Isotopic incorporation and the effects of fasting and dietary lipid content on isotopic discrimination in large carnivorous mammals

There has been considerable emphasis on understanding isotopic discrimination for diet estimation in omnivores. However, discrimination may differ for carnivores, particularly species that consume lipid-rich diets. Here, we examined the potential implications of several factors when using stable isotopes to estimate the diets of bears, which can consume lipid-rich diets and, alternatively, fast for weeks to months. We conducted feeding trials with captive brown bears (Ursus arctos) and polar bears (Ursus maritimus). As dietary lipid content increased to ∼90%, we observed increasing differences between blood plasma and diets that had not been lipid extracted (∆¹³C_{tissue-bulk diet}) and slightly decreasing differences between plasma δ¹³C and lipid-extracted diet. Plasma Δ¹⁵N_{tissue-bulk diet} increased with increasing protein content for the four polar bears in this study and data for other mammals from previous studies that were fed purely carnivorous diets. Four adult and four yearling brown bears that fasted 120 d had plasma δ¹⁵N values that changed by