ECOLOGY AND BIOLOGY OF THE PACIFIC WALRUS 



127 



80r 

 60 



40 

 20 



<D 0 

 O 

 k_ 

 Q) 

 CL 



40r 



20 



1 1 1 



Curvature 



1 



N = 180 Intact 

 82 Fractured 



N=I74 Intact 

 86 Fractured 



EL 



JD 



Divergence 



Convergence 



Fig. 86. Comparative shapes of intact (open bars) and fractured (crosshatched) tusks 

 of adult male Pacific walrus&s. 



abrasion and the slow rate of linear growth of the tusks in old age. Frontal 

 abrasion of the tusks is normal throughout the lifetime of these animals. Its 

 greatest effect is on or below the anteriormost part of the parasagittal arc. While 

 the tusks are rapidly growing in young animals, the area affected by that abrasion 

 is continually changing. However, when growth is very slow in the old animals, 

 the abrasion is on nearly the same area for several years in succession, resulting in 

 extreme reduction in thickness at that point. 



Abraded Surfaces of Tusks 



The form of the clinical crown of the tusks is continually altered by abrasion, 

 presumably during contact with the mud, sand, and gravel of the sea floor while 

 the animals are feeding. The principal effects of this are increased proximal-to- 

 distal taper and reduced length of the crown. The greatest effects of abrasion are 

 apparent on the anterolateral surface, near the distal end (Fig. 88a). There, the 

 outer layers of cementum and dentin are worn away, exposing the core to view. 



