FAUNA OF THE ALEUTIAN ISLANDS AND ALASKA PENINSULA 283 



on the chest. Naturally, it must clasp both of these objects to 

 its body during the roll, but it does this very adroitly and 

 casually, and it continues unconcerned with its meal. 



Mortality Factors 



The natural mortality factors affecting the sea otter are almost 

 unknown. The northern bald eagle has been suspected of preying 

 on young sea otters, and it is possible that this may occur on 

 rare occasions. But it is notable that in our study of the food 

 habits of this eagle (see under that species), not a single in- 

 stance of such predation was found upon examination of eagle 

 nests in the center of abundance of sea otters. It was concluded 

 that eagle predation on the sea otter must be negligible. 



Two mammals, the sea lion and the killer whale, have fre- 

 quently been mentioned as sea otter enemies, but we had little 

 opportunity to verify this. We rarely saw these animals near 

 any sea otters, and although occasionally we saw killer whales 

 cruising by the outer edge of a kelp bed, we could not identify 

 its prey. However, the killer whale is known to eat fur seals, 

 therefore it is reasonable to suppose that it will pick up a sea 

 otter when the opportunity is presented. At any rate, the sea 

 otter has demonstrated in recent years that it can increase in 

 numbers and extend its range when it is protected from human 

 hunters. Identification, and degree of predation, of its natural 

 enemies must be determined by thorough scientific study. 



It is a well-known fact that dead sea otters occasionally are 

 washed up on the beach. On our expeditions, we found a number 

 of skeletons on the beaches, from which blue foxes or eagles, or 

 both, had eaten the flesh. It is said that a sea otter sometimes 

 succumbs in the heavy surf in winter. Pups, as well as large 

 adults, are included in casualties thus recorded on the beach. In 

 the postwar years a higher mortality rate has become evident 

 and many dead sea otters have been found. The cause is not yet 

 known. 



From the evolutionary standpoint, the sea otter seems to be in 

 an intermediate or transitional stage. The peculiar dental spe- 

 cialization has been mentioned, as well as the prevalence of cavi- 

 ties in the molariform series. These cavities are present in fresh 

 specimens as well as in remains from old Aleut village middens. 

 Fisher (1940) has reported an instance of gastric perforations 

 in a sea otter found dead on a California beach. 



One cannot refrain from speculating whether the specializa- 

 tion in food, which involves hard and sharp mollusk shells, tests 

 and spines of sea urchins, barnacles, and similar materials that 



