168 



NORTH AMERICAN FAUNA 74 



5 cm 



Fig. 103. C()niparati\e mandibular nit'chanics in the walrus (upper) and sea otter (lower). 

 The approximate relati\e strength and direction of forces applied b\" the temporal 

 muscle (T) and by the masseter and pterygoid muscles (M), in relation to their moment 

 arms (mj, m2), suggest that the sea otter's mandible and jaw musculature are adapted to 

 exert a large crushing force (O) principalis- on the fourth postcanine, which is the largest 

 functional tooth in the jaw. The shape of the otter's mandibular ramus also indicates 

 maximal structural strength in the area of P3 and P4. Conversely, the walrus' T-M 

 musculature appears to be incaj^able of appK'ing a crushing force on the cheek teeth but 

 capable of exerting a weak percussi\e force in that area. The parabolic beam of the 

 walrus' jaw resembles tho.se of other otarioid pinnipeds and suggests adaptation to 

 maximal or most frecjuent occlusi\'e force (O) on its anterior end. Although the T-M 

 musculature can exert only a weak percu.ssix e force in that area, some additional pressure 

 probabK- can be exerted b\- the unusualh' powerful buccinator muscle (B). 



