334 



U. S. NATIONAL MUSEUM BULLETIN 217 



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Figure 17. — Mean body temperatures of arctic and subarctic birds and mammals. (From 

 L. Irving and J. Krog, Journ. Appl. Physiol., vol. 9, fig. 6, p. 677.) 



birds more variable than during the day (L. Irving, 1955) . At night 

 the birds were often awake and active when we approached their cages. 

 On several occasions, however, the body^ temperatures of some of them 

 were 3° lower than the resting level by day, and I concluded that in cold 

 nights, probably during sleep, northern birds may cool below their 

 daytime body temperature by about as much as Dawson (1954) re- 

 ported for birds in temperate regions. This nocturnal relaxation 

 of either minimum temperature level or regulation among northern 

 birds, however, is not enough to have much significance for the con- 

 servation of bodily heat in the Arctic. 



It is true that during hibernation at reduced body temperature 

 mammals save on the expenditure of metabolic heat (Hock, 1951). 

 At Barrow hibernating ground squirrels {Gitellus undulatus) were 

 often near 0° C. (Erikson, 1956). At this low temperature their 

 heat production is so meager that it cannot protect them from freezing 

 in air only a few degrees colder. Accordingly their winter survival 

 depends upon obtaining such effective insulation from a nest and bur- 



