. Depending on seal species, females exhibited either a greater or lower degree of individual specialisation during the inter-breeding period, reflecting species-specific biological constraints during that period. 5. These results suggest a significant impact of ecological interactions on the population niche width and degree of individual specialisation. Such variation at the individual level may be an important factor in the species plasticity with significant consequences on how it may respond to environmental variability.