Isotopic incorporation and the effects of fasting and dietary lipid content on isotopic discrimination in large, carnivorous mammals
A. Stricker, K.D. Rode, J. Erlenbach, C.T. Robbins, A. Cutting, S. Jensen, S. D. Newsome, S. G. Cherry, G. Stenhouse, M. Brooks, A. Hash, and N. Nicassio
Stable isotope analysis has become a ubiquitous method to assess wildlife diets. However, the utility of this method relies on understanding the relationship between isotopes in animal tissues and diets, and the temporal window that tissues represent. There has been considerable emphasis on understanding isotopic discrimination in omnivores, but discrimination may differ for carnivores, particularly those consuming lipid-rich diets. Here, we examined the effects of several factors important to applying stable isotopes for quantifying high-lipid diets of bears, including dietary lipid content and fasting behavior. We conducted feeding trials with captive grizzly bears (Ursus arctos) and polar bears (U. maritimus). We observed increasing ∆13C and decreasing ∆15N in plasma with increasing dietary lipid content up to 92% lipid for non-lipid extracted diets. We found that plasma δ15N values changed by 1.6-1.8‰ but δ13C exhibited no trend during fasts for four adult and four yearling grizzly bears. Our estimates for discrimination in blood, hair, and adipose tissue were most similar to other non-marine carnivores, such as mink (Neovison vison) and Arctic fox (Vulpes lagopus), fed fish-based diets, but differed from estimates for omnivores and some of the previous values applied to polar bears. Incorporation in red blood cells and whole blood was ≥ 6 months in larger subadult and adult bears which is considerably longer than previously measured in younger, smaller bears. Our results support the importance of considering the effects of variability in dietary lipid content and body size when estimating diets of carnivores.