ECOLOGY AND BIOLOGY OF THE PACIFIC WALRUS 



111 



those formed during years of rapid linear growth cover a much larger surface 

 area than any formed in years of slower growth. The rate of linear growth of the 

 tusks (hence, volume of cementum deposited) increases in the first 2 years, then 

 decreases in essentially three stages thereafter. I suspect that this is the key to the 

 rate of deposition of cementum on all of the other teeth. That is, the changes in 

 relative volume of cementum deposited on each of the cheek teeth appear to be 

 very similar to the changes in volume deposited on the tusks, where functional 

 need for perpetually renewed attachment is most evident. Hypercementosis has 

 been a consistent feature of the cheek teeth of odobenids only since the tusks 

 began to enlarge beyond the usual canoid condition (Repenning and Tedford 

 1977). 



Orientation and Shape of Tusks 



The tusks are deeply rooted in the anterior part of the maxillae and extend 

 downward from the corners of the mouth, on each side of the mandible. Their 

 orientation, relative to the horizontal plane of the skull, is not perpendicular; 

 they usually jut laterad and anteriad to some extent, rather than extending 

 straight downward (Fig. 75a, b). For example, in 27 adults the roots of the tusks 

 were inclined toward the midline of the skull at angles of 1 to 16° from the 

 sagittal plane (mean, 8.1°) and were inclined posteriad 9 to 20° (mean, 13.8°) 

 from the transverse plane. In each of these variables, there tended to be slightly 

 greater variation among males than among females, but the means for each sex 

 were comparable. 



Each tusk is more or less elliptical in cross section, usually with one shallow 

 longitudinal groove in the lingual (medial) surface and two in the labial (lateral) 

 surface. The long axis of the ellipse generally is oriented antero-posteriorly but 

 usually at an angle of 25 to 30°, rather than parallel to the midline (Fig. 75c). In 

 13 specimens, this angle ranged from 0 to 38° (mean, 28°), and the variation was 

 about the same in both sexes. 



Nearly all of the tusks that I examined had a slightly helical form, the right 

 canine with a dextral spiral and the left, sinistral (Fig. 75<i). The tusks of males 

 generally showed the least tendency to spiral. In nine males, the amount of spiral 

 per 50 cm of length ranged from none to 1/4-turn (0 to 90°), with a mean of 

 about 1/16-turn (22°); in 11 females, the range per 50 cm was from none to 

 1/2-turn (0 to 180°), with a mean of 1/10-turn (36°). 



The tusks are arched in the parasagittal plane, usually more so in females than 

 in males (Fig. 76, upper). I measured the radius of this arc on the posterior 

 surface of the tusks, using photographs of the lateral view of 30 skulls having the 

 tusks in place. Radii in 18 males ranged from 45.6 cm to more than 5 m (mean, 

 > 150 cm); radii in 12 females were from 22.6 to 142.5 cm (mean, 72 cm). 

 Expressed as a ratio of condylobasal length to radius of arc, the means for these 

 males and females were about 1:3.2 and 1:1.9, respectively. 



The tusks tend to be arched also in the paratransverse plane, though much less 

 than in the parasagittal plane. In frontal view, this slight paratransverse arc, 

 together with the helical form and parasagittal curvature, leads to the tendency 

 for distal convergence of the tips in most females (Fig. 76, lower). Because of 

 lesser helix and curvature in the males, their tusks tend to diverge. In the absence 

 of fractures, the tusks of many females that are 20 years old or older often are 



