ANIMALS THAT LIVE WITHOUT WATER I57 



requiring excretion. When dehydrated in this manner, 

 it dies in two or three weeks with an average weight 

 loss of about 34 per cent, but with no significant de- 

 crease in the actual water content of the body: death is 

 a result of a decrease in the volume of body fluid rather 

 than a change in composition. 



Whereas, in some uric-acid-excreting insects, water 

 may be saved from excretion by storing uric acid in the 

 body throughout the life of the individual, in the kanga- 

 roo rat (as in all other mammals), in which protein 

 nitrogen is converted to urea, no such storage mecha- 

 nism exists and the urea is excreted as fast as it is formed. 

 So long as the filtration rate remains at a constant value, 

 this urea, with sodium chloride and other urinary con- 

 stituents, automatically sets the minimal rate of urinary 

 water loss. Hence the degree to which the urine can be 

 concentrated becomes of paramount importance. The 

 kidney of the kangaroo rat can concentrate to the great- 

 est extent of any known mammal, reaching osmotic con- 

 centrations 17 times that of the plasma, as compared 

 with the maximal figures of 5.6 in the seal, 4.5 in the 

 dog, and 4.2 in man. The concentration of urea in the 

 urine reaches 23 per cent, that of sodium chloride 8.7 

 per cent, as compared with about 6.0 and 2.3 per cent, 

 respectively, in man. The urine is, in fact, so concen- 

 trated that it is apt to soHdify after it is withdrawn from 

 the bladder. 



This extraordinary concentrating power enables Di- 

 podomys to drink even sea water, the only mammal on 

 record that can do this. But to induce it to drink sea wa- 

 ter, which it finds unpalatable, it is necessary to dehy- 

 drate it first by feeding it a diet so high in protein 

 nitrogen that it cannot otherwise maintain itself in water 

 balance. Since the kangaroo rat can concentrate sea wa- 

 ter two and a half times, something more than one-half 

 of the water of the sea water is in theory available for 

 urine formation. It would be expected that the poorly ab- 

 sorbable salts, magnesium and sulfate, would cause diar- 



