New model for estimating trophic position in mammalian carnivores based on bone collagen individual amino acids nitrogen stable isotopes

Trophic position, or the level occupied by an organism in the food chain, is a key characteristic for understanding its role in the recent or past ecosystem. Nitrogen stable isotopes in selected amino acids of bone collagen, in particular in the glutamate and phenylalanine, allow for quantification of the trophic position in fossils and thus, for reconstruction of the ecological history of species and landscapes. Applying this method for carnivorous mammals was limited until now due to the lack of a key parameter, the trophic discrimination factor (TDF) for collagen, representing an isotopic shift in amino acids from prey bone collagen to carnivore bone collagen. In this study, we determined this parameter in two carnivorans: the modern wolf and the Pleistocene cave hyena. We measured the glutamate and phenylalanine δ¹⁵N values in bone collagen of these carnivores and their prey, applying five models to estimate the mean diet. We proposed a new model of calculating the TDF for collagen-based data, with herbivores regarded as a baseline level. We found collagen–to–collagen glutamate-phenylalanine TDF in both studied carnivorans and in all models of the mean diet similar to each other, +4.8 ‰ on average. This value is lower than that determined previously in invertebrate proteins or in plant–to–vertebrate tissue systems. Our results allow for more accurate reconstruction of trophic position in carnivores, including paleontological and archaeological materials where bone collagen is the only available tissue. We applied our new herbivore model and collagen-based TDF to re-asses the trophic structure of an exemplary Late Pleistocene assemblage, and found that the Upper Paleolithic humans at this site occupied a higher trophic position than previously determined.