The effects of sex, tissue type, and dietary components on stable isotope discrimination factors (Δ13C and Δ15N) in mammalian omnivores

We tested the effects of sex, tissue, and diet on stable isotope discrimination factors (Δ13C and Δ15N) for six tissues from rats fed four diets with varied C and N sources, but comparable protein quality and quantity. The Δ13C and Δ15N values ranged from 1.7–4.1 ‰ and 0.4–4.3 ‰, respectively. Females had higher Δ15N values than males because males grew larger, whereas Δ13C values did not differ between sexes. Differences in Δ13C values among tissue types increased with increasing variability in dietary carbon sources. The Δ15N values increased with increasing dietary δ15N values for all tissues except liver and serum, which have fast stable isotope turnover times, and differences in Δ15N values among tissue types decreased with increasing dietary animal protein. Our results demonstrate that variability in dietary sources can affect Δ13C values, protein source affects Δ15N values even when protein quality and quantity are controlled, and the isotope turnover rate of a tissue can influence the degree to which diet affects Δ15N values.