62 



Com'parative Animal Physiology 



Krogh extended the calculation to the walrus and whalebone whales which 

 feed largely on marine invertebrates that are more concentrated than fish and 

 more concentrated even than the blood of the mammals themselves. Less 

 water would be lost from the lungs in the whale because, owing to the in- 

 creased pressure under water, it can probably extract more of the oxygen and 

 thus would not need to saturate so much air. Assuming a little less water loss 

 by feces than in the seal, Krogh calculates that the kidney could easily excrete 

 the salt and urea with the water a\'ailable from its food. Freezing point de- 

 pressions of 1.83-2.49 have been observed in urine of the pollack whale 

 (Morimura, from Smith--*''). Dolphins given hypertonic salt solution by 

 mouth had feces isotonic with blood, and in several hours 53 per cent of the 



TABLE 6. CONCENTRATIONS OF BLOOD AND URINE 

 OF VARIOUS MARINE MAMMALS 



salt and 84 per cent of the water was e.xcreted. ''* The blood chloride changed 

 very little, hence there must have been a shift of tissue water. 



Mammalian urine can be more concentrated than sea water, hence the 

 body should be able to extract some water from sea water and excrete salts. 

 Indeed, Gamble ^'^ has found that, in man, utilization of sea water is not 

 hmited by the kidney but by the gastrointestinal system. It would be of 

 interest to know the gastrointestinal tolerance of sea water in marine mammals. 



In summary, reptiles, birds, and mammals have very low surface perme- 

 ability to water. Birds and mammals are able to excrete a urine which is more 

 concentrated than the blood. The primitive water-excreting function of the 

 glomerulus has become diverted to a filtration-reabsorption system to excrete 

 waste products with little loss of water. Hence the animals are able to get 

 enough water by food or by drinking to supply their meager demands. The 

 embryos are essentially aquatic but are protected against evaporative loss, in 

 birds bv the shell, and in most mammals bv retention in uterine fluids. 



CONCLUSIONS 



When animals are arranged according to their osmotic performance (Table 

 7), certain correlations with their distribution and taxonomic relationships 

 are apparent. Many of the differences among animals from various habitats 

 are differences in the degree of development of a capacity rather than in the 

 specific kind of capacity. 



Success in fixing in the ocean may accompany either osmotic lability or 



