ECOLOGY AND BIOLOGY OF THE PACIFIC WALRUS 



119 



Table 13. Measurements and estimated sum of linear increments for 

 tusks of 28 immature male Pacific walruses. 



Length (mm) 



Age 



Clinica.1 



Clinical 



Lost by 



• 



Estimated sum 



(years) 



crown 



root 



abrasion 



of increments 



0.0 



0 



51 



0 



51 



0.0 



0 



52 



0 



52 



0.0 



0 



54 



0 



54 



0.3^ 



0 



65 



0 



65 



0.4a 



10 



72 



0 



82 



1.0 



19 



96 



0 



115 



1.0 



22 



95 



0 



117 



1.1 



29 



79 



2 



110 



1.2a 



14 



98 



0 



112 



1.2 



33 



90 



2 



125 



1.3a 



31 



105 



1 



137 



1.5 



63 



121 



5 



189 



2.0 



51 



110 



9 



170 



2.0 



70 



124 



6 



200 



2.0 



64 



133 



6 



203 



2.0 



89 



135 



7 



231 



2.0 



90 



97 



6 



193 



3.0 



106 



142 



5 



253 



3.0 



89 



135 



7 



231 



3.0 



127 



140 



10 



277 



3.5 



152 



146 



19 



317 



3.6 



105 



115 



18 



238 



4.0 



108 



149 



13 



270 



4.0 



152 



162 



25 



339 



4.0 



159 



143 



20 



322 



5.0 



184 



165 



50 



399 



6.0 



209 



175 



25 



409 



9.0 



292 



172 



50 



514 



'Walruses of known age reared in captivity. 



Rate of linear growth. — The length of the annual increments of tusk growth 

 can be measured as the distance between the successive annular ridges on the 

 root, the latest increment being most proximal. In immature walruses, distinct 

 annulations usually are not recognizable with certainty, but because the tusks 

 are only lightly abraded, they make up nearly the entire sum of linear incre- 

 ments. The few millimeters of length that have been lost through abrasion can be 

 estimated by visual comparison with tusks of similar form from younger animals. 

 Using those measurements (Tables 13, 14), I derived estimated annual linear 

 increments of tusk growth for the pre-adult years. Annual increment rates for the 

 adult years were measured directly as the distances between annular ridges. 



I obtained the longest series of measured increments from tusks that I 

 decalcified, then stripped away the cementum (Fig. 84). Shorter series were 

 measured on longitudinally sectioned tusks; the shortest series were measured on 



