heat gained from the surroundings by conduction and radiation. The heat gain from 

 the surroundings of a physical body is proportional to the effective surface area. 

 Also the metabolic heat of mammals is approximately proportional to the body sur- 

 face, and consequently the total heat to be dissipated is roughly proportional to the 

 surface area. 



Table 1 gives a summary of rates of evaporation found under actual desert con- 

 ditions in a few mammals of different body sizes. It will be seen that the dog loses 

 body water at a rate which is more than twice as high as that of the donkey if cal- 

 culated on the basis of body weight. However, we have just found that the total 

 heat gain should be approximately proportional to the surface area of an animal, and 

 in the table we note that the water loss per surface area actually is nearly the same 

 in these animals, irrespective of their body size. 



It would be permissible to extend this reasoning to animals of even smaller or 

 larger body size in order to estimate how much water should be evaporated in order 

 to keep the body temperature constant in a desert climate similar to that actually 

 ejqserienced in the observations given in Table 1. Such calculations are, of course, 

 very rough approximations, and give only an order of magnitude of the expected 

 rates of evaporation. 



The results of such calculations are given in Table 2. In the last column it 

 will be seen that an increase in body size from the donkey to the camel causes a re- 

 duction in the rate of evaporation to not quite half the value. On the other hand, in 

 small animals the rate of evaporation will increase rapidly with diminishing size. 

 Since the relationship is an exponential function, the rate of increase gives a loga- 

 rithmic curve as shown in Fig. 1. A mouse attempting to maintain constant body 

 temperature in the hot desert would have to use water in an amount exceeding 20% 

 of its body weight per hour. This amount of water loss is fatal, and here we find the 

 explanation for the fact that mammals of small body size usually do not sweat or in 

 other ways use water for heat regulation. If exposed to the heat for any length of 

 time there would be a choice of evaporation and death from dehydration, or no eva- 



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