Sn. Atkinson et Ma. Ramsay, THE EFFECTS OF PROLONGED FASTING OF THE BODY-COMPOSITION AND REPRODUCTIVE SUCCESS OF FEMALE POLAR BEARS (URSUS-MARITIMUS), Functional ecology, 9(4), 1995, pp. 559-567
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.