THE PELVIC LIMB 



served, was sati c fied with determining that there was movement of the 

 feet. Consequently we must content ourselves for the time being with 

 a judgment of the swimming method according to anatomical evidence. 



A brief examination of the external features of the sea otter will show 

 that the fore feet are of small area but that the hind feet are much en- 

 larged, with the fifth toe the longest and the others successively shorter, 

 while the sole is densely haired. The remainder of the hind limb is 

 unusually short. The tail is of moderate length, extending to the rear 

 about the same distance as the tips of the toes when the limbs are trailed, 

 and is said to be slightly expanded in a lateral direction. 



The anterior feet of Enhydra are too small to be of any practical 

 use for swimming, and for the same reason the tail is not capable of act- 

 ing alone. The hind feet are, however, sufficiently modified so that we 

 can be sure that they furnish the chief propulsive force. Precisely how 

 fast the animal can swim I have no idea, but according to accounts sev- 

 eral men in a light skiff have all they can do on a long pull to catch 

 up with one. It seems that they may be able to travel as fast as ten 

 miles an hour, possibly with higher speed for a short distance. One 

 can be sure that they are at least reasonably speedy swimmers, too, be- 

 cause otherwise a pelagic mammal of this size would quickly be ex- 

 terminated by sharks and killer whales, even though kelp beds are their 

 favorite habitat. 



If this probability be conceded, then they cannot swim by direct 

 thrust-and-recovery movements in alternation by the hind feet, for, as 

 previously described, even moderately high speed cannot be attained 

 thereby. On the contrary some motion must be employed involving 

 oblique action of the feet against the water, comparable to the swimming 

 actions of the seal. Can the sea otter swim in the same way as the seal, 

 with lateral motions of the feet held palm to palm ? Indications are that 

 it does not do so for the following reasons. In the seal the feet are 

 shaped like the tail of a fish with lunate posterior border, and for the 

 same reasons. Both first and fifth digits are elongated to accomplish this. 

 If the sea otter swam in a similar fashion it is sufficiently specialized for 

 it to show definite indications of the same development. This, however, 

 might be complicated by the presence in this animal of a tail of moderate 

 length. The tail of the seal is too short to be of any consequence, but 

 it is possible that in the sea otter this member, of the same length as 

 the feet, might in a mechanical sense take the place of the uppermost 

 toe held against it in swimming posture. But the fifth toe is the longest 

 and in order to act with the tail so that the posterior border of the swim- 



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