While not designed to study such patterns in mothers, early work on human nursing did not detect an isotopic effect in lactating women (Fogel et al. 1997). In contrast, a study of wild horses showed that lactating females had lower δ15N values than other adults (males, nonlactating females) and used mass balance calculations to argue that this 15N-depletion is the expected result of the nitrogen balance perturbations associated with lactation (Koch 1997). Further support for this trend was reported in CSF-1R inhibitor Kurle (2002), where blood δ15N values of a single lactating northern fur seal were approximately 1‰ lower than those for nulliparous females. Fuller

et al. (2004) reported δ15N variations among pregnant human females. They found that δ15N values dropped from conception to birth, and that the magnitude of the drop correlated to the birth weight of the baby as well as the amount of weight gained by the mother. If these phenomena occur in marine mammals, they would reduce Δ15Ntissue-diet values for growing

or pregnant females. Expectations for lactating females are more complex and may Olaparib molecular weight depend on whether animals feed or fast while lactating (i.e., income vs. capital breeders). The δ18O value of a biomineral depends on the temperature at which it forms and the 18O value of the body fluid from which it precipitates (discussion below based on Clementz

and Koch 2001 and Koch 2007). For mammals there is a constant offset between the 18O value of body water and phosphate (∼+18‰), and between the phosphate and carbonate components of bioapatite (∼+8‰), close to values predicted for isotopic equilibrium at typical body temperatures. Physiology affects the 18O value of body water by altering the fluxes of oxygen into and out of the body, as well as fractionations associated with transport and/or transformation of oxygen-bearing compounds. Ingested water is a major flux of oxygen into marine mammals and includes preformed water in food, seawater consumed incidentally when eating, and water taken by active drinking (mariposia). The proportion of water gained from these sources varies widely among marine mammals (Ortiz 2001), yet as these processes 4-Aminobutyrate aminotransferase do not strongly fractionate oxygen, these fluxes should all have 18O values close to that of seawater (0‰ V-SMOW). Metabolic water generated by oxidation of food dry matter may contribute to marine mammal body water. This water may be 18O-enriched relative to ingested water, as atmospheric O2 is much heavier than ingested water (∼+21‰ V-SMOW). Finally, there is evidence in cetaceans for a substantial flux of water across the skin (Hui 1981, Andersen and Nielsen 1983); it is unlikely that this process greatly fractionates oxygen isotopes, though the issue has not been studied.