THE METABOLIC ACTIVITY OF VERTEBRATES 73 



than tropical species at their normal habitat temperature 

 (30° C). Extrapolation of the tropical curve gives values at 

 0° C. which are 30-40 times lower than those at the normal 

 tropical temperature. This indicates, therefore, that the meta- 

 bolic rate of the arctic forms is ten times the value expected had 

 there been no acclimation. Such acclimation is rare among 

 terrestrial species. The precise relationship between the accli- 

 mation temperature and the animal's metabolic rate is com- 

 plex and varies from species to species. The general conclusion is 

 that poikilothermic animals do have ways in which they are 

 adapted to changes in environmental temperature, especially 

 long-term changes. The limitations on their geographical distri- 

 bution, therefore, are not as great as might have been expected. 



(ii) Homoiotherms 



Among homoiothermic animals the effect of a lowered 

 environmental temperature is to increase the metabolic rate and 

 so raise their heat production and contrasts with the decreased 

 oxygen consumption found amongst cold-blooded forms. There 

 is, however, a range of temperature (the neutral range = 27°- 

 31 ° C. in man) over which there is no change in metabolic rate 

 with environmental temperature. Above the thermo-neutral 

 range the oxygen consumption rises just as it does below it. The 

 temperature below which an increase in metabohc rate occurs 

 is referred to as the critical temperature (Tc). This is lower for 

 animals which live in arctic conditions partly because of their 

 large size but also because of their thick coats. In general small 

 mammals live in tropical regions to which they are acclimated 

 and have higher critical temperatures. Mammals with lower 

 critical temperatures have less steep temperature/metabolism 

 curves, i.e., the increase in metabohsm for a given fall in tempera- 

 ture is less for arctic forms than tropical forms. Thus the air 

 temperature can only be reduced to 28° C. before the racoon 

 increases its metabolic rate in order to maintain its body 

 temperature, whereas an Eskimo dog pup maintains a constant 

 body temperature until that of the environment falls as low as 

 -25° C. 



