ECOLOGY AND BIOLOGY OF THE PACIFIC WALRUS 



121 



Fig. 84. Exposure of linear den- 

 tinal increments in a tusk from an 

 adult male Pacific walrus. Right, 

 basal part of untreated tusk on 

 which five annular ridges are 

 visible near the apical end 

 (upper); left, decalcified slab 

 sawn from the opposite side of 

 the same tusk, showing five addi- 

 tional ridges that were exposed 

 by removal of the cementum. 

 (Photo by F. H. Fay) 



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sum of linear increments. The results (Tables 17 and 18) suggest that the rate of 

 decrement is higher from tusks of males than from those of females, and the 

 highest rates in both sexes are in the weanling to subadult years, when the tusks 

 are most slender and divergent and the tips jut forward to or beyond the anterior 

 plane of the snout. 



Because the tusks of females are more slender than those of males, one might 

 expect them to be shortened more rapidly by abrasion, but this does not seem to 

 happen. The high rate of decrement from the tusks of the immature animals sug- 

 gests that the tips have the greatest exposure to abrasive materials at that age. 



Fracture. — Minor breakage or flaking of the tips of the tusks occurs rarely in 

 immature walruses, infrequently in adult females, and very frequently in adult 

 males. This may account in part for the apparently higher overall rate of linear 

 decrement from the tusks of males. The causes of this flaking are not completely 

 known. Where males haul out on rocky shores in summer, impact against the 

 rocks may be a major cause. Many of the bulls residing at Round Island in 

 summer showed signs of recent fracture, and J. W. Brooks, K. W. Kenyon, and I 

 retrieved several fragments of tusks from among the rocks there. The smallest 

 fragments were flakes a few millimeters long; the largest was nearly the entire 

 clinical crown broken off near the base. J. Harkey, W. Sholes, and E. H. Miller 



