REGULATORY MECHANISMS 405 



on a regulation and balance of the amounts of heat produced 

 and lost. Poikilothermous animals have a temperature only 

 slightly higher than that of the environment. Some seem 

 to be able to raise their temperature slightly for a period by 

 muscular contractions, such as the python when it is coiled 

 round its eggs. But these animals have no means of combating 

 really cold external temperatures, during which they must 

 either hibernate or die. Within limits, the hotter the tempera- 

 ture, the better are the conditions for poikilothermous forms. 

 Some lizards, however (Varanus, Uromastix), when exposed 

 to great heat, increase their rate of breathing very considerably, 

 and so resort to panting. Panting results in the lungs getting 

 rid of large quantities of water vapour, and as heat is absorbed 

 in the conversion of water into vapour, panting means loss of 

 heat also. Uromastix, which inhabits deserts, is dark in colour 

 up to a temperature of 41 ° C, but as the temperature rises 

 above this point, it tends to become white. Since dark colours 

 absorb heat and light colours reflect it, Uromastix has a peculiar 

 mechanism which tends roughly to regulate its intake of heat 

 from the environment. This method, however, is quite 

 different from that of homothermous animals, birds, and 

 mammals. In the first place, the homothermous animals have 

 an external covering which is a bad conductor of heat ; this 

 takes the form of feathers in birds, hairs in terrestrial mammals, 

 and oil or blubber in birds and mammals which lead an aquatic 

 existence. The effect of such a layer is to minimise the loss 

 of heat by radiation. Next, they have more efficient respiratory 

 and vascular systems, notably a four-chambered heart with 

 complete separation of the arterial and venous circulations. 

 In the Monotreme Echidna, the temperature is regulated by 

 varying the amount of heat produced, but it has no method of 

 varying the amount of heat which it loses. It has no sweat- 

 glands, no increase in the amount of blood in the skin (vaso- 

 dilatation), and it does not resort to panting. The heat- 

 production of Echidna varies according to the difference 

 between its temperature and that of the environment. How- 

 ever, this regulation is not very efficient, for if the environ- 

 mental temperature varies from 35 to 5 C, the temperature 



