AQUATIC MAMMALS 



umbrella. Undoubtedly the important features, except for the webbing, 

 of this equipment were evolved before the avian ancestry became aquatic. 

 But in mammals the only terrestrial types roughly approximating this 

 condition are saltatorial sorts of a desert habitat which would never be 

 expected to become aquatic. It seems apparent that every mammal which 

 has ever become highly aquatic originally had a shank but slightly nar- 

 rower than the foot, and there seems to be no stimulus furnished by an 

 aquatic existence for narrowing the shank. If the foot and shank in- 

 crease in breadth together, as is frequently the case, so that a greater 

 surface may be presented to the water, the shank would then be pro- 

 ductive of too much retardation through recovery during swimming and 

 the result would be impossibly inefficient. If the shank should become 

 shortened or withdrawn into the body contour, as is now the case in 

 the Pinnipedia, not only would even a greater amount of resistance 

 during recovery take place, but the propelling feet would be so close 

 to the body that they would lack requisite leverage and the animal would 

 fiddle along at a slow pace. Hence, if an aquatic mammal is going to 

 swim at speed by means of the hind feet it will have to evolve some 

 other method of using them than by the alternate thrust-and-recovery 

 of the duck. And this has been accomplished by the seals in the way 

 in which they place the feet sole to sole and oscillate them from side to 

 side. It may also be so after a different fashion in the sea otter. This 

 method of swimming is the most effective of any that uses the hind 

 limbs as the sole method of propulsion, but it could not be highly de- 

 veloped in a mammal with a long tail for the reason that the latter 

 would quickly outstrip the feet as a propeller. Just how and why the 

 feet of the seals adopted this style of function while those of the sea-lion 

 did not is a question about which one can only speculate. 



If a mammal that is highly aquatic swim chiefly by means of the tail 

 the hind feet will not play any important part as equilibrators for the 

 reasons that they they are too near the propulsive organ to be efficient, 

 and as but one pair of equilibrators in the same plane seems necessary 

 in mammals, the fore feet, being better situated for the purpose, will 

 assuredly take over this function. The only mammal swimming chiefly 

 by the fore feet and with a tail suitable for equihbration is the platypus. 

 It is probable that its hind feet were at one time a greater aid to active 

 propulsion than they are now. Whether these members are now really 

 necessary for assisting the tail to function in equilibration is unknown. 

 But it is requisite that a flat-bodied animal such as the platypus, or marine 

 turtle, swimming by partly or chiefly vertical movements of the anterior 

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