312 WHALES 



From Fetcher's experiments it also emerged that the kidney of these 

 animals can, for very short periods, release urine with a fairly high salt 

 concentration, but from other experiments on whales and dolphins it 

 appears that the salinity of the urine generally corresponds with that of 

 the blood and other body fluids. The kidney cannot therefore be likened to 

 the special salt-excreting cells of bony fishes or to the kidneys of some 

 desert mammals (e.g. the kangaroo rat) whose urine is particularly 

 saline. The only way, therefore, in which Cetaceans can get rid of their 

 surplus salt is to pass large quantities of urine with a consequent waste of 

 water - something these animals which, as we have seen, lose water through 

 their mouths and intestines as it is, and which have to be very parsi- 

 monious in their water economy, can ill affoi'd. 



Fortunately, not all Cetaceans are equally handicapped in this respect. 

 Those which feed on mammals or birds, like the Killer Whale, and those 

 which feed on fish like most porpoises and dolphins, do not take in highly 

 concentrated salt solutions with their food, and their main problem is 

 their inability to make good any water losses by drinking fresh water as we 

 do. While they cannot help swallowing some sea-water with their food, 

 they do seem to keep it down to an absolute minimum. Seals are said to 

 swallow no sea-water at all, but all we can meanwhile say about Cetaceans 

 is that Mysticetes which feed on krill and those Odontocetes which feed 

 on cuttlefish are in a particularly unfortunate position in that, by eating 

 non-vertebrates, they swallow a diet with the same salinity as sea-water. 

 They will therefore have to pass particularly large quantities of urine and 

 limit all other water losses as much as possible. In this they are greatly 

 helped by the fact that they do not lose water through their skin, which, 

 as we have seen, is devoid of sweat glands. Moreover, the air they inhale 

 is so saturated with water vapour that there is little water loss in the lungs. 

 Irving has calculated that four-fifths of the herring which dolphins eat 

 consists of water, of which a maximum of 20 per cent is lost by the produc- 

 tion of faeces and by exhalation, so that 80 per cent can go into the produc- 

 tion of urine. In addition, Cetaceans, because of their high metabolic rate, 

 oxydize vast quantities of food with the consequent liberation of large 

 quantities of water, particularly since they oxydize mainly fats whose 

 combustion releases more water than that of carbo-hydrates or proteins. 

 Combustion of fat and minimum evaporation through the skin 

 are probably also the reasons why some desert animals can go without 

 water for such long periods. Clearly, desert and salt water environments 

 are similar in more than one respect. 



While various scientists including the physiologist Krogh (1939) agree 

 that Cetaceans must get rid of their surplus salt by the excretion of vast 

 quantities of urine, there is no experimental evidence that they do so in 



