1. In a recent paper, Caut, Angulo & Courchamp (2008, Functional Ecology, 22, 255) experimentally measured isotope discrimination factors for rats Rattus rattus. In their study, values for their discrimination factors spanned a much larger range than previously reported in the literature and were found to be negatively related to the stable isotope composition of the diet that the rats were fed.
2. In a subsequent meta-analysis, Caut, Angulo & Courchamp (2009, Journal of Applied Ecology, 46, 443) confirmed the trends they had found in their previous study and pointed to a method for obtaining adequate values for discrimination factors when they could not be measured experimentally.
3.Synthesis and applications. We argue that the discrimination factors determined by Caut et al. (2008) were an artefact of experimental design. We also argue that the reported linear relationships between the stable isotope composition of the diet and isotope discrimination factors in their follow-up meta-analyses (Caut et al. 2009) do not reflect relevant trends that can be extrapolated to the field and that the method they proposed for obtaining adequate values for discrimination factors should be used with considerable care.