THE SEA OTTER IN THE EASTERN PACIFIC OCEAN 



61 



searching for food, stroking or paddling movements of the hind 

 flippers are also used. 



Unlike other marine mammals, the sea otter habitually swims 

 on the surface when it is moving from one area to another. It 

 usually swims on its back and paddles with alternate strokes of 

 its hind flippers (fig. 30). In this position the long fifth digit dips 

 most deeply into the water and in combination with the other 

 webbed digits produces forward motion. 



Thus, the fifth digit is a useful adaptation to two unusual 

 methods of swimming in a mammal: (1) Progression beneath the 

 surface through undulations of the hind flippers and tail in the 

 vertical plane, and (2) progression on the surface by paddling 

 while resting on the back. Whether the fifth digit became elongated 

 as an adaptation to swimming beneath the surface or as the 

 dominant member used in surface propulsion is difficult to surmise. 



The sea otter infrequently swims belly down at the surface, 

 with the head and shoulders above the surface (fig. 86). At such 



Figure 30. — Three adult males in typical swimming attitude. The forepaws 

 are folded across the chest or pressed palmside down against the chest. 

 Forward progress is obtained by alternate strokes of the hind flippers. 

 Maximum sustained surface speed is about 2.5 km per hour (1.5 knots). 

 (KWK 62-27-14) 



