l6o FROM FISH TO PHILOSOPHER 



available water is significant, because no potential source 

 of water gain can be ignored. Nor can any potential 

 source of water loss be ignored— to which end, in some 

 species of kangaroo rats, the animars cheek pouches, by 

 which it carries home its sun-dried food, are lined with 

 fur. 



Man's water requirements in the desert assumed poten- 

 tial military significance in World War II, and were 

 studied extensively by a group from the University of 

 Rochester under die direction of E. F. Adolph. By far 

 the greater part of the water lost by man at high tem- 

 peratures is in the form of sweat, and sweating is, of 

 course, highly variable, depending on temperature, hu- 

 midity, and physical activity. At 100° F. water loss by 

 sweating (including respiratory loss) ranges from 7.2 

 hters per day, when the subject is sitting clothed in good 

 shade, to more than 24 liters per day when he is walking 

 clothed in the sun and carrying a 33-poimd pack, and 

 to 28 hters per day when walking naked in the sun with- 

 out a pack. (These figures are to be compared with one 

 liter or less as the minimal quantity of water required 

 for urine excretion.) Since sweat contains considerable 

 quantities of salt, men sweating under desert conditions 

 eat excessive quantities in order to compensate for salt 

 loss, the daily loss reaching, in the extreme, 10 to 15 per 

 cent of the total quantity in the body. Some men ac- 

 cumulate a visible crust of salt on the skin during a day's 

 activity in the sun. One of the notable adaptations to a 

 hot climate (and to hot weather) is that the salt content 

 of the sweat diminishes after a week or so of sweating, 

 though never enough to prevent significant loss of salt 

 from the body. But even under extreme conditions, the 

 kidneys maintain the salt: water ratio (and hence the 

 osmotic concentration of the body fluids) within very 

 narrow limits. 



Depending on the dietary intake of salt and protein, 

 700 to 900 cc. per day of urinary water are needed for 



