AQUATIC MAMMALS 



lengthy periods of time, then it seems that a hairy border was in some 

 way stimulated to growth by the endlessly repeated lashing of the tail 

 from side to side during propulsion through the water. This motion 

 results in considerable friction by the water along both the upper and 

 under side 1 of the tail and this may have been the activating agent, 

 throughout a very long period of time in order to have final effect. 



A similar possibility may explain the development in insectivores 

 and rodents of a tail flattened transversely and expanded in the vertical 

 plane. Any stimulus that increased growth of hair upon the dorsal 

 and ventral borders of the tail would presumably result, at a later stage 

 of aquatic specialization, in the deposition of more subcutaneous tissue 

 along these borders. And this is mainly fatty or even partly glandular, 

 rather than muscular or bony. But this is pure speculation, for as we 

 cannot even find out what really causes baldness in man one can hardly 

 speak authoritatively on the reasons governing hair growth upon the tail 

 of a shrew. A rather baffling coincidence in the tails just considered is 

 the fact that although hairy caudal keels evidently precede (usually) 

 flattening of the member itself, in those forms in which the latter is the 

 case the tail is no hairier above or below than upon the sides. 



Of those insectivores and rodents whose tails are flattened in the 

 horizontal plane Desmana, Potomogale and Ondatra are the only ones 

 in which this character is really pronounced, although Galemys and 

 Umnogale approach the conditions existing in the first two genera re- 

 spectively. The tails of Desmana and Galemys are somewhat swollen 

 posterior to the base, as is usual in the family Talpidae, this being due 

 to the presence of bodies that are largely fatty but which may also be 

 glandular in character. In the muskrat (Ondatra), however, the tail 

 is unusually slender, the skin not only being rather thin but without 

 any sign of subcutaneous fat and so closely adherent to the muscular 

 and tendinous tissue that this member is quite difficult to skin. In these 

 aquatic moles and in the muskrat the base of the tail is not enlarged 

 but is abruptly smaller than the hind quarters, approximately as in the 

 common brown rat. In the muskrat at least the tail describes sharp 

 sinuous movements throughout a considerable arc during aquatic pro- 

 pulsion. This motion follows oscillation of the hind quarters while the 

 hind feet are kicked alternately and is, undoubtedly, chiefly involuntary. 

 The flattening of the tail, however, has progressed to the point where 

 this member is a definite, although still a secondary, aid to swimming, 

 and but a slightly greater specialization in this direction would enable 

 it to constitute a propulsive organ equal in importance to the hind feet. 



[188] 



