ANIMALS THAT LIVE WITHOUT WATER I59 



latter suffers the disadvantage of greater water loss 

 through the skin despite the fact that, like Dipodomys, 

 it does not sweat. A long nose and dry skin contrive to 

 keep total extra-renal water loss to only slightly more 

 than half that of the white rat, and to probably one- 

 third or less than that of man under moderate, resting 

 conditions. 



When total water loss (that which is evaporated from 

 the lungs and skin and excreted in the urine, plus a small 

 loss in the feces) is balanced against total water intake 

 (the preformed water of the food plus metabohc water), 

 the issue of net water balance— the issue of life or death 

 —hinges importantly on the relative humidity of the at- 

 mosphere in which the kangaroo rat hves. Dipodomys 

 has excess water available to it at all relative humidities 

 above lo per cent. During the driest part of the year 

 the humidity above the ground in the Arizona desert 

 is usually near this critical limit, and consequently the 

 animal cannot afford to risk loss of water by exposure 

 to the direct sun: with air temperatures as high as 

 104° F. and ground surface temperatures reaching 

 157° F., it must remain in its burrow except at night. 

 The average depth of the burrow is only two feet, but 

 the temperature rarely exceeds 86° F. and the humidity 

 averages 30 per cent. The humidity in a small, closed 

 pocket of air at a depth of only 12 inches is 100 per cent, 

 but the humidity of the burrow is reduced by ventila- 

 tion, and some species keep ventilation to a minimum by 

 closing the entrance of the burrow during the daytime 

 hours. 



The humidity of the burrow is no less important in 

 respect to the storage of food. Pearled barley, with which 

 these quantitative observations were carried out, yields 

 53.6 cc. of metabolic water per 100 grams. If dried in 

 the desert air, the free or preformed water would be 

 negligible, but after storage at a humidity of 30 per cent 

 the free water increases to 10 cc. per 100 grams, increas- 

 ing the total available water to 63 cc. This increase in 



