The effects of sex, tissue type, and dietary components on stable isotope discrimination factors (Δ<sup>13</sup>C and Δ<sup>15</sup>N) in mammalian omnivores

<div><p>We tested the effects of sex, tissue, and diet on stable isotope discrimination factors (Δ<sup>13</sup>C and Δ<sup>15</sup>N) for six tissues from rats fed four diets with varied C and N sources, but comparable protein quality and quantity. The Δ<sup>13</sup>C and Δ<sup>15</sup>N values ranged from 1.7–4.1 ‰ and 0.4–4.3 ‰, respectively. Females had higher Δ<sup>15</sup>N values than males because males grew larger, whereas Δ<sup>13</sup>C values did not differ between sexes. Differences in Δ<sup>13</sup>C values among tissue types increased with increasing variability in dietary carbon sources. The Δ<sup>15</sup>N values increased with increasing dietary δ<sup>15</sup>N values for all tissues except liver and serum, which have fast stable isotope turnover times, and differences in Δ<sup>15</sup>N values among tissue types decreased with increasing dietary animal protein. Our results demonstrate that variability in dietary sources can affect Δ<sup>13</sup>C values, protein source affects Δ<sup>15</sup>N 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 Δ<sup>15</sup>N values.</p></div>