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 \permil 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.