In some cases, accretionary tissues (e.g., Hobson and Sease 1998, Niño-Torres et al. 2006, Newsome et al. 2007b, 2009a), continuously growing but metabolically inert tissue (e.g., Schell et al. 1989, Lewis et al. 2006, Newsome et al. 2009b), or a suite of tissues assumed to have different isotopic incorporation rates (e.g., Sinisalo et al. 2008) have been analyzed to construct a longitudinal

record of dietary or trophic level variation. Mother-to-offspring transfer of nutrients during pregnancy and nursing has been the focus of several recent isotopic studies (Jenkins et al. 2001, Polischuck et al. 2001, Newsome et al. 2006, Stegall et al. 2008, York et al. 2008). Isotopic methods are particularly useful in evaluating mother-to-offspring nutrient transfer because lactating mothers catabolize their tissues to produce check details 3-MA concentration milk; nursing offspring are consuming their mother’s tissues and thus are feeding a trophic level higher than their mothers. For carbon isotopes, this prediction is complicated by the fact that milk can have a high concentration of 13C-depleted lipid. An animal that produces milk with a high-lipid content, such as an otariid with milk that is 15–50 weight% lipid (Costa 2002), feeds

its young a food source with a relatively low δ13C value. There are no pronounced differences in δ15N value between lipids and associated proteins, so the consumption of lipid-rich milk would not affect 15N-enrichment. Thus, nursing offspring should have δ15N values 3–5‰ higher and δ13C values either lower or similar to their mothers, depending on milk lipid content. Isotopic studies of nursing and recently weaned marine mammals have used samples from ontogenetic series of bones and/or annuli in dentin from sectioned teeth. For pinnipeds, analysis of dental annuli in Steller sea lions and California sea lions (Zalophus californianus) shows that nursing young have higher δ15N values (2‰–3‰) and lower δ13C values (1‰–2‰) than adult females

上海皓元医药股份有限公司 (Hobson and Sease 1998, Newsome et al. 2006, York et al. 2008). York et al. (2008) used isotopic and growth line data from canines to argue that weaning age increased and growth rate decreased in Steller sea lions from the 1960s to the 1980s, perhaps due to a reduction in available resources. Ontogenetic series of modern northern fur seal bones from the Pribilof Islands (southeastern Bering Sea) show that preweaned and recently weaned pups (aged 2–6 mo) have δ15N values that are approximately 5‰ higher than juveniles aged 12–20 mo (Newsome et al. 2006). Furthermore, adult female δ15N values are 2‰–3‰ lower than young pups (aged 2–6 mo), but significantly higher than those of juveniles. The δ13C values of the ontogenetic series show no trend with age.