Comment: Use of stable isotopes to determine diets of living and extinct bears

Notes 2300 Isotopic biogeochemistry of ancient bon collagen is a powerful tool for determining the diet of extinct species. It has been increasingly widely used during the last few years (e.g., Bocherens et al. 1994; Matheus 1995; Grocke and Bocherens 1996). This seemingly simple approach is nonetheless useful only when basic requirements are fulfilled. It appears that Hilderbrand et al. (1996), in their study of extinct cave bear diet using collagen carbonand nitrogenstable isotopes, have left unanswered several key points, which precludes valid conclusions, first by misquoting previous work and second by publishing data that are not interpretable as they stand in the paper. The first point is the misquotation of the conclusions of Bocherens et al. (1994) concerning the cave bear diet. Indeed, Hilderbrand et al. (1996) leave the impression that the interpretation of isotopic-analysis results obtained by Bocherens et al. (1994) is incorrect and that the results presented in that paper, combined with their isotopic-analysis results, indicate an omnivorous diet with a significant meat component for extinct European cave bears. In Bocherens et al. (1994), one key result is that in bears, modern and fossil, isotopic values measured on dentine and bone are not at all equivalent from a palaeodietary point of view. In mammals with brachyodont teeth, dentine collagen reflects a milk diet and thus a higher trophic level than that of the adult. Thus, isotopic-analysis results from such dentine will indicate carnivory, even in species where adults are plant eaters. A crucial consequence is that only adult bone collagen can be used as reflecting adult diet. The discussion in Hilderbrand et al. (1996) mixes all values, from dentine and bone, without taking this point into account. Thus, concluding that the values published in Bocherens et al. (1994) indicate omnivory in cave bears is clearly incorrect. A second point is the fact that the approach used by Hilderbrand et al. (1996) presented several flaws in sampling and assessing the validity of the isotopic measurements, which makes the interpretation of these results highly problematical. From the “sample collection” section of that paper, it appears that Pleistocene samples come from numerous different sites whose age is not known and that are spread all over Europe. It is well known that isotopic signatures are strongly influenced by environmental factors, such as aridity, soil acidity, canopy effect, and season of growth (e.g., Heaton et al. 1986; Ambrose 1991; van der Merwe and Medina 1991; van Klinken et al. 1994; Rodiere et al. 1996). These effects lead to a very large range of isotopic values for a given species when individuals come from different places and different times (e.g., Fizet et al. 1995; Bocherens et al. 1997). An illustration of this high variability can be given by comparing the nitrogen-isotope compositions of large bovine species (Bos and Bison) in three western European upper Pleistocene sites in Belgium (Sclayn: δ15N = 4.7 ± 0.4‰; Bocherens et al. 1997), France (Marillac: δ15N = 6.7 ± 0.9‰; Fizet et al. 1995), and Italy (Paglicci: δ15N = 7.9 ± 1.4‰; Iacumin et al. 1997). It is thus clear that the data presented in Hilderbrand et al. (1996) cannot be interpreted in a palaeodietary perspective, because exact geographical and time origins are not provided in the paper. Also, the lack of individual age indication for the bear samples in Hilderbrand et al. (1996) makes it possible that high δ15N values are due to young age and suckling. Another problem with the isotopic-analysis data from Hilderbrand et al. (1996) is the total absence of information on ancient collagen integrity. It is well known that collagen is altered in ancient bones, and only when chemical purity is assessed can isotope-analysis results be considered with confidence to reflect biogenic signatures (DeNiro 1985; Ambrose 1990; Bocherens 1997). Classical ways of checking collagen purity are to use yield of extraction, %C, %N, and C/N ratio of extracted collagen, and in some cases amino acid analyses. None of these data are provided in Hilderbrand et al. (1996), which precludes any serious interpretation of the presented isotope-analysis results. Finally, so far as the isotopic-analysis data on cave bears presented in Hilderbrand et al. (1996) are concerned, valuable palaeodietary information might be obtained if (i) collagen indegeneity was assessed according to the chemical criteria mentioned above, (ii) time and geographical origins were provided for all samples, (iii) comparisons were made only among herbivores, carnivores, and bears from the same site and the same time period, and (iv) only isotopic-analysis values from adult bone collagen were considered.