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 summer/autumn may fast for up to 8 months in addition to meeting the nutritional demands of gestation and lactation. We quantified the nutritional costs of this prolonged 'reproductive fast' and examined the effect of variation in maternal body condition on reproductive success. 2. Prior to entering dens, pregnant females were obese, containing as much as 1 kg of fat/kg of lean body mass (LBM). Among bears, LBM increased with body fat mass. This accumulation of LBM may be necessary in order to transport the large fat stores required for fasting, and may also provide a pool of protein essential for reproduction. 3. While fasting, bears lost 43% of body mass. Of the total energy expended on maintenance and reproduction, 93% was drawn from fat stores. This dependency upon fat conforms to the pattern of nutrient metabolism seen among other species adapted to prolonged fasting. Maternal metabolic rate was less than the predicted resting metabolic rate which illustrates the effectiveness of denning as an energy-conserving strategy. 4. Body fat was critically important for reproductive success. In particular, offspring body weight was very strongly related to the size of maternal fat stores before denning. Fatter bears produced heavier cubs which would be more likely to survive. 5. Among bears, pre-denning body condition was positively associated with age. Within the observed range (4-21 years), age-specific reproductive success should thus be highest among older bears. Such an effect could arise if: (1) the body condition of individual bears tends to improve with age and experience or (2) animals of poorer quality and condition die at a younger age.