THE METABOLIC ACTIVITY OF VERTEBRATES 



75 



10 12 1 U 



Midnight 



Fig. 22. 



Oxygen consumption of a humming bird and a shrew throughout a 

 24-hour period (after Pearson, 1957). 



homoiothermic condition is not reached for several months. 

 During its pouch life (65 to 70 days) a change in environmental 

 temperature from 20° to - 10° C. leads to a lowering of the 

 internal body temperature by as much as 20°. In the adult 

 similar changes do not affect the deep body temperature at all. 

 Perhaps the most striking variations from homoiothermy are 

 found in small mammals and birds which are unable to maintain 

 their body temperature when that of the environment falls at 

 night. The humming bird is a well-known example and in fig. 22 

 the oxygen consumptions of a humming bird and a masked shrew 

 of approximately the same size are plotted for 24 hours. During 

 the night the external temperature falls and the metabolic rate 

 of the shrew increases slightly. The humming bird on the other 

 hand shows a very marked drop in oxygen consumption asso- 

 ciated with a fall in body temperature near to that of the environ- 

 ment. In the middle of the night it was living at a metabohc level 

 Qnly 1/1 5th as rapid as during the day. In this torpid state the 



