ARCTIC METABOLIC ECONOMY 341 



homoiothermism (H. Krog, M. Monson, and L. Irving, 1955). 

 Porcupines in winter maintained body temperature and related metab- 

 olism to cold below their critical temperature better than they did 

 in summer (L. Irving, H. Krog, and M. Monson, 1955). In general, 

 well adapted arctic mammals preserve their body temperature very 

 accurately over ranges of cold which are far beyond the tolerance of 

 tropical forms (L. Irving and J. Krog, 1954). As already men- 

 tioned, the small mammals find in their habits and behavior the sup- 

 plemental protection from the arctic climate which physiological 

 adaptation or adjustment cannot provide for them. 



Stability of Basal Metabolic Rate 



Although we have now no measurements of the temperature in the 

 burrows occupied by arctic tree squirrels, weasels, lemmings, mice, 

 voles, and shrews, these animals while resting probably are not exposed 

 to temperatures much lower than their critical temperature. If this 

 expectation is confirmed, they can rest without their metabolic ex- 

 penditure exceeding the basal level appropriate to animals of their 

 size. We have not found the basal metabolic rates of small or large 

 wild arctic mammals to differ from those of tropical or temperate 

 forms in any manner related to climate (Scholander, Hock, Walters, 

 and Irving, 1950 ; H. Krog and M. Monson, 1954 ; H. Krog, M. Mon- 

 son, and L. Irving, 1955; L. Irving, H. Krog, and M. Monson, 1955). 

 Mammalian basal metabolism, like body temperature, is a character- 

 istic not modifiable for climatic adaptation ; and individuals of three 

 species {Vulpes vulpes, Erethizon dorsatum^ I'amiasciurus hudsoni- 

 cus) did not change basal metabolism significantly in passing from the 

 cold subarctic winter to the mild weather of summer at Anchorage 

 (L. Irving, H. Krog, and M. Monson, 1955) . 



After white laboratory rats have been exposed for a month to air 

 about 0° C. their resting metabolism is elevated some 60 percent above 

 the level characteristic of them when they are kept in a warm labora- 

 tory. This enabled them to survive cold better than the rats from a 

 warm room, but their reactions to low temperatures were not as well 

 balanced nor as effective as those of wild brown rats, living out of 

 doors in the Alaskan winter, whose basal metabolism did not differ 

 significantly from that of normal white rats. Although the white rats 

 acquired some extra tolerance of cold with their elevated resting meta- 

 bolic rate, it appeared that this adjustment resembled the physio- 

 logical alterations that compensate individuals for pathological con- 

 ditions or unnatural stress but which could not be regarded as a natural 

 adaptation proper for a race of animals (H. Krog, M. Monson, and 

 L. Irving, 1955). 



