CHANGES IN THE BODY-COMPOSITION OF FASTING POLAR BEARS (URSUS-MARITIMUS) - THE EFFECT OF RELATIVE FATNESS ON PROTEIN CONSERVATION

Bears in the genus Ursus, in particular polar bears, are thought to be highly efficient at avoiding a net loss of body protein during their prolonged seasonal fasts. Most data supporting this idea, however, ori ginate from studies in which captive bears apparently maintained a con stant lean body mass (LBM) while fasting. Using deuterium oxide diluti on, we quantified changes in the body composition of free-ranging pola r bears during an annual period of fasting. For a sample of 10 adult a nd subadult male polar bears, body condition at the start of the study ranged from 0.12 to 0.58 kg of fat/kg of LBM. Subsequently, over an i nterval of 79 d, losses of LBM and fat varied considerably among indiv iduals In contrast to previous studies on fasting bears, catabolism of protein appeared to meet a significant proportion of maintenance ener gy demands in some individuals. When converted into energetic units, b etween 74% and 99% of the observed losses in body energy content were attributable to the catabolism of body fat. Furthermore, bears that de rived the highest proportion of their energy needs from lipid stores w ere those that were relatively fat when first captured. We suggest tha t in polar bears, and ursids in general, the previously noted ability to minimize protein loss during extended fasts is in part dependent on the extraordinary magnitude of fat stores that some individuals can a ccumulate in seasons when food is abundant.