340 U. S. NATIONAL MUSEUM BULLETIN 217 



another day. We crossed the tracks of an occasional lemming while 

 traveling some 20 miles at some distance from shore over the sea ice 

 and I fomid one about four miles from land dead but undamaged 

 by predators. These forays of lemmings turned out badly but they 

 show that while active these small and poorly insulated mammals 

 can for some hours withstand cold far below their critical tempera- 

 ture. 



It is common to see the tracks of the arctic least weasel {Mustela 

 rixosa) in the snow of the arctic forest and over the tundra. I have 

 never followed them successfully very far before losing them where 

 they apparently returned beneath the snow. The tracks of the larger 

 weasel {Mustela erminea) I have followed for a mile without reach- 

 ing the end of its continuous journey in cold winter air. They are 

 larger and have somewhat thicker fur than the least weasel, but I 

 doubt if their insulation could protect them with resting metabolism 

 in air colder than 0° C. Both species of weasels regularly expose 

 themselves while active, and they can probably pursue their intensive 

 activity for several hours at a rate sufficient to preserve their body 

 temperature by the heat of activity, although their insulation is not 

 sufficient for them long to endure cold while at rest. 



Although some species have been observed to raise their metabolic 

 rates about six times in cold, others managed only about a 3-fold in- 

 crease during several hours of experimental exposure to cold. As yet 

 no climatic pattern in the possession of this faculty is shown among 

 tropical and arctic, small and large forms of mammals and birds, 

 but that it is, in part, a matter of metabolic disposition, is suggested 

 by the fact that aggressive wild brown rats {Rattus norwegicus) of 

 Fairbanks raised their metabolism in experimental cold nearly twice 

 as much as white laboratory rats (H. Krog, M. Monson, and L. 

 Irving, 1955). And we have as yet no criteria for deciding whether 

 the metabolic increase originates in other tissues than the visibly active 

 muscles. 



When the increase of metabolism below the critical temperature 

 accurately supplies the heat lost and maintains normal body tempera- 

 ture the line relating metabolism to air temperature is straight. 

 This is the situation postulated in the description of homoiotherms 

 as heat machines (Scholander, Hock, Walters, Johnson, and Irving, 

 1950). Hart (1962) found the body temperature of mice to be well 

 preserved by their muscular activity for a certain amount of cold 

 below their critical temperature. White rats regulated their body 

 temperature during short experimental periods in air temperatures 

 10° or 20° below their critical temperature, but in colder air their 

 body temperature dropped and the line showing elevation of 

 metabolism progressively fell below the theoretical line for 



