PHYSICAL AND CLIMATIC FACTORS 417 



Temperature may vary in several different ways, either in 

 space or in time, or in both. So the tropics and the temperate 

 and polar regions differ in temperature, as do day and night 

 or summer and winter. 



Homothermous animals are largely independent of tem- 

 perature variation in the outer environment since they live in 

 a constant internal environment of their own. However, the 

 outer environmental temperature has a bearing on their size. 

 This follows readily from a consideration of the ratios of surface 

 to volume at different sizes. The surface increases as the 

 square, but the volume increases as the cube of the linear 

 dimensions, so that there is relatively more surface in small 

 animals than in large ones. The importance of this for 

 homothermous animals is that the amount of internal heat 

 produced (by metabolism) and lost (by radiation) varies 

 relatively with the surface. So, of two dogs weighing 20 and 

 3 J kg. respectively, the former will have a surface of 7,500 sq. 

 cm., the latter 2,423 sq. cm. For every kg. of dog, there is 

 in the large dog 375 sq. cm., and in the small one 757 sq. cm. 

 of surface, and the amount of heat given off from the dogs 

 per kg. is twice as high in the case of the small dog as in the 

 case of the large one. 



Small homothermous animals therefore radiate relatively 

 more heat from their surface than large animals, and this 

 heat-loss has to be compensated by relatively more active 

 metabolism and intake of food. In spite of the fact that 

 mammals and birds grind their food up small (in the mouth 

 in mammals : in birds, in the gizzard) so that the processes of 

 digestion are accelerated, a stage of smallness is reached when 

 the animals have to spend all their time feeding. Shrews and 

 humming-birds are of about this size. If they were smaller 

 than this they would need to consume quantities of food which 

 they would not have time to eat. Especially true is this of 

 regions in which because of seasonal variation the days are 

 short for a period in each year. The ratio of surface to volume 

 therefore establishes a minimum limit of size for homothermous 

 animals in a given outer environmental temperature. 



In cold climates, such as prevail in polar regions, homo- 



2 E 



