56 THE BIOLOGY OF MARINE ANIMALS 



enlarged proventriculus, responsible for secreting large amounts of lipoids. 

 It appears that the secretion of stomach oil is confined to the breeding 

 season and that it is used in feeding the young. Adult fulmars also show a 

 preference for fatty foods. A highly adaptive lipoid metabolism is charac- 

 teristic of animals which are exposed to water shortage. The metabolic 

 oxidation of fats yields far more water than other foodstuffs, and probably 

 is an important factor in maintenance of water balance among pelagic 

 birds (76, 90). 



Mammals. Osmotic relations in marine mammals are similar to those 

 in pelagic birds. The osmotic pressure of the blood is slightly higher than 

 that of terrestrial species, but still well below sea water (Fig. 2.17), and 

 active processes of osmoregulation are necessary to maintain the salt and 

 water balance of the animal. Water is obtained largely from the food. 

 Both birds and mammals that feed upon marine vertebrates obtain a food 

 of relatively low salt content (about 1 % NaCl), not markedly different from 

 the concentration of their own body fluids (fish-eating birds, seals, killer 

 whales). Those species that feed upon marine invertebrates (some birds, 

 crab-eating seals, walruses, whalebone whales) are in a rather different 

 category since the salt concentration of their prey is practically equivalent 

 to sea water, and the margin of water left for their functional needs is 

 reduced. 



Water is lost by evaporation at the respiratory surfaces, in the faeces 

 and in the urine, but loss through the latter two routes is reduced by 

 resorption in the rectum and production of a hypertonic urine. A further 

 loss of water occurs during lactation and the production of a concentrated 

 milk (rich in fat), in whales and seals, is related to the necessity for water 

 economy (26, 77, 91, 124). 



In the mammalian kidney the urine is concentrated by resorption of 

 water in a special segment of the distal convoluted tubule. Measurements 

 of osmotic pressures and chloride concentrations in the urine of marine 

 animals show values of A 0-73-4-50 and 10-413 mM CI for seals and 

 A 1-83-3-41 and 75-820 mM CI for whales. The majority of values lie 

 within the range encountered in terrestrial species. In the harbour seal 

 Phoca vitulina it has been found that the rate of urine formation is low 

 between meals but increases markedly after a meal of fish. Loss of water 

 in the urine is thus curtailed between meals, and the kidneys are enabled 

 to function more effectively after feeding when additional water is avail- 

 able for renal excretion. Marine mammals drink little sea water : seals are 

 able to swallow their prey while submerged without taking in salt water, 

 and whalebone whales use their huge fleshy tongues to press out the water 

 from the crustaceans trapped in the baleen plates. It appears, then, that 

 marine mammals keep down their salt content by avoiding the ingestion 

 of salt water, by excreting minimal amounts of water and by the produc- 

 tion of a hypertonic urine (47, 55, 79, 128). 



The data derived from seals which have been fed upon fish (herring) 

 indicate that there is sufficient water available from this source to provide 



