Testing the niche variation hypothesis in pinnipeds

Abstract

Individual trophic specialisation (ITS) has many important consequences for ecological and evolutionary processes. An old hypothesis highlighting the relevance of ITS is the niche variation hypothesis (NVH), which proposes that populations composed of trophically specialised individuals have a wider population niche than populations composed of more generalist individuals. Pinnipeds are a good model to test the NVH because they are mostly generalist species, living in dense colonies, and exhibiting variation in trophic niche width among populations. We tested the NVH in pinnipeds using longitudinal isotopic data: published δ13C and δ15N data obtained from sequential sections of whiskers (vibrissae) from individuals belonging to 14 populations. To the best of our knowledge, this is the first study evaluating the NVH by using longitudinal isotopic data. We compiled studies that published raw data on δ13C and δ15N for pinniped whisker segments to estimate the within-individual and the between-individuals components of variance, the total niche width (TNW), and ITS. One-dimensional (i.e. δ13C or δ15N) and multi-dimensional (i.e. δ13C and δ15N) analyses were used. Also, we tested whether an evolutionary component would affect the degree of ITS among pinniped species with different levels of shared ancestry. Our results indicate that, in line with the NVH, pinniped populations composed of more specialised individuals tend to show wider trophic niches. When analysing each sex separately, the hypothesis is supported for females but not for males. We believe that physiological and behavioural differences between sexes may explain this result. In females, high TNW is mainly related to greater differentiation among individuals, while in males, it is mainly related to high diversity of resources consumed by all individuals.