114 



NORTH AMERICAN FAUNA 74 



Fig. 77. Comparative views of shape of the tusks in adult male (upper) and female 

 (lower) Pacific walruses. (Photos by K. W. Kenyon and K. J. Frost) 



sectional diameter at the gingiva and lesser diameters both proximally and 

 distally. This increase in relative size at the gingiva is mainly a function of 

 increasing extent of overlap of the annual layers of cementum, as noted earlier. 



The tusks of males also are stouter and more elliptical than those of 

 comparable length from females (Figs. 79, 80). In both sexes, the cross-sectional 

 area of the tusks at the gingival margin increases with increasing length, but it 

 usually is about 20% greater in males than in females for any given length. In 

 adult males, the ratio of the short to the long axis of the cross-sectional ellipse 

 usually is about 1:1.5, whereas in females it is about 1:1.4. In both sexes, the 

 ratio is least in the immature and greatest in the very old individuals. 



Linear Growth, Abrasion, and Fracture of Tusks 



The tusks grow and erupt continually, in which respects they are comparable 

 to the incisors of rodents, elephants, and a few other mammals. Their growth is 

 cumulative; new increments are added at the proximal end of the tusk, resulting 



