Diet: tissue stable isotope fractionation of carbon and nitrogen in blood plasma and whole blood of male reindeer Rangifer tarandus

Estimates of diet derived from stable isotope analyses are sensitive to the accuracy of corrections made for diet-tissue fractionation. In particular, diet-tissue fractionation in reindeer Rangifer tarandus may be expected to differ significantly from the generic values often used in stable isotope dietary calculations, given the known values obtained from other ungulates and the complex digestive system and nutrient recycling characteristic of the species. We fed domestic reindeer a homogenous artificial diet of known isotopic value in order to directly determine diet-tissue isotopic fractionation of carbon and nitrogen, the main elements used in stable isotope dietary analyses. Diet-tissue fractionation values for blood plasma were +3.5 +/- A 0.1aEuro degrees (delta(13)C) and +4.2 +/- A 0.3aEuro degrees (delta(15)N). Diet-tissue fractionation values for whole blood were +3.7 +/- A 0.2aEuro degrees (delta(13)C) and +2.5 +/- A 0.3aEuro degrees (delta(15)N). Metabolic turnover rates were clearly sufficient for complete tissue replacement over the period of artificial feeding for blood plasma, but may not have been so for whole blood, especially for delta(15)N. Our values, except for whole blood delta(15)N, differ considerably from the generic values often used in dietary studies and interspecific comparisons of trophic niche. The results underline the importance of obtaining as specific as possible fractionation values for the species, tissue, and in some cases sex and physiological status of animals under examination, and the potential problems associated with assuming 'generic' fractionation values when comparing species, especially where digestive processes are dissimilar.