ECOLOGY AND BIOLOGY OF THE PAGIFIC WALRUS 103 



Fig. 69. Pressure-induced resorption of alveolar bone in a walrus reared in captivity 

 on a shell-free diet of fishes and clam meats. Arrows indicate sites of occlusion of ?£ and 

 P3 with the maxillary gingiva adjacent to P^ and P^. Ventral views with mandible in 

 place and teeth occluded (left) and with mandible removed (right). (Photos bv G. C. 

 Kelley) 



not available to the animals reared in captivity, or by some feeding procedure 

 that the captives did not or could not employ. 



The skulls of several of the captives also showed the same kind of "basin-like 

 excavations" in the alveolar bone, lateral to the upper teeth, which Cobb 

 (1933:657) attributed to "pressure ... by a hard substance in the food" 

 (meaning clam shells). In these captives, as well as in the free-living walruses, I 

 found that such depressions in the bone occurred only where the mandibular 

 teeth occluded with the maxillary gingiva (Fig. 69). Although these basins are 

 indeed sites of pressure-induced resorption and remodelling of the bone, the 

 causative pressure appears to be from the teeth themselves, rather than from any 

 intervening, hard substance. 



