ANIMALS THAT LIVE WITHOUT WATER 165 



flow and filtration rate (and of course the urine flow) 

 are reduced to very low levels. A considerable quantity 

 of blood is thus diverted from the kidneys and made 

 available for circulation elsewhere. It may be inferred 

 that some sort of a diving reflex is present in all the trulv 

 marine mammals, since they all can remain below the 

 water for some period of time, and it is possible that 

 the evolution of the diving reflex paved the way for the 

 evolution of the metabolic control of renal function, the 

 conjoint adaptation making it possible for these mam- 

 mals to Hve exclusively in the sea. 



So far as is known, terrestrial birds are either drinkers 

 of fresh water or, among desert forms, able to live on 

 water derived from food. The maximal osmotic concen- 

 tration of the urine in the chicken (unfortunately the 

 only bird studied to date) is only some 60 per cent 

 greater than that of the blood (recall that the osmotic 

 urine/plasma concentration ratio in the kangaroo rat 

 may reach the spectacular value of 17). If this limita- 

 tion apphes to other birds, as seems to be the case from 

 fragmentary data on the maximal concentrations of so- 

 dium and chloride in the urine, the avaflability of water 

 or very moist food must be important in determining 

 their habitat, despite their excretion of uric acid, because 

 salts and nitrogenous substances that cannot be con- 

 verted to uric acid will demand considerable water for 

 their excretion. 



Some birds, however, have solved the problem of 

 spending weeks or months— even years— at sea, returning 

 to the land only to lay and hatch their eggs. Among the 

 truly marine birds are the shearwater (Puffinus), the 

 petrel {Fulmarius) , and the albatross (Diomedea); 

 while many coastal birds, such as the gulls (Larus), 

 the guillemots {Uria), auks (Plautus), and cormorants 

 (Phahcrocorax) appear to be independent of any fresh- 

 water supply. Until recently it was thought that these 

 marine forms were also entirely dependent on metabohc 



