Stable isotope analyses of tooth annuli reveal temporal dietary records: An example using steller sea lions

Stable isotope analysis of teeth of marine mammals can provide valuable information on trophic level and source of feeding. However, the isotopic analysis of whole teeth presents only an average dietary estimate for individuals across the period of growth of that tooth. While such analyses can be valuable, particularly in the case of fossil material, in contrast, isotopic analysis of individual annuli of teeth can provide dietary information for each year of tooth growth, in some cases representing the whole of the animal's life. We measured stable-carbon isotope ratios (C-13/C-12) in the inorganic (hydroxyapatite) and stable-nitrogen isotope ratios (N-15/N-14) in the organic (primarily collagenous) components of individual tooth annuli of 18 male Steller sea lions (Eumetopias jubatus) obtained from archived collections from the Bering Sea and Gulf of Alaska and from single northern fur seals (Callorhinus ursinus) and northern elephant seals (Mirounga angustirostris) from the central Aleutian Islands and eastern Gulf of Alaska, respectively. In several individuals, we detected considerable variation in stable isotope values among annuli, up to 6.1 parts per thousand for delta(15)N and 5.1 parts per thousand for delta(13)C values. Enrichment in delta(15)N and depletion of delta(13)C values in the first annulus may correspond to dietary inputs from mother's milk during the period of suckling. Other variations among years may be caused by dietary changes or movements of individuals between regions differing in isotopic signatures of foodweb primary production. Our study indicates that the isotopic analysis of individual tooth annuli represents a fine-scale cool for dietary reconstructions involving marine mammals, and cautions against the use of whole-tooth material averaged over several annuli.