AQUATIC MAMMALS 



scrape of so ponderous a bulk against a sharp rock would lay the ani- 

 mal completely open. And yet many hundreds of pounds may be ap- 

 plied to a cable passed through an incision in this whole layer without 

 it tearing away. 



The constitution of the blubber layer in the Cetacea certainly appears 

 unusual, but whether its precise histological structure is or is not unique 

 among the Mammalia has not been determined. At any rate the mere 

 presence of a considerable layer of fatty tissue beneath the integument of 

 aquatic mammals is not surprising. Many land mammals store up just 

 such a supply of extra fuel both for warmth and to tide them over periods 

 of food shortage. Any carnivore must be prepared to live through times 

 of hunger and this should be true in the case of large whales which re- 

 quire such a prodigeous quantity of food, often of small size, and which 

 travel so widely. Any sort of mammal which experiences times of 

 plenty alternating with periods of hunger will quickly acquire the abil- 

 ity to accumulate a reserve of fat. Some sort of insulation of the body 

 is probably of critical necessity to an arctic marine mammal, for although 

 it never experiences an aquatic temperature lower than a few degrees 

 below freezing, the conductivity of the water is 27 times that of air. 

 But there are probably other advantages derivable from the presence of 

 a fat layer in such mammals. The tropical sirenians surely have little 

 need for insulation and yet they are not only abundantly supplied with 

 fat, but this seems singularly inefficient in insulating the body, for the 

 manatis of Florida are known quickly to succumb to any unusual low- 

 ering of the air temperature. Furthermore, we cannot know just how 

 advantageous it is to an aquatic mammal to have the added buoyancy af- 

 forded by an extensive deposit of fat. The blubber layer itself may be 

 significant of nothing more than stated above, but the excessive oiliness 

 of other parts of the body in marine mammals and of the proneness of 

 the Odontoceti to develop fat organs of some sort seems to point to the 

 probability of all this fat and oil serving some other and very important 

 physiological purpose of which we do not know. This question will re- 

 ceive further consideration elsewhere. 



It is perhaps proper here to discuss the question of a dorsal fin, al- 

 though little can be stated in this connection. Some whales are without 

 it (Balaenidae, Rhachianectes, Physeteridae, Delphinapterus, Monodon, 

 Phocaena) ; 4n some it is barely indicated and too small for function, 

 while in most odontocetes it is quite large, culminating in the killer 

 whales, in which it may attain a length of several feet. All that it is 

 now safe to say is that all aquatic vertebrates are prone to develop such 



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