110 



NORTH AMERICAN FAUNA 74 



Fig. 74. Development of annual 

 length increments in tusks of male 

 Pacific walruses. Linear growth 

 completed (gg) in each month is 

 expressed as percentage of the 

 total increment in the previous 

 year (gj). Bars indicate range 

 and mean ±2 S.E. (open) and 

 ± 1 S.D. (solid) for each monthly 

 sample; numerals are sample sizes. 

 Curve is estimated mean, based 

 on assumed asymptote of 100%. 



Month 



even of teeth that are not gingivally erupted. The deposition of cementum on all 

 of the teeth commences in the first summer after birth, about the time when the 

 tusks first penetrate the gingiva but well before any of the other teeth have begun 

 to emerge. On the tusks, each increment of cementum is about as thick as its 

 predecessor, but on the cheek teeth, the second year's increment usually is thicker 

 than the first, the third is about as thick as the second, and the fourth and all 

 subsequent increments are thinner than their precedessors. Those formed on the 

 cheek teeth in the first 8 to 10 years usually are about twice as thick as those 

 formed in the next 10; those in turn are much thicker and less clearly delineated 

 than the layers formed in old age. These differences in thickness of increments 

 suggest either changes in dental function or changes in an intrinsic physiological 

 rhythm affecting cementum deposition. 



Cementum serves primarily to attach the tooth in its socket by embedding the 

 dental ends of the periodontal fibers, the opposite ends of which are anchored in 

 the alveolar bone. When the tooth shifts in position, as in eruption or in response 

 to changing stresses, the old fibers are replaced and more cementum is added for 

 renewed attachment. While the teeth continue to function, cementum is 

 continually added in response to such changes (Kronfeld 1938; Scott and Symons 

 1961). In man, who has an annual cycle of growth (Tanner 1962) but no other 

 outstanding annual physiological or dietary cycles, the cementum tends to 

 increase in thickness at a more or less constant rate (Zander and Hiirzeler 1958). 

 That is, the annual deposits are about equal in thickness. 



In walruses, cementum is deposited on the teeth in an annual cycle, but only 

 on the tusks are the annual layers equal in thickness. Those on the other teeth are 

 unequal, and their variation in thickness does not appear to be correlated with 

 any known characteristics of their function or development. The closest relation 

 of the thickness of layers on the cheek teeth seems to be with the changing 

 annual rates of linear growth of the tusks. Of all of the teeth, the tusks have the 

 greatest need for continual renewal of periodontal attachment, because of their 

 rapid growth and distad migration. Although the cementum increments on the I 

 tusks are all of about the same thickness, they are not all of the same volume, for 



140 

 120 

 100 



o 



o 80 



60 

 40 

 20 

 0 



I / 



55 



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