THE METABOLIC ACTIVITY OF VERTEBRATES 71 



into account the upper lethal temperature is five or more degrees 

 lower. It is likely that denaturation of the proteins is involved at 

 these temperatures but this cannot be the case in arctic fish 

 which may have upper lethal temperatures as low as 10° C. 



(i) Poikilotherms 



Although the metabolic rate of cold-blooded animals follows 

 the environmental temperature this is not to say that there is no 

 control at all or that they are always cold. They do not have 

 exactly the same temperature as the environment, especially 

 when this is fluctuating. Smaller individuals follow the air 

 temperature with less time lag than larger forms which tend to 

 maintain a temperature which is the mean of any fluctuations 

 in the environment. One eff'ect of this is that small poikilotherms 

 are able to take advantage of brief periods of sunlight to become 

 warmer. Many reptiles bask in the sun early in the morning, 

 burrow later in the day to avoid excessive heat, reappear again 

 in the cool evening and finally burrow again as it becomes cold 

 during the night. At the cold end of the range a serious danger is 

 that freezing of the body fluids may lead to death because of the 

 effects of ice crystal formation disrupting the protoplasmic struc- 

 ture, especially during warming-up. Some small animals may 

 be frozen and survive de-freezing but few adult vertebrates are 

 in this category, though mammalian sperm and red blood cells 

 can be stored when frozen for many months if pre-treated with 

 glycerol, frozen quickly, and stored below — 40° C. Hamsters 

 have been maintained frozen at temperatures of - 3° to - 14° C. 

 for more than 50 minutes and survived when warmed gradually 

 and given artificial respiration. Some arctic fish are known to 

 live in waters with temperatures of - 1-7° C. although the freez- 

 ing point of their blood is —0-9° C. If fish under such conditions 

 are seeded with ice crystals they freeze at once and it is evident 

 that the blood is in a super-cooled state. Fishes inhabiting the 

 deep fjords of Labrador do not normafly come into contact with 

 such crystals and in this way survive the low winter and summer 

 temperatures. Those living at the surface, however, live in 

 temperatures of 5° C. in summer and -1-5° C. in winter. It 



