Discrimination factors (Δ15N and Δ13C) in an omnivorous consumer:: effect of diet isotopic ratio

1. Naturally occurring stable isotopes in resources and their consumer allow the estimation of nutritional flows between the two and have been much used to improve our understanding of the nutritional ecology of free-living animals. 2. The difference in isotopic composition between an animal and its diet is represented by a discrimination factor. Carbon and nitrogen flows are estimated by calculating the discrimination factors in stable isotope ratios (delta N-15 and delta C-13), which are presumed to be c. 3 parts per thousand and 1 parts per thousand heavier in the consumer tissues than those in their resources, respectively. 3. The discrimination factor is known to vary according to species, tissue, age, growth rates and food quality, but the estimation of discrimination factors is difficult and a fixed discrimination factor is usually used in diet reconstruction. It has also been suggested that discrimination factors could vary linearly with the diet isotopic ratio. If this linear relationship could be demonstrated using regression, this would provide an adequate method for the estimation of discrimination factors. In order to understand how diet isotopic ratios affect the discrimination factor, we investigated the pattern of its change in nitrogen (Delta N-15) and carbon (Delta C-13) in different tissues (liver, muscle and hair) of an omnivore species, the rat Rattus rattus. We fed captive rats with diets of the same nutritional quality but on different isotopic ratios. 4. First, discrimination factors for Delta N-15 and Delta C-13 showed great variability, ranging from -1.46 parts per thousand to 4.59 parts per thousand and from -8.79 parts per thousand to 0.64 parts per thousand, respectively. Discrimination factors depended on both diet isotopic ratio and tissue. 5. We also show that isotope ratios in shaved hairs showed a turnover during the first month, and then stabilized during the second month. Using shaved hairs has the potential to be an effective non-lethal method for determining resource shifts in non-specialist consumers. 6. Finally, we demonstrated, for all tissues, a decrease of Delta N-15 and C-Delta 13 with an increased values of delta N-15 and delta C-13, respectively. These relationships allow us to propose a framework to estimate discrimination factors from diet isotopic ratios by means of regression models.