To the Editor:
A recent paper Richards et al. (1) shows the highest δ15N values of fossil collagen in adult cave bears found until now. The reviewing of previous papers makes them propose, as the only viable explanation, an omnivorous feeding for these specimens. However, they erroneously quote the hypothesis shown in our paper (2) in which we propose a mechanism to explain the high δ15N values in cave bears due to urea recycling during hibernation (3). This error is of critical importance, because the authors rule out our hypothesis, understanding this mechanism explains low δ15N values due to cold weather conditions. On the contrary, long hibernation periods—longer in cold climates—would result in higher δ15N values.
This hypothesis is supported by recent studies on present bears in which bone tissue remodeling during hibernation, in spite of disuse, has been proven (4, 5), as well as the keeping of muscle protein, with an approximately 1‰ rise in δ15N after several months of hibernation (6), even though the quantification of this effect in fossil remains has not been possible so far.
The difference between the values obtained in “Peçstera cu Oase” and the more frequent ones in the rest of sites (1) is practically equivalent to two trophic levels. An occasionally omnivorous feeding alone cannot account for this difference. Taking into account the overwhelming evidence that the special physiology of bears during hibernation plays a decisive role in bone tissue metabolism, we believe the values shown by Richards et al. (1) must be explained considering other hypotheses apart from omnivorous feeding.
Footnotes
The authors declare no conflict of interest.
References
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