THE EFFECTS OF PROLONGED FASTING OF THE BODY-COMPOSITION AND REPRODUCTIVE SUCCESS OF FEMALE POLAR BEARS (URSUS-MARITIMUS)

1. In total, pregnant Polar Bears that enter maternity dens in late su mmer/autumn may fast for up to 8 months in addition to meeting the nut ritional demands of gestation and lactation. We quantified the nutriti onal costs of this prolonged 'reproductive fast' and examined the effe ct of variation in maternal body condition on reproductive success. 2. Prior to entering dens, pregnant females were obese, containing as mu ch as 1 kg of fat/kg of lean body mass (LBM). Among bears, LBM increas ed with body fat mass. This accumulation of LBM may be necessary in or der to transport the large fat stores required for fasting, and may al so provide a pool of protein essential for reproduction. 3. While fast ing, bears lost 43% of body mass. Of the total energy expended on main tenance and reproduction, 93% was drawn from fat stores. This dependen cy upon fat conforms to the pattern of nutrient metabolism seen among other species adapted to prolonged fasting. Maternal metabolic rate wa s less than the predicted resting metabolic rate which illustrates the effectiveness of denning as an energy-conserving strategy. 4. Body fa t was critically important for reproductive success. In particular, of fspring body weight was very strongly related to the size of maternal fat stores before denning. Fatter bears produced heavier cubs which wo uld be more likely to survive. 5. Among bears, pre-denning body condit ion was positively associated with age. Within the observed range (4-2 1 years), age-specific reproductive success should thus be highest amo ng older bears. Such an effect could arise if: (1) the body condition of individual bears tends to improve with age and experience or (2) an imals of poorer quality and condition die at a younger age.