| Summary: | Brown bears give birth and nurture their young for the first 3–5 months while fasting in hibernation. During this period, bears use body reserves to support the energy and protein costs of reproduction. Limited reserves may restrict reproductive investment. We developed a model to assess the energetic costs of reproduction during hibernation and to determine how maternal condition, length of lactation, litter size, and length of hibernation affect brown bear reproductive success. Increasing litter size by one cub was more costly than increasing the lactation period by 14 days. For hibernating females, ∼73% of their mass loss during lactation was lean mass. Minimum levels of fat reserves necessary to support reproduction varied from 19% to 33% depending on number of cubs and length of lactation while denned. Reproduction was not possible if body fat content was below 19% and length of hibernation was over 120 days. While the importance of fat or energy reserves at the beginning of hibernation has been long recognized, the importance of consuming high-protein foods after hibernation may be equally important. Therefore, more attention should be given to understanding pre- and post-hibernation nutritional factors affecting reproduction and the trade-offs between available food resources, maternal condition, and reproductive investment and success in wild bear populations. Ursus arctos; Hibernation; Reproductive costs; Reproductive trade-offs; Energetics; Fat reserves;
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