76 VERTEBRATE RESPIRATION 



animal was practically insensible and scarcely moved, and this 

 represents a considerable metabolic economy over the 24-hour 

 period. It also enables the animal to conserve a great deal of 

 body water. Shortly before daybreak the body temperature 

 spontaneously returns to normal, as does the metabolic rate, 

 and the bird is soon searching for food. Similar torpidity is found 

 in bats, poorwills (a desert bird), and swifts ; the body tempera- 

 ture of the bat being almost the same as the environment when 

 it is asleep during the day. 



(iii) Hibernation 



The state of torpor is very similar to that of hibernation found 

 in many mammals and some birds. Hibernation must be distin- 

 guished from the deep winter sleep of bears and other large 

 mammals for in the latter the body temperature may be held 

 at 31°-34° C. although the surrounding air is at —25° C. True 

 hibernation involves a marked change in the temperature regu- 

 lating mechanism and affects the whole metabolic activity of the 

 animal. Not only does the body temperature fall but also the 

 metabolic rate, heart rate, and respiration, as shown below for 

 a Marmot. 



The entry into hibernation may be gradual but in some ground 

 squirrels it has been shown to be a complex process which may 

 take 21 days. The plot of brain temperature shown in fig. 23 

 indicates the way in which the diurnal lowering of temperature 

 during the night is made use of in this respect. By a series of 'test 

 drops' the temperature on alternate nights is successively 



