ECOLOGY AND BIOLOGY OF THE PACIFIC WALRUS 



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(e.g., Bel'kovich and Yablokov 1961:52; King 1964:40; Walker et aL 1964:1297). 

 Some doubt was cast by Brooks (1954:59) on the importance of the tusks in 

 feeding, for he observed that "an old bull walrus . . . that had no tusks 

 whatever" was in "healthy condition," from which he cautiously concluded that 

 "the tusks may not be absolutely necessary for food procurement." I have seen 

 several adult walruses that lacked tusks, and numerous others in which the tusks 

 were very short. The physical condition of each animal was comparable to that 

 of its tusk-bearing companions, seemingly confirming that tusks are not essential 

 for obtaining food. 



Mansfield (1958a:69) judged that the predominant anterolateral abrasion of 

 the tusks suggested that the animal uses them for stirring up the benthic 

 sediments while "standing on its head and rotating its tusks from side to side." 

 Loughrey (1959:40) observed further that the abrasion pattern "seems to indicate 

 that [the walrus] . . . ploughs or scrapes the bottom with its tusks by a sideways 

 and forward thrusting motion of the head." He rejected as untenable Johansen's 

 (1912) report of the animals moving "backward" and raking the bottom with 

 their tusks, for he observed that the posterior surfaces were not abraded in accord 

 with that procedure. My findings also offer no support for the digging and raking 

 hypothesis. 



Close inspection of the surfaces of the tusks has revealed that the direction of 

 movement of the majority of abrasive particles is from proximal to distal, as 

 would occur if the animal moved forward along the bottom, dragging its tusks in 

 the sediments. This pattern is consistent also with that of abrasion on the vibris- 

 sae. Furthermore, the site of maximum abrasion of the tusks is on the anterior- 

 most arc, which in adults usually is some distance away from the tips and on a 

 plane with the front of the snout. The position of the tips relative to that plane 

 seems to determine their rate of linear decrement, for the highest rates occur in 

 young animals, in which the tips extend to or beyond the rostral plane. 

 Thereafter, as the tusks increase in length and begin to recurve, the brunt of the 

 abrasive force is taken up on the shafts, rather than the tips, and the rate of linear 

 decrement diminishes. This probably is the explanation for the lower rate of 

 decrement in females than in males, for the tusks of females tend to be more 

 strongly arched than those of males; hence their tips should be less affected. 



Evidence of side-to-side motion, as hypothesized by Mansfield (1958a), was 

 absent on the specimens that I examined. Virtually all of the signs of abrasion 

 indicated forward motion of the animal, with tusks and snout pressed against the 

 bottom. The characteristics of the skin on the face, the abraded surfaces of the 

 vibrissae, and the abrasion on the tusks all indicated to me that digging, if done 

 at all, is accomplished by "rooting" with the upper edge of the snout, rather than 

 with the tusks. 



Relation to Social Behavior 



The primary role of the tusks probably is a social one. The importance of 

 tusks, horns, antlers, and other such appendages as signals of social rank in 

 terrestrial mammals has been recognized in a general way for a long time and 

 confirmed by several detailed studies (Verheyen 1954; Espmark 1964; Geist 

 1971). Although such appendages often function secondarily as weapons, their 

 primary value is in ritualized dominance-threat displays in which fighting per se 



