Individual foraging specialization and sexual niche segregation in South American fur seals

Abstract

Individual variation in habitat and resource use has been reported for many top predators. This variation becomes important when comparing individuals taking into account sex, size, or age classes, since it can influence population dynamics and stability. We evaluated the individual variation and sexual/geographical isotopic niche overlap of the South American fur seal (SAFS) from the western South Atlantic. Whiskers of adult individuals from Brazil (n = 19), Uruguay (n = 29), and Argentina (n = 5) collected between 2005 and 2016 were serially sampled, resulting in 1001 samples, and their carbon and nitrogen isotopic ratios were analyzed longitudinally. According to its length, time integrated by whiskers ranged between 1.4 and 5.6 years. Males had δ13C (− 14.5 ± 0.6‰) and δ15N (18.9 ± 1.2‰) values significantly higher than females (δ13C = − 15.2 ± 0.5‰, δ15N = 17.8 ± 1.2‰). Females from Uruguay and Brazil were isotopically similar, displaying a large isotopic niche overlap (65.2–84%). Contrary, moderate isotopic niche overlaps were observed between males from Uruguay and Brazil (40.1–48.4%), and Uruguay and Patagonia (22.3–27.8%), indicating the use of different prey and/or feeding grounds. The WIC/TNW index of individual specialization pointed a significant specialization in males (0.38 for δ15N and 0.39 for δ13C). Females, on the other hand, are more generalists compared to males (0.53 and 0.71, for δ15N and δ13C, respectively). Differences in the ecological opportunity between sexes can account for these variations. Our study points out that trophic generalist populations of SAFS are composed of specialist and generalist individuals.