JANSKY 



(Hart, 1950; Jansky, 1959 a), and in golden hamsters (Jansky, 19 59c), 

 kept at laboratory constant temperatures, the working oxygen con- 

 sumption increased with decreasing temperature. Oxygen consump- 

 tion during work was added directly to the cold thermogenesis, giving 

 two parallel curves (Figure 2). These results confirmed the validity 

 of the addition theory and showed clearly that the highest values of 

 metabolism could be measured only after simultaneous application 

 of cold and work. 



Contrasting results have been obtained on two wild rodents , the 

 common vole ( Microtus arvalis) and the bank vole ( Clethrionomys 

 glareolus) (Jansky, 1959b), kept for a short period at naturally fluc- 

 tuating temperatures. Both species are closely related and have the 

 same average weight (18 gm). In the common vole, the typical addi- 

 tion relationship between working and resting heat production in the 

 cold was observed. In the bankvole,onthe other hand, the metabol- 

 ism during maximal work below the thermoneutral zone did not in- 

 crease to the same extent as that during rest with decrease in tem- 

 perature. Therefore the heat production during work partially sub- 

 stituted for the cold thermogenesis. This was particularly clear when 

 the maximalrunningspeedsof the two species of voles are consider- 

 ed (Figure 3). Although there was a slight increase in running speed 

 with lowering of temperature from 25 C to 5 C, in both species, 

 there was a decrease in the difference between working and resting 

 metabolism in the bank vole but not in the common vole (Figure 2). 

 When the lowest temperature was reached, both the running speed 

 and the difference between working and resting metabolism were 

 greatly reduced. The common vole had a greater range for maximal 

 work than did the bank vole in both heat and cold. 



These experiments have shown that the metabolic differences 

 among various species of mammals do not depend on the weight of 

 the animals or on their phylogeny. The only obvious difference be- 

 tween the common vole and the bank vole lies in the ecology of both 

 species and in their acclimatization to various temperatures. Bank 

 voles live in forests in deep burrows and build well- insulated nests 

 and are not, therefore, exposed directly to the effect of low tem- 

 peratures. Common voles, on the other hand, live in meadows in 

 superficial burrows almost without nesting material and are forced 

 more often to endure extreme temperatures. This is reflected in 



178 



