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, originate from studies in which captive bears apparently maintained a constant lean body mass...

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Bibliographic Details
Published in:Physiological zoology 1996-03, Vol.69 (2), p.304-316
Main Authors: Atkinson, Stephen N., Nelson, Ralph A., Ramsay, Malcolm A.
Format: Article
Language:English
Subjects:
Bears
Black bears
Blood
Body composition
Body fat
Body protein
Catabolism
Fasting
Grizzly bears
Marine
Polar bears
Ursus maritimus
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Summary: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, originate from studies in which captive bears apparently maintained a constant lean body mass (LBM) while fasting Using deuterium oxide dilution, we quantified changes in the body composition of free-ranging polar bears during an annual period of fasting. For a sample of 10 adult and 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 interval of 79 d, losses of LBM and fat varied considerably among individuals. In contrast to previous studies on fasting bears, catabolism of protein appeared to meet a significant proportion of maintenance energy demands in some individuals. When converted into energetic units, between 74% and 99% of the observed losses in body energy content were attributable to the catabolism of body fat. Furthermore, bears that derived the highest proportion of their energy needs from lipid stores were those that were relatively fat when first captured. We suggest that 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 accumulate in seasons when food is abundant.
ISSN:0031-935X
1937-4267
DOI:10.1086/physzool.69.2.30164186