Blix A S, Lentfer J W
Am J Physiol. 1979 Jan;236(1):R67-74. doi: 10.1152/ajpregu.1979.236.1.R67.
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.
12月下旬出生时，北极熊体型很小（700克），没有毛发保暖，十分无助。它可能具有一定的代谢产热能力，依赖母熊以及出生时所在的雪洞来获得热保护。在阿拉斯加巴罗角对一个人工北极熊洞穴的微气候进行了研究，发现只要引入相当于一只成年北极熊的热源（200瓦），洞穴内温度就能保持在0摄氏度左右。4月初幼崽离开洞穴时，体重已增长到约10千克，毛发保暖良好，但几乎没有皮下脂肪。幼崽的静息代谢率很高（4.6瓦·千克-1），这靠高脂肪的北极熊乳汁来维持。它的较低临界温度约为-30摄氏度，环境温度为-45摄氏度时，代谢仅增加33%。幼崽能耐受2.3千瓦·平方米的风冷，直肠温度没有明显下降或应激反应。如果将幼崽浸入冰水中，直肠温度在30分钟内会下降11摄氏度。研究得出结论，由于毛发保暖和较高的代谢产热，幼崽能够耐受空气中极低的温度，但无法长时间应对冰水的寒冷。
