It has been seen that even light clothing in man cuts down evaporation by a 

 major fraction of the total. The reduction in evaporation corresponds to the re- 

 duced heat gain from the environment, because we can assume that the metabolic 

 heat was the same. The insulation value of animal fur is considerably higher than 

 that of 'light clothing', and one can expect a considerable advantage in heat and 

 water economy due to the insulation of fur. 



The advantage of increased surface insulation is of course limited. It is true 

 that an infinite insulation of the surface would reduce the heat gain from the en- 

 vironment to zero. This is of course not biologically possible, and furthermore, 

 there should remain means for dissipating the metabolic heat. 



The only means of dissipating heat in an environment warmer than the body sur- 

 face is by evaporation of water. The heat is bound at the site of evaporation, and 

 here we will find some relations of importance to the effectiveness of evaporative 

 cooling. 



A diagram of the animal surface is sketched in Figure 3. Water will appear in 

 the form of sweat on the surface of the skin. It will be seen that the water either 

 could wet down the fur and evaporate from the outer surface of the fur layer, or it 

 could evaporate at the skin surface and diffuse as water vapour through the fur to 

 the surrounding air. Since heat is bound at the location where water changes from 

 the liquid state to vapour, there is a considerable advantage if the sweat evaporates 

 at the skin surface without wetting the fur. The heat of evaporation will be taken 

 from the body as well as the outside air, and the amounts would be in reverse pro- 

 portion to the insulation of the layers between the source of the heat and the site of 

 evaporation. The fur layer between the site of evaporation and the hot environment 

 is a great advantage in reducing the amount of heat that reaches the site of evapora- 

 tion from the environment. However, if evaporation took place at the surface of the 

 fur, the fur layer would be a disadvantage by reducing the transport of heat from the 

 body and it would provide no insulation between the site of evaporation and the hot 

 environment. 



It will now be clear that the most economical use of water for heat dissipation 

 includes the fur, and an increase in the insulation is advantageous as long as it 

 does not interfere with the dissipation of the water vapour. Furthermore, the eco- 

 nomy in the use of water will depend upon the ease with which heat is transported 

 from the body to the skin surface, i.e., the circulation in the skin and the insulation 

 value of subcutaneous tissues. An increase in the subcutaneous adipose tissue 

 would by its insulation properties directly disfavour an advantageous distribution of 

 the heat flow to the site of sweat evaporation. It can perhaps be assumed that there 

 would be reason to consider the distribution of adipose tissue in desert mammals 

 from this viewpoint. The thin skin and particularly the localization of depot fat in 

 e.g. the hump of the camel and the brahma cattle and the tail of the fat- tail sheep 

 may indicate the possibility that this distribution may have a value in the heat and 

 water economy as outlined above. 



The principles outlined in this paper are an attempt to make it clear that active 

 heat regulation in desert animals of small body size is a nearly impossible propo- 



186 



