Stable carbon and nitrogen isotope variations in canine dentine growth layers of Kerguelen southern elephant seals

Abstract

Foraging behaviour of mammals, namely the change in distribution and trophic lev- els from juvenile stage to adulthood, can be investigated by measuring δ 13 C and δ 15 N stable iso- topes for layers deposited in a growing tooth. For the first time, we describe geographic differ- ences in the ontogeny of foraging strategies and in the niche partitioning process according to sex and age of a highly sexually dimorphic species: the southern elephant seal Mirounga leonina. Canines from 8 males and 6 females were analysed for δ 13 C and δ 15 N stable isotope signatures. To assess intra-individual variability, instead of analysing collagen we analysed the bulk dentine within each of the 4 growth layers deposited annually. The δ 13 C signature revealed that, in indi- viduals of 1 to 4 yr of age, teeth of both males and females exhibited large intra-individual varia- tion in δ 13 C, suggesting that juveniles were foraging over a broad range of marine habitats encom- passing both sub-Antarctic and Antarctic waters. Four out of the 6 teeth taken from females were collected on individuals younger than 4 yr, preventing investigation of longer-term changes. A δ 13 C pattern emerged for males older than 4 yr: individuals became resident to either a sub- Antarctic (-17‰) or an Antarctic (-20‰, both values reported as deviations from the Vienna PeeDee Belemnite standard) foraging habitat, with a decrease in intra-individual variability. Up to the age of 4 yr, juvenile males were at a slightly higher trophic level than juvenile females, but by the age of 4 yr, while their δ 13 C signature revealed that they were faithful to their foraging habitat, males exhibited a significant increase in their trophic levels, as shown by their δ 15 N signature.