126 NORTH AMERICAN FAUNA 74 



Table 18. Estimated rates of decrement from tusks of female Pacific walruses. 



Tusk length (cm) Decrements (cm) 



Age Sum linear 



(years) Total^ increments^ Total Annual rate 



0 



4.3 



4.3 



0.0 



0.0 



1 



9.2 



9.2 



0.0 



0.0 



2 



14.3 



15.0 



0.7 



0.7 



3 



18.8 



20.4 



1.6 



0.9 



4 



22.4 



25.5 



3.1 



1.2 



5 



26.0 



30.2 



4.2 



1.1 



6 



29.5 



34.5 



5.0 



0.8 



7 



32.6 



38.3 



5.7 



0.7 



8 



35.7 



41.6 



5.9 



0.2 



9 



38.5 



44.5 



6.0 



0.1 



10 



41.1 



47.0 



5.9 



0.0 



11 



43.6 



49.3 



5.7 



0.0 



12 



45.6 



51.4 



5.8 



0.0 



13 



47.8 



53.4 



5.6 



0.0 



14 



50.0 



55.2 



5.2 



0.0 



15 



51.8 



57.0 



5.2 



0.0 



16 



53.6 



58.6 



5.0 



0.0 



17 



55.0 



60.2 



5.2 



0.0 



18 



56.4 



61.5 



5.1 



0.0 



19 



57.3 



62.6 



5.3 



0.0 



20 



58.0 



63.5 



5.5 



0.0 



21 



58.7 



64.1 



5.4 



0.0 



22 



59.0 



64.9 



5.9 



0.0 



23 



59.1 



65.0 



5.9 



0.0 



24 



58.9 



65.3 



6.4 



0.4 



25 



58.7 



65.6 



6.9 



0.5 



^Total lengths are estimated means from Fig. 83. 

 ^^Sum of linear increments from fitted curves in Fig. 85. 



Fractures tended to occur most often in males who had curved, highly divergent 

 tusks (Fig. 86). Fractured tusks also tended to occur more often in mature males 

 (94/267) than in subadult males (10/103). This difference probably reflects 

 increasing likelihood of fracture with increasing age. Major fracture of the tusks 

 of females seems to occur less often than in males, possibly because females avoid 

 haulouts on rocky shores or engage in less vigorous fights. In a sample of 79 adult 

 females sighted at sea, only 2 showed fractures; in both, these were of one tusk. 



I saw bilateral fractures of the tusks about 35 to 40 cm distal to the gingiva in 

 several old females (Fig. 87, right). The site of these fractures was at or just distal 

 to the most anterior point in the parasagittal arc. In old females, the loss of tissue 

 by frontal abrasion in that area results in great reduction in thickness of the 

 shaft. Sometimes the shaft is thinner there than near the tip (Fig. 87, left). 

 Moderate pressure or light impact are sufficient to break the tusk at this weak 

 point, as I discovered accidentally while examining one dead specimen in the 

 field. The development of the weak point seems to be due to two factors: frontal 



