104 



NORTH AMERICAN FAUNA 74 



Reduction in length of many teeth takes place also by resorption of tissue at the 

 proximal end. This is a relatively minor cause of dental destruction, usually 

 amounting to removal of no more than a millimeter or two of the cementum. As 

 in man, such resorption takes place most often on the proximal third of the root 

 and is found mainly in middle-aged or older individuals. The principal 

 "exciting" causes of such resorption in human teeth are trauma and systemic 

 disturbances, often associated with old age (Henry and Weinmann 1951). 



As the crowns of the functional cheek teeth are worn away by attrition and 

 abrasion, the teeth migrate distad, maintaining occlusal contact with their 

 antagonists. Functional teeth that are unopposed, due to loss of an antagonist, 

 also continue to migrate distad. These often develop a very high, polished crown 

 that occludes eventually with the opposing gingiva. The vestigial incisors, P4, 

 and the molars, which are small, irregular in occurrence, and often unopposed, 

 are more variable in their reactions to lack of an antagonist. The lower incisors 

 and V usually are resorbed in place, without ever becoming gingivally erupted. 



and the molars also seldom penetrate the gingiva. When they do erupt, they 

 rarely extend beyond the level of full exposure of the anatomical crown. 12 often 

 becomes gingivally erupted, and continues to migrate distad until the tooth is 

 completely destroyed by incisive abrasion. 



Form, Development, and Abrasion of Tmks 



The permanent upper canine teeth (C^) of the walrus are recognized as "tusks" 

 because of their great elongation, enlargement, and exposure. Their large size 

 also identifies them as organs of great functional importance to the individual, 

 hence of great adaptive significance for the evolutionary success of the species. In 

 the past, the functional role of the tusks was more a subject for speculation than 

 of definitive study. For at least 200 years, they were thought to function mainly 

 in the acquisition of food, presumably being used as a digging or raking device 

 for unearthing the clams. Recently, Miller (1975a, 1976) demonstrated that their 

 major importance is as social organs. 



Early in the present study, I observed that the supposed clam-digging function 

 of the tusks did not seem to be upheld by the characteristics of their relative 

 growth, abrasion, and placement in the skull. Hence, I devoted much effort to 

 examining their structure and form and investigating their growth and abrasion. 

 The objective of that work was to obtain indirect evidence of their function. The 

 following are my findings. 



Basic Form and Structure 



At birth, the incipient tusks consist of simple, hollow cones of dentin, capped 

 by enamel, and contained entirely within their alveoli (Fig. 70). At that time, 

 each tusk is about 4 to 6 cm in overall length, the distal half of which comprises 

 the anatomical crown. Gingival eruption of the tip of the crown begins during 

 the first summer to autumn of extrauterine life; full gingival eruption of the 

 anatomical crown is completed about 1 year later. In the 1-year-old walrus, the 

 overall length of each tusk is more than double that in the neonate, and the 

 enamel-covered portion exposed beyond the gingiva resembles the typical upper 

 canine of a large carnivore (Fig. 71). Because this crown does not extend below 



