Assimilation and discrimination of hydrogen isotopes in a terrestrial mammal.

Stable isotope analysis has revolutionized the way ecologists study animal resource use from the individual to the community level. Recent interest has emerged in using hydrogen isotopes (H/H) as ecological tracers, because they integrate information from both abiotic and biotic processes. A better physiological understanding of how animals assimilate hydrogen and use it to synthesize tissues is needed to further refine this tool and broaden its use in animal ecology. We conducted a controlled-feeding experiment using laboratory mice (Mus musculus) in which we varied the hydrogen isotope (δH) values of water and the proportions of dietary protein and carbohydrates among nine experimental treatments. For each tissue, we calculated the percent of hydrogen derived from water and the percent hydrogen derived from dietary protein versus carbohydrates using linear relationships and isotope mixing models based on accompanying carbon isotope (δC) data. The net discrimination (∆H) between mice tissues and potential water and dietary sources of hydrogen differed among tissues. ∆H was positively correlated with dietary protein content in red blood cells (RBC) and muscle, but negatively correlated in liver and plasma. We also report the first estimates for hydrogen isotope discrimination factors (∆H) for different sources of hydrogen (∆H, ∆H, and ∆H) available for tissue synthesis. This research provides a foundation for understanding how diet quality (e.g., protein content) influences hydrogen isotope assimilation and discrimination in different tissues of a terrestrial mammal, which is a first step towards using δH as a tracer of resource use in free-ranging mammals.