MAMMALIAN COLD ACCLIMATION 



winter. Consequently they must rely for protection on nest building, 

 huddling, and other devices as shown by Sealander (1952), Pearson 

 (1960), and Barnett (1956). Microclimatic observations by Pruitt 

 (1957) in Alaska have shown that the environment of the boreal red 

 back voles (Glethrionomys rutilus) 3-9 inches below the moss sur- 

 face ranges for the most part between -5 and +2 C when the air 

 tempeature falls to -40 C and below. Nevertheless, temperatures 

 in this range are well below the thermoneutral range of most small 

 mammals as shown by Hart (19 53), Kalabukhov (1940), Morrison and 

 Ryser (1951), Morrison, Ryser, and Dawe (1959), Pearson (1960), 

 Smirnov (1958), and various other workers, suggesting that meta- 

 bolic compensation is necessary to maintain homeothermy under 

 these conditions. Evidently the protection afforded to Peromyscus 

 maniculatus during the winter in the Ottawa area is likewise insuf- 

 ficient to prevent cold exposure since metabolic acclimatization to 

 winter conditions was pronounced (Hart and Heroux, 1953). Unpub- 

 lished observations of Hart and Heroux have likewise shown season- 

 al metabolic acclimatization in wild dump rats, and similar obser- 

 vations have been made on short tailed shrews. Details of the sea- 

 sonal metabolic changes in rats reviewed by Hart (196 2b) are beyond 

 the scope of this review. 



Semi- aquatic Fur Bearers 



Certain fur bearing mammals such as beaver, otter, and musk- 

 rats are dependent on an aquatic environment for their food and 

 shelter, butspendonly a small portion of their total life in the water. 

 These mammals are protected from the cooling effect of the water 

 by a layer of air trapped in the fur. In general, very little is known 

 concerning temperature regulation in this group of mammals. How- 

 ever, observations of the author (Hart, 1962a) onmuskrats (Ondatra 

 zibethicus ) in air and in water illustrate some of the problems in 

 the temperature regulation of a semi- aquatic mammal. 



Muskrats tested in air at various temperatures for about 1 hour 

 showed an increased heatproductionattemperatures below a critical 

 level of approximately 10 C and a gradual lowering of body tem- 

 perature which became pronounced below -40 C (Figure 7). No 

 appreciable seasonal changes were observed. 



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