ECOLOGY AND BIOLOGY OF THE PACIFIC WALRUS 



95 



p3 p2 pi i3 ^^^^^ f 



V^XV^V^ Wers slightly 3 



p p p C behind lowers 

 •^4 "^3 ^2 1 



SV^HM P^slightly 1 

 behind lowers 



End to End 

 evenly spaced 



10 



/V/V/H)^ P^.VsHghtly 1 

 ahead of lowers 



WrVV/V I^^lpfpz slightly 

 ahead of lowers 



Mixed 



/V/VrV P'P'P' ^''^^^'y 



' ' ' " ' ' ' ahead of lowers 



A^/VAf'/V Wers sMghtly 

 ' ' ' 1 ' W 1 QhgQ^ lowers 



^yWyAy uppers slightly 

 ahead of lowers 



Fig. 63. Frequency of occurrence (f) of eight occlusal patterns identified in the cheek 

 teeth of Pacific walruses (10 males, 20 females). 



and intermediate occlusion, in which the lower teeth tended to be ahead (1) or 

 behind (11) the upper (Fig. 63). An extreme example of intermediate or 

 "overlapping" occlusion, in which each lower tooth but P4 (which was absent) 

 occluded with two uppers, was illustrated by Cobb (1933: Plate IV). Cobb 

 concluded that overlapping occlusion, because of its resemblance to that in other 

 mammals, was more primitive than the end-to-end type. 



The cheek teeth are incapable of chewing or grinding, other than by vertical 

 action, for the mandible is hinged in such a way as to prevent side-to-side or 

 forward-and-back motion. Furthermore, the mandibular symphysis, the "third 

 joint" of the jaw (Scapino 1965), becomes solidly ankylosed at an early age. 

 Thus, the occlusive stresses on the teeth are exclusively vertical-percussive. 



Although the cheek teeth of the walrus generally have been regarded as 

 "crushers," presumably for breaking the shells of clams, they differ greatly from 

 the cheek teeth of other mammals in which crushing is known to be the primary 

 or secondary function. First, the crowns are more or less conical and unicuspid; 

 second, their protective enamel cap is extremely thin and fragile, and usually is 

 lost entirely before the animal reaches adulthood; third, they lack a cingulum, 

 which would provide added protection to the peridental gingiva from damage 

 by the sharp edges of broken shells; and fourth, they all are single-rooted and 

 mostly set obliquely, which would seem to be comparatively disadvantageous for 

 absorbing the stresses of pounding and crushing. 



With early loss of the protective enamel, each tooth is composed only of dentin 

 and cementum, both of which are appreciably softer in the walrus than the same 

 tissues are in the teeth of other carnivores. Since they also are softer than the 

 shells of most of the mollusks that this animal consumes, one would expect the 

 teeth to be worn down very rapidly if clam-crushing were a regular function. Yet 

 the rate of decrement from the crown by attrition and abrasion is remarkably 

 low, as is shown in the next section of this report. Furthermore, in the absence of 

 a protective cingulum, one would expect the peridental gingiva and bone, 

 especially about the premolars, to show either extensive damage or counteracting 

 adaptive changes in structure, yet neither is evident. With the exception of P 



