TROPICAL CLIMATES AND BIOLOGY — CARTER 441 



Lastly, I will take an example from physiology, and from an en- 

 vironment very different from the swamps and equatorial regions we 

 have so far considered. My example is the problem how small mam- 

 mals are able to satisfy their needs for water in desert conditions. This 

 has been studied by B. and K. Schmidt-Neilsen [13]. 



It has always been difficult to understand how such animals as 

 jerboas and desert rats can survive without drinking in deserts where 

 the temperature may rise to 130° F. (54° C.) at midday. There are, 

 however, some characters of the desert environment and the animals' 

 biology that go part of the way to help us to understand their ability 

 to do so. 



First, the most striking characteristic of a desert climate is the large 

 diurnal range of temperature, very hot at midday but cool and even 

 near the freezing point at dawn. Dew is frequent in many desert 

 climates, and Buxton [14] showed that grass blowing about on the 

 surface of a desert and apparently entirely dry contained water to 

 50 percent of its weight at midday, presumably derived from the dew 

 of the previous morning. Secondly, these animals are largely 

 nocturnal ; they avoid the extremes of midday heat in burrows. 



They may obtain some water by eating the grass or from dew and 

 may reduce their water loss by sheltering, but it seems unlikely that 

 they can wholly maintain their water balance in these ways. The only 

 other supply of water available to them is the metabolic water formed 

 in the oxidation of their food; 1 g. fat yields 1.07 g. water in its 

 oxidation, 1 g. carbohydrate 0.56 g., and 1 g. protein 0.40 g. Schmidt- 

 Neilsen set out to determine whether this was a sufficient supply. 



It should be noted that, as we should expect, desert animals are 

 adapted in several ways to economy of water. They do not control 

 their body temperature by sweating. Their sweat glands are reduced, 

 and control of temperature by sweat is in fact impossible for small 

 animals in desert conditions, for the amount of water loss required is 

 far too large in proportion to their body weight. Sclmnidt-Neilsen 

 finds that for a man of 70 kg. in the temperatures of a desert in day- 

 time 1.47 percent of his body weight must be evaporated per hour, 

 for the kangaroo rat {Dipodornys^ 0.1 kg.) 12.8 percent, and for a 

 mouse (0.02 kg.) 21.5 percent. 



Then, again, their urine is more concentrated than that of other 

 mammals. Comparable results are given in the following table. 



Concentration in urine 



Electrolytes N. Urea M. 



Man 0.37 (2.2%) 1.0 (6%) 



Rat {R. norvcgicus) 0.6 (3.5%) 2.5 (15%) 



Dipodomys 1.2 (7%) 3.5 (23%) 



Also, very little water is lost in the feces. The feces of Dipodomys 

 have a water content only one-quarter of that of the rat's feces. 



