Hydrogen and oxygen stable isotopes are important tracers in migratory, food web, and paleoecological applications. Unlike carbon and nitrogen stable isotopes, for which only dietary sources contribute to tissue isotope values, hydrogen and oxygen can be derived from diet, free water in food, drinking water, and (for O) atmospheric O2. As paired measurements of H and O isotopes are becoming more common, there are mounting examples of weak relationships between tissue and environmental water isotope composition in one or both isotopes, which complicates predictions of migratory origin and paleoecological interpretations. We leveraged several published studies to develop a mass balance model accounting for input and output fluxes in order to predict organismal δ2H and δ18O body water values and the isotopic composition of keratin tissue. The model permits the evaluation of ecological and physiological conditions on tissue isotope ratios, including characteristics such as body mass, water flux, relative humidity, and dietary composition.
Results/Conclusions
We explored sensitivity in several model parameters to determine the relative effects of differing organismal and environmental characteristics on animal tissue H and O isotope ratios. Relative humidity has a much greater effect than temperature on keratin isotope composition, and may contribute to substantial variability across a large geographic range. Herbivores that do not drink have higher δ2H and δ18O values as a result of evaporative water loss in plant leaf water that affects the isotopic composition of food water and dietary macromolecules. In addition, as the water content of the food decreases, so do the keratin δ2H and δ18O values. Finally, dietary composition can be an important driver of additional variation in δ2H and δ18O values, particularly if preferential routing of dietary proteins occurs during tissue synthesis. Overall, we demonstrate that predictions of H and O isotope ratios in a range of organisms can be improved by accounting for organismal ecophysiology. In addition, we expect this model to be useful understanding how observed H and O isotopic composition measured in wildlife may offer insight into the diet and physiology of individuals.