sition because of the lafge quantities of water that would be required for evapora- 

 tion. They avoid the heat problems by underground life and nocturnal habits. On 

 the other hand, the larger animals cannot lead an underground life, but due to their 

 large size the problem of heat regulation is less severe. In the absence of exact 

 knowledge based on experimental work, a working hypothesis can be based on a 

 simple statement of the physical laws that govern heat exchange between the ani- 

 mal body and its surroundings. 



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AIR TEMP 



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frsrxtrsrs BODY TEMP 



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Figure 3. 

 A simplified diagram of the temperature gradients at the surface of an animal when the am- 

 bient temperature is higher than the ix)dy temperature. The temperature gradients indicated by 

 the solid line ASB and the broken line AS' B, respectively, show in which direction heat flow 

 will occur under different circumstances. If sweat evaporates at the surface of the skin (S) 

 without wetting the fur the temperature gradients will be as given by the solid line ASB. If 

 sweat evaporates from the surface of the fur (S' ) the gradients will be as shown by the broken 

 line AS' B. In both cases heat flow to the site of evaporation from each side, but it is evi- 

 dent that much less water is required to maintain the gradients indicated by ASB than by 

 AS' B. In order to maintain constant body temperature the total heat flow along the gradient 

 BS must equal the metabolic heat. In the case of evaporation from the surface of the fur, 

 this gradient would have to be extended to S', requiring a lower temperature at S' than at S. 

 This lower temperature would further increase the steepness of the gradient AS', which 

 governs the heat flow from the air to the site of evaporation. The heat flow along the less 

 steep gradient AS (when water is evaporated at the skin surface) is much lower, and the steep- 

 ness (and the heat flow) will decrease as the insulation value of the fur layer increases. It 

 is further evident that a reduction of the insulating value of the skin itself (reduction of the 

 distance BS) will permit a steeper gradient to be set up for the heat flow from the body to the 

 surface, without a simultaneous increase in the steepness of the gradient AS. In other words, 

 the heat flow from the body to the surface is facilitated by a thin skin of low insulation value. 



187 



