AJP - Regulatory, Integrative and Comparative Physiology, Vol 236, Issue 1 67-R74, Copyright © 1979 by American Physiological Society
Modes of thermal protection in polar bear cubs--at birth and on emergence from the den
A. S. Blix and J. W. Lentfer
At birth in late December the polar bear is small (700 g), uninsulated, and
helpless. It probably has a modest capacity for metabolic heat production
and depends on the female and a snow den in which it is born for thermal
protection. The microclimate of an artificial polar bear den was
investigated at Point Barrow, AK, and the temperature therein found to stay
around 0 degrees C provided a heat source (200 W) equivalent to an adult
polar bear was introduced. When the bears desert the den in early April the
cub has grown to about 10 kg and has a well-developed fur insulation, but
almost no subcutaneous fat. The cub has a high resting metabolic rate (4.6
W.kg-1), which is supported by the fat polar bear milk. Its lower critical
temperature is about -30 degrees C, and an ambient temperature of -45
degrees C results in only a 33% increase in metabolism. The cub can
tolerate a wind chill of 2.3 kW.m2 without apparent stress of drop in
rectal temperature. If the cub is immersed in ice water rectal temperature
drops 11 degrees C in 30 min. It is concluded that the cub can tolerate
extremely low temperatures in air due to fur insulation and high metabolic
heat production, but is unable to cope with the chill of ice water for any
prolonged period of time.
