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

† Contribution to Special Section ‘Stable Isotopes in Mammals’.

Abstract

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.

Keywords:

• animals
• carbon-13
• diet
• food
• isotope ecology
• natural isotope fractionation
• nitrogen-15
• protein turnover
• rats
• δ¹³C
• δ¹⁵N

Acknowledgements

The authors extend their thanks to D. Casper, M. Hite, and J. Waldron for help with rat care; Unisea Corporation in Dutch Harbor, Alaska for donating fish meal; D. Andreasen, A. Bowman, J. Curl, S. Kim, and C. Marcotte for laboratory help; and J. Estes, P. Raimondi, and two anonymous reviewers who provided important comments on the manuscript.

Funding

Support was provided to C.M.K. by funds from the Environmental Protection Agency, CDELSI, Myers Trust, USFWS/National Fish and Wildlife Foundation, Sigma Delta Epsilon, the STEPS Institute for Innovation in Environmental Research, and UCSD. The experiments reported here were approved by the UCSC Chancellor's Animal Research Committee.

Notes

† Contribution to Special Section ‘Stable Isotopes in Mammals’.