THE VOICE OF THE DESERT 1 Qg 



kidneys have to do is to get rid of any excess solids, includ- 

 ing salt. But all vertebrates, including of course all those 

 which Hve on land, have to maintain in their bodies blood 

 of a density which must not vary except within small limits 

 and which, as it happens, is considerably less dense than 

 that of sea water. 



There are those who believe that its density corresponds 

 to that of the once less salty sea at the time when the first 

 vertebrates developed. But however that may be, the kid- 

 neys have to see to it that the density remains constant by 

 ehminating excess water or excess minerals as .the case 

 may be. To get rid of excess salts they must dehver to the 

 bladder for excretion a fluid much denser than the blood. 

 But there is a limit to what they can do in preparing this 

 concentrated fluid. Men and nearly all animals will quickly 

 die if they drink a quantity of sea water, because it is far 

 denser than any fluid their kidneys can concentrate and it 

 simply increases that oversaltiness of the blood of which 

 thirst is a sign. So far as is known, only one animal can 

 drink sea water, for though sea birds have sometimes been 

 assumed capable of it, they probably are not. The one ani- 

 mal who can — as the reader has probably guessed — is our 

 hero Dipodomys. 



He does not want to waste water merely to get rid, as 

 he must, of excess mineral matter. Hence he has developed 

 an unparalleled capacity to concentrate urine. Man can- 

 not concentrate a fluid to more than 4.2 times the osmotic 

 concentration of his blood plasma, but the kangaroo rat 

 is capable of concentrating to 17 times the density of his 

 blood and that is considerably denser than sea w^ater. 

 Hence if he is forced by a special diet to drink when only 

 sea water is available, he can quench his thirst with that. 



