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NORTH AMERICAN FAUNA 74 



where it appears as a shallow depression. The depression usually is highly 

 polished and has a few, fine striae parallel to the long axis of the shaft. Abrasion 

 apparently takes place there as a consequence of movement of the mandible, 

 presumably while feeding, and probably is hastened by the presence of benthic 

 sediments caught between the lower jaw and the tusk. 



Medial abrasion of this type probably takes place continuously throughout the 

 lifetime of the animal. During the preadult years, when linear growth of the 

 tusks is most rapid, this abrasion smooths and polishes the medial surface of the 

 shaft and creates slight undulations in it. On the tusks of many adult males, 

 however, a series of shallow depressions is produced, the regularity and 

 dimensions of which suggest direct relation with the annual cycle of linear 

 growth of the tusks. 



Much deeper, more irregularly spaced depressions of this kind often are 

 present along the medial surface of the tusks of middle-aged and older females. 

 The spacing of these, mostly 3 to 7 cm apart, does not correlate with the annual 

 cycle of linear growth but suggests that they are formed at 2- to 5-year intervals. 

 Since such intervals correspond to those between pregnancies, I suspect that these 

 deep depressions are produced during periods of intensive feeding, probably 

 during the late prenatal period. In a pregnant walrus held in captivity ("Petula": 

 Marineland of the Pacific), maximal food intake took place from February to the 

 time of birth in June (E. D. Asper, personal communication). 



I found the deepest depressions of medial abrasion on the tusks of very old 

 individuals. These penetrated through the cementum and well into the dentin. 

 Apparently, they resulted from continuous abrasion over a period of several 

 years on virtually the same locus, for the tusks of these animals all showed linear 

 growth rates of less than 3 mm per year. That this extreme abrasion can lead to 

 weakening of the tusks was indicated by fractures at that locus in each of two 

 very old bulls that I observed on Round Island, 



Missing and Anomalous Tusks 



Of 1,106 specimens, most of them adult males, only 17 (1.5%) lacked one 

 tusk, and 2 (0.2%) lacked both tusks (Fig. 92). Nine of the missing tusks 

 appeared to have been shed, probably due to pulpitis; six of the remainder had 

 been broken off at or just distal to the gingiva. In each of the two animals with 

 no tusks, one tusk apparently had been shed and the other broken off, 



I recorded seven instances of anomalous tusks in free-living walruses and was 

 able to determine that four of these were due to abscess of the pulp (Fig, 93); at 

 least three of the remainder were supernumeraries not caused by abscess (Fig. 

 94), Two of the four pulpal abscesses were caused by bullet wounds; the causes of 

 the other two were not determined. Other instances of tusk anomalies have been 

 reported by Colyer (1936) and Caldwell (1964), 



The nature of the abrasion on anomalous tusks suggested that the tusks are 

 dragged through the benthic sediments, rather than used for digging. For 

 example, in one young male that had one normal tusk and one which jutted 

 anteriad, the anomalous tusk had been abraded to about half the length of the 

 other, though they were about equal in basal circumference. The tip of the 

 anomalous tusk was strongly beveled, like the tips of the superior mystacial 

 vibrissae. In each of the specimens that had supernumerary tusks, the anterior- 



