Pitfalls in comparing modern hair and fossil bone collagen C and N isotopic data to reconstruct ancient diets: a case study with cave bears (Ursus spelaeus)

Hervé Bocherens
Aurora Grandal-d’Anglade
Keith A. Hobson

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Stable isotope analyses provide one of the few means to evaluate diet of extinct taxa. However, interpreting isotope data from bone collagen of extinct animals based on isotopic patterns in different tissues of modern animal proxies is precarious. For example, three corrections are needed before making comparisons of recent hair and ancient bone collagen: calibration of carbon-13 variations in atmospheric CO2, different isotopic discrimination between diet–hair keratin and diet–bone collagen, and time averaging of bone collagen versus short-term record in hair keratin. Recently, Robu et al. [Isotopic evidence for dietary flexibility among European Late Pleistocene cave bears (Ursus spelaeus). Can J Zool. 2013;91:227–234] published an article comparing extant carbon (δ13C) and nitrogen (δ15N) stable isotopic data of European cave bear bone collagen with those of Yellowstone Park grizzly bear hair in order to test the prevailing assumption of a largely vegetarian diet among cave bears. The authors concluded that cave bears were carnivores. This work is unfortunately unfounded as the authors failed to consider the necessary corrections listed above. When these corrections are applied to the Romanian cave bears, these individuals can be then interpreted without involving consumption of high trophic-level food, and environmental changes are probably the reason for the unusual isotopic composition of these cave bears in comparison with other European cave bears, rather than a change of diet. We caution researchers to pay careful attention to these factors when interpreting feeding ecology of extinct fauna using stable isotope techniques.