ECOLOGICAL FACTORS 385 



or Sunda tiger. On the other hand, homoiothermal animals which dis- 

 pense with these heat retainers entirely or in part are present only 

 in the tropics. Such forms include the hairless dog, the Somali rodent, 

 Heterocephalas philippsi, of Somaliland (Fig. Ill), the sparsely haired 

 buffalo, and the monkeys with bare faces, hands, and rumps. Birds 

 with featherless necks are almost entirely confined to the warmer 

 regions. 



Subcutaneous fat is especially abundant in aquatic homoiothermal 

 animals, notably in penguins, seals, and whales. In the seal, 50 kg. of a 

 body weight of 115 kg. is hide and fat. Such fat is common among 

 terrestrial animals, especially in cold regions and in the winter; this is 

 notably true of reindeer, red deer, musk oxen, wild hogs, bears, and 

 badgers. The effectiveness of fat as a thermal insulator is evidenced 

 by the fact that a seal causes no visible melting after remaining for 

 hours upon an ice floe, and that a dead walrus has a high body tempera- 



fife? 



Fig. 111. — Heterocephalus philippsi, X %• After Thomas. 



ture even after 12 hours in ice-cold water. In many animals that store 

 fat in warm climates, the fat is localized, as in the fatty humps of 

 camels and zebus or the fat tails and rumps of Mediterranean sheep. 



The Bergmann principle. — Under identical conditions all 

 homoiothermal animals give off equal amounts of heat per unit of 

 surface. 24 Two dogs with body weights of 20 kg. and 3.2 kg. each 

 were found to have surfaces of 7500 and 2423 sq. cm. respectively; 

 for each kilogram there are 375 sq. cm. of surface in the large one, 

 757 sq. cm. in the smaller one; the heat production for 1 kg. of mass 

 amounted to 45 calories for the large dog, 88 calories for the small 

 one, per unit of time, which agrees closely with the corresponding 

 value of the surfaces. 25 Thus a decrease of surface is of advantage for 

 the maintenance of the body heat in a cold environment. 



Whether mammal or bird, a homoiothermal animal under other- 

 wise similar conditions has an advantage when its body surface is pro- 

 portionately small. In otherwise similar bodies, the larger one has the 

 smaller surface in proportion to mass, since volume and mass increase 



