AQUATIC MAMMALS 



that its lateral aspects become broader than the vertical ones, either 

 by the acquisition of a hairy fringe above or below or a flattening of 

 the member in the transverse plane ; and of course with such a specializa- 

 tion the efficiency of the tail as a swimming organ is thereby increased. 

 This is the rule, but there are some exceptions. Presumably Chironectes, 

 Myocastor and the Australian water rats are sufficiently aquatic to have 

 developed a flattened tail, but the latter is perfectly terete. Whether they 

 actually swim by some slight variation in the movements of the hind feet 

 that does not introduce a stimulus for tail flattening, or whether the 

 caudal appendage merely has unusually inherent resistance to such change 

 but will later accomplish it, is unknown. 



The speed which the hind feet operating upon the kick-and-recovery 

 principle are capable of imparting to the body is very definitely limited, 

 as previously discussed. In mammals with a tail of respectable length 

 there will almost certainly come a time (save possibly in the case of 

 the beaver) when the tail and its musculature will have become suffi- 

 ciently specialized so that it can propel the body at a faster pace than 

 can the hind feet alone. The latter will then quickly fall into disuse 

 unless they be employed for other important purposes. It is not un- 

 likely that the tails of Desmana and Ondatra are now almost sufficiently 

 specialized to accomplish more speedy swimming than the feet, but 

 for a very long time at least the latter will continue a necessity to both 

 animals for terrestrial activities. 



Most of these small mammals discussed are not sufficiently modified 

 for there to have occurred marked change in any part of the leg above 

 the ankle. Dobson (1882), however, has remarked that in Desmana 

 the femur is but little more than half the length of the tibia, and the 

 superficial biceps and semitendinosus have broadened considerably (fig. 

 48), both of these alterations thus being in the direction followed by 

 the seal. The gluteal muscles are also unusually robust, as in so many 

 other aquatic mammals. In all aquatic mammals having an enlarged 

 hind foot one will almost certainly find that the shank and thigh muscles 

 are correspondingly strong. 



(b) Next for discussion is the case of such aquatic mammals as em- 

 ploy all four feet in swimming but have a short tail, involving the polar 

 bear, capybara, aquatic rabbits, hippopotamus and tapir. All have great 

 dependence upon the land and in all the tail is insufficient in size to 

 act as a rudder or stabilizer, and in all but the rabbit the hind feet are 

 not much larger than the fore feet. The latter may thus function ef- 

 fectively to overcome the asymmetrical, wabbling motion that the kick- 



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