120 



NORTH AMERICAN FAUNA 74 



Table 14. Measurements and estimated sum of linear increments for tusks 

 of 24 female Pacific walruses. 



Length (mm) 



Age 



Clinical 



Clinical 



Lost b\' 



Estimated sum 



(years) 



crown 



root 



abrasion 



of increments 



0.0 



0 



30 



0 



30 



0.0 



0 



42 



0 



42 



0.0 



0 



42 



0 



42 



0.0 



0 



44 



0 



44 



0.0 



0 



45 



0 



45 



0.0 



0 



45 



0 



45 



0.0 



0 



45 



0 



45 



1.0 



13 



79 



0 



92 



1.0 



29 



67 



0 



96 



1.2a 



25 



75 



11 



111 



1.3a 



22 



75 



3 



100 



2.0 



67 



108 



6 



181 



2.6 



60 



97 



6 



163 



3.0 



79 



102 



10 



191 



3.4a 



28 



101 



100 



229 



3.5 



89 



104 



11 



204 



4.0 



99 



103 



18 



220 



4.0 



105 



114 



16 



235 



4.0 



140 



127 



19 



286 



4.0 



124 



111 



10 



245 



5.0 



152 



133 



19 



304 



6.0 



177 



183 



25 



385 



7.0 



216 



146 



29 



391 



8.8 



245 



131 



100 



476 



'Walruses of kno\\ n age reared in captivit\-. 



tusks that had not been treated by either of those methods. Average annual 

 increments from the tusks of 11 males and 11 females (from which two or more 

 measurements were made) ranged from 33.4 mm at age 13 to 3.5 mm at age 33 

 for males (Table 15) and from 43.0 mm at age 8 to 2.5 mm at age 25 (Table 16). 

 Possibly, the increments from the females are atypical, for they were from the 

 only tusks in a large series that had well expressed annular ridges on the roots. 



The incremental rates derived graphically from Tables 13 and 14 and from the 

 mean increments in Tables 15 and 16 were plotted, and smoothed curves were 

 fitted to them (Fig. 85). The results for both sexes suggest that growth in length 

 of the tusks accelerates for about 2 years after birth and then decelerates in three 

 or four stages during the remainder of life. The final stage of rapid deceleration 

 seems to begin about the 28th or 29th year in males and about 10 years earlier in 

 females. In both sexes, the maximum increment (second year) is about 6 to 8 cm; 

 the most usual rate in adulthood is between 1 and 3 cm per year. In very old 

 animals, the increment may be as little as 1 mm per year. 



Rate of decrement. — To obtain an estimate of relative rates of decrement of 

 tissue from the tips of the tusks at different ages, I compared total length with the 



