442 



ANNUAL REPORT SMITHSONIAN INSTITUTION, 1961 



5 mq 



10 mg 



15 mg 



20 mg 



mg HaQ/ 



/liter oir 



Figure 1. — The water balance of Dipodomys. (After B. and K. Schmidt-Neilsen.) 



Putting these facts together Scliinidt-Neilsen draws the diagram 

 given in figure 1 for the water balance of Dipodomys eating pearl 

 barley without any supply of water beyond that present in the barley 

 as preformed water and that produced in the metabolism (oxidation 

 water). It shows that the water balance is positive at all relative 

 humidities greater than 10 percent. Remembering that the animals 

 spend most of the daytime in burrows, we may conclude that they are 

 sufficiently adapted to live permanently in a desert climate without 

 drinking. The rat is not quite in balance at a relative humidity of 100 

 percent. It is of mterest to note that a similar balance was worked 

 out by Krogh [15] for the seal {Phoca) living in the sea, which is for 

 a mammal a "dry" environment since the osmotic pressure of sea water 

 is greater than that of mammalian tissues and water must leave the 

 body by diffusion. Krogh found that the seal was also in balance, but 

 in neither case is any allowance made for water loss by the female when 

 she is giving milk. 



I hope that these examples will have shown that tropical biology 

 offers us many problems that repay investigation, and that the results 

 are often not those that we should expect from our knowledge of 



