52 



NORTH AMERICAN FAUNA 68 



in the pulp chamber beneath the large pit that has resulted in obliteration 

 of the pulpal horn. This phenomenon is ordinarily considered as a defense 

 reaction against trauma or caries. Looking at these pits and the more ad- 

 vanced damage in the other skulls, we are tempted to conclude that some sort 

 of peculiar attritional sequence is represented. The later flattening can be 

 more easily explained in the usual terms of wear and tear. I suspect that a 

 rather large-scale study would be required before the facts could be ascer- 

 tained as to how the early pitting occurs. 



There is a good bit of calculus on the teeth. The bone resorption in an adult 

 female (KWK 59-153) suggests that there may have been periapical ab- 

 scesses in the lower left molar region, probably as a result of pulp exposure 

 from the attrition. The bone damage in the upper left posterior region could 

 be explained in the same way, or equally well as an outcome of periodontal 

 disease. There are also other areas of bone damage. 



Tooth attrition and its relationship to feeding habits and 

 mortality at Amchitka Island are discussed elsewhere. It is ap- 

 parent that a dense otter population at Amchitka has resulted in 

 a shortage of soft-bodied invertebrate food species. Hard-shelled 

 forms are eaten in large quantities. Sea otters often chew mussels 

 (Mytilus) and sea urchins (Strongylocentrotus) . The sounds of 

 shells crushed by the teeth are often audible for many meters. 

 Also, sand is ingested with food organisms and sand grains are 

 often found imbedded in teeth. 



Dental attrition is less prevalent in sea otters from areas having 

 uncrowded populations than from the crowded Amchitka area. 

 A captive female sea otter that was fed primarily on soft foods 

 showed no dental attrition when she died at the age of about 

 7 years. It is thus concluded that dental attrition of the kind found 

 at Amchitka is the result of a diet that includes many hard- 

 shelled organisms with which abrasive sand, and even rocks are 

 taken (see Food and Feeding Behavior). 



Age Determination 



The chronological ages of certain wild animals may be as- 

 certained from anatomical specimens. In pinnipeds the teeth show 

 rings or annuli (Scheffer, 1950a) which are related to cyclical 

 annual phases of fasting and feeding. Similar growth layers are 

 found in the ear plugs and baleen plates of whales (Ichihara, 1966) 

 and in bones (Chapskii, 1952). To a limited degree of accuracy, 

 eye lens weights may indicate age in long-lived mammals, such as 

 the fur seal (Bauer, Johnson, and Scheffer, 1964). 



In any study of age determination in mammals it is desirable 

 to have a collection of known-age specimens. The isolation of sea 



