ECOLOGY AND BIOLOGY OF THE PACIFIC WALRUS 



107 



and abrasion. Though these tusks are hardly long enough to be used as digging or 

 raking devices, it is at this age that most of the young become fully weaned and 

 nutritionally independent of the mother. 



By the end of the third year, about half of the tusk is clinically erupted, and 

 more than half of the enamel has been removed from the anatomical crown by 

 abrasion. Usually by the end of the fourth year, the enamel is absent or only a 

 tiny plaque remains on the medial surface, near the tip of the tusk. From that 

 time forward, the tusks are composed only of dentin and cementum. Eventually, 

 the core of globular dentin becomes exposed near the tip, as the outer parts of the 

 tusk are abraded away (Fig. 70). 



In a longitudinal section of the tusk, one can see that each major increment of 

 dentin takes the form of a hollow, truncated cone, in which the walls are 

 thinnest proximally and thickest distally (Figs. 70, 72). Each conical increment 

 of dentin that is formed in the pre-adult years is slightly larger in diameter 

 proximally than its predecessor; those formed during adulthood are about equal 

 in diameter. Each successive increment tends to be shallower than the preceding 

 one, and the amount of globular dentin also tends to be less. In the tusks of very 

 old animals, the latest increments are very thin and shallow and often lack the 

 globular dentin. At that stage, the tusk is nearly solid to its base, and has only a 

 thin, lenticular pulp at its proximal end. 



That these major dentin increments are deposited on an annual basis was first 

 suggested by Tomes (1904:195-196) and recently by Scheffer (1950) and Laws 

 (1953a), who observed that the annular ridges on the root occur at regular 

 intervals (Fig. 73). This regularity suggested that the ridges were formed by an 

 annual rhythm of growth, like that in fur seals and elephant seals and that they 

 might be counted for age determination. However, they usually are not apparent 

 on the tusks of females or of immature males. Furthermore, on the tusks of adult 

 males, the ridges usually are visible only on the proximal half or less of the root; 

 more distally, they are obscured by overlying deposits of cementum. I confirmed 

 that the ridges are produced annually by noting the position of the most proximal 

 ridge, relative to the proximal end of the tusk, in animals taken in different 

 months (Fig. 74). Apparently the ridge is formed in mid- to late winter, in the 

 mating season, during temporary slowing or cessation of growth (cf . Kubota et 

 al. 1961). 



The pattern of deposition of cementum on the exterior of the tusks is rather 

 different from that on the other teeth, in that all of the layers are of about the 

 same thickness, and each overlaps only part of its predecessor (Figs. 70, 72). The 

 proximal end of each annual layer of cementum is in contact with the proximal 

 layer of dentin, formed in the same year. The distal end of each cementum layer 

 overlaps one or more of the layers deposited in previous years. The extent of that 

 overlap is inversely proportional to the rate of linear growth of the tusks, being 

 least (about 60 % ) in the youngest animals, in which linear growth is most rapid, 

 and greatest (more than 95%) in the oldest animals which have very slow 

 growth. The cumulative thickness of cementum on the tusks is directly related to 

 the extent of that overlap. Because of this, the greatest cumulative thickness of 

 cementum in all tusks always is at or near the gingival margin (Figs. 70, 72). The 

 thickness of cementum at that location in tusks of middle-aged males is about 2 to 

 2.5 mm; in very old males it may attain about 4 mm. In females, both the 



