Ontogenetic changes in feeding habits of northern elephant seals revealed by δ15N and δ13C analysis of growth layers in teeth

Stable isotope analysis is useful for examining the feeding strategies of mammals. Isotopes in the annual deposition growth layers of dentine in teeth permit assessment of ontoge- netic dietary shifts in individuals, because this metabolically inert tissue is not resorbed after deposition. Profiles of stable isotope ratios of nitrogen (δ 15 N) and carbon (δ 13 C) from the teeth of northern elephant seals Mirounga angustirostris, aged between 1 and 11 yr, were described from stranded individuals in San Benito and Magdalena Islands, Mexico, between 2000 and 2009. δ 13 C and δ 15 N values differed with age in each sex indicating shifts in feeding habits throughout life, as well as in their physiological condition. Although steady δ 13 C values within each individual sug- gest fidelity to feeding grounds, differences in δ 13 C values between males and females were likely a reflection of sexual segregation on the feeding areas, which begins during the juvenile stage. Mean δ 15 N values of males (18.3‰) and females (18.2‰) would suggest that both groups feed at a similar trophic level; however a combination of differences in benthic versus pelagic and longi- tudinal foraging habitats would be masking real sex differences in trophic level, estimated for the species at 4.6. δ 15 N and δ 13 C values varied markedly between individuals of the same sex, sug- gesting the potential existence of diverse feeding strategies leading to a resource partitioning in this species. The alleged fetal growth layer has a δ 15 N value 1.8‰ higher than the average of the adult female stage, suggesting that the fetal layer is more likely the product of the intensive nurs- ing during the first month of life.