METABOLISM 205 



these losses could probably be made good widi only half an ounce of meat 

 a day, and we all know that we would quickly starve to death if we ate no 

 more than that. We eat food, not only to replace cells, but also to obtain 

 energy for the work our body does. The greater the work done, the greater 

 our need for food. Our body may be likened to an internal combustion 

 engine, with a large part of our daily diet acting as the fuel. However, a 

 car needs no petrol when it stands in the garage, while even the idlest of 

 stay-a-beds must eat to keep alive. Even while he is supine on his back, 

 his heart continues to beat, his respiratory system continues to function, 

 and his body temperature is maintained. After all, man lives in an 

 environment some 36° F. colder than his body, and if his temperature 

 dropped below a certain critical level he would quickly die. No matter 

 how warmly we dress, we constantly lose a certain amount of heat which, 

 just like the heat given out by a stove, must be replenished by burning 

 combustible materials - coal or food. To supply the necessary energy for 

 keeping our organs working when our body is at rest, and to supply the 

 necessary heat, we need about 1,800 great calories of food a day, i.e. 

 about t^vo-thirds of our requirements for sedentary or other light work. 

 In other words, most of our food is needed to maintain tempei'ature at its 

 normal level. 



Heat is lost primarily through the skin and the greater our external 

 surface, the more heat is lost. For this reason we hunch up when we are 

 cold, and stretch out as far as possible when we feel hot. Now, we have seen 

 that the heavier an animal, the smaller its surface area and its heat losses 

 per unit of body weight. Big animals will therefore lose relatively much 

 less heat than small ones and will therefore need relatively less food. 

 Cetaceans have the additional advantage that their streamlined form, i.e. 

 the absence of protruding limbs, pinnae, etc., reduces their relative surface 

 area still further, as a result of which their metabolism is particularly 

 eflficient. On the other hand, they have to overcome far greater environ- 

 mental difficulties than terrestrial animals, particularly since water 

 conducts heat about twenty-seven times as well as air. Our own body 

 cools down much more quickly in water than in still air of the same 

 temperature. Moreover, when a whale swims, the water flows past its 

 body at speed, with the result that further great heat losses are incurred, 

 just as we are cooled off by a breeze. Nor can whales find any form of 

 shelter or even curl up to decrease their surface area, as, for instance, dogs 

 do when they are cold. If we bear in mind that a normal human being 

 loses consciousness after three hours in water at about 60° F., and after 

 only fifteen minutes in water at about 32° F., we will appreciate what 

 difficulties aquatic mammals have to contend with. 



The best solution is, of course, a thick layer of insulating material. Thus 



