418 EVOLUTIONARY MORPHOLOGY 



thermous animals tend to be large. They profit by their 

 relatively small surface from which they lose heat, and also by 

 the fact that they do not require to spend all their daylight 

 eating as they would if their surface/volume ratios were large 

 and they were small in size. On the other hand, tropical 

 homothermous animals can afford to be small and to have 

 large surface/volume ratios. The intensity of heat radiation 

 is less than in polar regions because of the higher temperature 

 of the air, and small size enables them to get rid of their heat. 

 Also, there is ample food, and daylight to eat it in, to make up 

 for the heat lost. Homothermous animals as small as humming- 

 birds could not live in really cold climates. 



It is worth noticing that fat, which is a poor conductor of 

 heat, forms a layer underlying the skin in the animals inhabiting 

 polar regions (seal, penguin), and so assists in minimising the 

 amount of heat lost by radiation. When, on the other hand, 

 fat is stored by homothermous animals living in hot climates, 

 it is not distributed under the skin all over the body, where it 

 would interfere with heat- radiation, but it is localised and 

 forms humps as in the camel or the zebu. 



Whereas homothermous animals tend to be large in polar 

 regions and small in the tropics, poikilothermous animals show 

 precisely the opposite tendency, and for the same reasons. 

 The reptile depends on the outer environment for its heat. In 

 cold climates, when it is not hibernating, it is to its advantage 

 to absorb as much as possible of what heat there is. This is 

 assisted by a large surface/ volume ratio, and consequently a 

 small size. Effectively, it is found that the fish, amphibia, and 

 reptiles inhabiting cold climates are smaller than their relatives 

 living under warmer conditions. For in tropical climates, 

 these animals can afford to be large. The giant frogs, turtles, 

 lizards, snakes, and crocodiles of the tropics illustrate this 

 point well. The huge size of the reptiles in the Jurassic 

 period must have been made possible by the hot conditions 

 which prevailed then. 



It is further to be noticed that in tropical regions, the 

 poikilothermous animals can compete successfully with the 

 homothermous ; whereas in polar regions, the homothermous 



