14 THE FUTURE OF ARID LANDS 
3. Drought resistance in animals is different from drought 
resistance in plants in which large quantities of water are im- 
pounded within the plant body and growth is continued when no 
other water is available. Although animals resist drought differ- 
ently the term drought resistance can still be used since normal 
activities are maintained. Dr. Schmidt-Nielsen’s studies of the 
camel and the donkey in Algeria will do much to explain the 
physiological processes by which these animals are able to resist 
desert conditions. (See pp. 370-380). He has found no reason to 
agree with the common idea that the camel carries in pouches in 
his stomach a reserve of water to be used when no drinking water 
is available. Drought resistance in animals rests upon the physio- 
logical processes by which animals are able to concentrate the 
urine, stop perspiration, lose little water in the feces, endure 
dehydration, and still remain active. 
The methods by which the kangaroo rat, jaroba, and some other 
animals avoid loss of water is by voiding nearly dry feces and 
concentrating the urea in the urine to as high as 23%. Some 
birds and reptiles can void urea and nitrates in solid form. Ex- 
tensive studies of the kidney functions of these animals have 
been made and are under way to try to determine the exact 
means by which such concentration is accomplished. This ability 
might well be a deciding factor in enabling some animals to escape 
death by drought in an arid desert. 
Drought resistance in animals has been accomplished in some 
groups not by impounding water but by the physiological process 
of developing metabolic water (11). Dry fat when oxidized pro- 
duces 1.1 grams of water from each gram of fat. A gram of starch 
or sugar produces 0.6 gram, and a gram of protein 0.3 gram of 
water. The water formed by the oxidation of the hydrogen in dry 
food can be used to carry on the life functions. Protein has a 
great disadvantage as a source of water for it requires the loss 
of much water to remove the urea that is formed. 
A great amount of work has been done on the kangaroo rat to 
determine the water balance under desert conditions (7 and 14). 
These animals can be kept on dry pearled barley almost in- 
definitely without any other food or water if the relative humidity 
