ECOLOGY AND BIOLOGY OF THE PACIFIC WALRUS 



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arms, and the muscles themselves are large and strong, capable of accom- 

 modating a large vertical force on the principal postcanine teeth (cf. Smith and 

 Savage 1959). The mandible of the sea otter also is structurally adapted for that 

 kind of occlusive force, inasmuch as it shows great thickening in an inverted arc, 

 roughly from the coronoid to P4, the largest crushing tooth in the jaw (Fig. 103). 

 In the walrus, the moment arm of the temporal muscle is somewhat smaller than 

 that of other otarioid pinnipeds, and that of the masseter-pterygoid is much 

 smaller. The temporal muscle is of moderate to large size; the masseter is very 

 small, and the pterygoid not much larger. The mandible is greatly thickened 

 along the anterior ramus of the coronoid, and this structural strength is carried 

 forward in an inverted arc to the symphysis and incisive area, rather than to the 

 canine-postcanine toothrow. The bone supporting the base of the postcanine 

 teeth is the thinnest part of the mandible. Each mandibular ramus of the walrus 

 is a nearly perfect example of a cantilevered parabolic girder, shaped to accom- 

 modate occlusive force principally on its anterior end (cf. Thompson, 1942:996), 

 with compensatory force exerted mainly by the temporal muscle. The pattern of 

 trabeculae within the mandible also seems consistent with that observation (Fig. 

 104). Additional upward force probably is provided by the buccinator muscle, 

 which is unusually large in the walrus and is situated about midway along the 

 jaw, where it has a large moment arm. 



The mandibular symphysis of the sea otter is not firmly ankylosed, as it is in 

 the walrus, but is joined by a soft fibrocartilaginous cushion which "seems to 

 function to facilitate fitting the teeth to the object that is to be crushed" (R. P. 

 Scapino in Kenyon, 1969:43). Scapino notes further that "this joint consistently 

 shows high mobility ... in those large carnivores that are powerful crushers." 

 In the walrus, the symphysial joint is fused and immobile, and there is no lateral 

 or vertical flexibility at the condyles. The mandible is capable only of vertical 

 motion. 



The incisive area is covered by an extraordinarily tough, firm gingiva, unlike 

 that on any other part of the mouth but closely resembling the skin on the palms 

 and soles of the flippers. This rough, cornified surface seems admirably suited for 

 grasping and holding slippery prey; I believe that it functions also to hold 

 molluscan shells while their contents are removed by suction. 



The hypothesis that walruses use suction to remove the soft parts from the 

 shells of mollusks was proposed first by Vibe (1950), who deduced this from 

 comparison of the stomach contents with intact mollusks. Vibe observed that 

 those bivalves whose shells are open at both ends, even when tightly shut, could 

 easily be sucked out, since "there is no great vacuum to conquer." He also 

 suggested that gastropods might be removed from their shell in the same way. 

 From a few simple tests done with walruses in captivity, I conclude that Vibe's 

 hypothesis is tenable. Those tests were as follows: 



• In June 1958, I was present in the New York Aquarium at a regular feeding 

 of Olaf , a 3-year-old male walrus that had been in captivity since infancy (Coats 

 and Atz 1958). At the time of my visit, his daily ration consisted entirely of soft- 

 shelled clams {Mya arenaria) that were prepared for him only by removal of the 

 shells. I selected one of the larger of those shell-free clams; holding it firmly in 

 my fist with only the siphon exposed, I offered it to the walrus. He took my fist in 

 his lips and, in an instant, removed the siphon by suction and swallowed it. 



