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HIBERNATION / AESTIVATION - Editorial Comment

Editorial Comment

(Editorial Overview Text Replicated on Overall Species page - Ursus maritimus - Polar bear)
  • Pregnant female polar bears den and hibernate for a period of four to eight months, from mid-October or early November to between February (in the south) to as late as May in the north. During this time they do not eat, drink, urinate or defecate, but they give birth to and suckle their cubs.
  • Other polar bears may spend seven weeks to nearly three months in a den in autumn, possibly to avoid heat-stress during the ice-free period, and/or in winter to avoid inclement weather or to reduce energy expenditure when food availability is reduced.
  • Polar bears appear to be able to enter a hibernation-like physiological state any time of the year if food is unavailable.

(References are available in detailed literature reports below)

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Hibernation / Aestivation

Source Information

  • Pregnant females den from mid-October or early November to March or April. (B406.37.w37)
  • In southern areas, break out occurs from February, while in the north, some wont come out until May. (V.w100)
  • Only pregnant females den; other polar bears remain active through the winter except perhaps in particularly bad weather. (J332.71.w1)
  • Denning by polar bears is primarily a reproductive strategy; pregnant females enter a den for the whole winter, but other polar bears remain active, hunting seals on the sea-ice. (B490.27.w27)
  • All polar bears may remain in snow shelters for 0.5 to 4.0 months and fast for prolonged periods, in a manner similar to dormancy, if food is unavailable. (J332.81.w1)
  • In Manitoba, pregnant females appear to begin hibernation in an earth den but burrow into snow drifts during the winter. (J332.71.w1)
  • In most areas, pregnant females den once drifts are large enough that they can construct a den, so that the timing of den entry depends on annual snow and ice conditions. They leave the dens in spring, once the cubs are able to survive the climate outside the den. (B490.27.w27)
  • Pregnant female polar bears hibernate for up to eight months and during this time nurse their cubs to reach about 10-12 kg by the time they emerge from the den. (J343.52.w3)
  • Cubs emerge from the den and follow their mother from late March/early April. (B285.w4)

Metabolism

  • Hyperphagia and accumulation of body reserves in the period when seals are most available is essential for accumulation of energy reserves by female polar bears prior to seclusion in maternity dens. (J46.226.w1)
  • During hibernation, metabolism averages 73% of predicted basal metabolic rate and may fall as low as 40% of basal rate. (J345.10.w2)
  • During hibernation, polar bears do not eat, drink, urinate or defecate. Fluid levels are maintained using water produced from catabolism of fat. Lean body mass is maintained, as are mineral levels and blood electrolyte balance. Pregnant females may den for four to eight months. (B490.27.w27)
  • During hibernation, 27 pregnant females lost an average of 127 kg, representing 43.5% of their pre-hibernation weight and an average loss of 0.74 +/- 0.06 kg/day. Weight loss in individual bears varied from 50 - 225 kg. (J46.234.w1)
  • During hibernation, females may reduce their metabolic rate to as low as 50% normal, although a 30% reduction is more usual. Body temperatures of three females placed into a simulated maternity den at temperatures similar to natural temperatures were recorded as 35.4-37.2 C. The standard metabolic rates of the bears was calculated as 2186-2740 kcal/day, 65-82% of predicted SMR for the bears' weights. For one female, from an initial weight of 285 kg, she lost 98 kg over 109 days, while birthing two cubs and raising them to about 10 kg each. It has been calculated that an average size female would need to have fat deposits of 20-30% of body weight for successful denning. At breakout, cubs have a lower critical temperature of about 0 C; at lower temperatures, down to -30 C, physical activity was used to maintain body temperature. In the maternal den, if the temperature dropped to -20 C, the female would have an increased energy cost if the cubs were to grow and develop; temperatures in a covered maternity den were measured remaining within two degrees of freezing while external temperatures varies from -15 to -35 C. Once the female is in the den, if a snow layer freezes and reduces oxygen exchange, the female must dig past the ice layer to maintain a suitable oxygen concentration in the den. In the earthen dens in the Hudson Bay area, maternity dens without ventilation holes have been observed for periods up to two months; the combined earth and snow chambers of dens in this area may assist in ventilation. (P17.57.w1)

Facultative metabolic shift

  • It appears that polar bears are able to alter their metabolism to a hibernation-like pattern if food-deprived at any time of year. (B490.27.w27)
    • Experimentally, polar bears were held and fasted for about 38 days, then fed for three day. Serum urea and creatinine were measured. In the first experiment, the serum urea: creatinine ratio rose from 11.0 (SE 2.6) in the fasting polar bears to 32.0 (SE 3.2) on the first day after feeding and dropped back to 22.8 (SE 3.2) by three days post-feeding. In the second experiment, the pre-feeding urea:creainine ratio was mean 15.8 (SE 2.3), rising to 61.2 (SE 10.6) after feeding for three days and dropping back to mean 29.2 (SE 5.1) seven days post-feeding. A low serum urea to creatinine ratio is also seen in hibernating Ursus americanus - American black bears. The findings suggested that polar bears can rapidly move from a fasting serum urea:creatinine ratio to a higher ratio if fed, and can rapidly return to a fasting Urea:creatinine ratio once food is withheld, suggesting that polar bears may demonstrate urea conservation similar to that seen in hibernating black bears and that they may be able to change between a feeding and a fasting metabolism, based on whether or not food is available, throughout the year. (J457.6.w1)

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Authors & Referees

Authors

Dr Debra Bourne MA VetMB PhD MRCVS (V.w5)

Referee

Andrew Derocher (V.w100)

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