ECOLOGY AND BIOLOGY OF THE PACIFIC WALRUS 169 



Fig. 104. Longitudinal section of the mandible of a 5-year-old female, showing orienta- 

 tion of trabecLilae. (Photo by F. H. Fay) 



leaving the remainder of the clam still in my hand. I repeated this with the same 

 result. Thus, it was clear that the siphons of large clams could be removed by 

 suction alone, even by a walrus with no prex ious experience in benthic feeding. 



• In the second test, performed about 1 year later by G. C. Ray (personal com- 

 munication), Olaf was given several intact specimens of \I. arenaria, but these 

 were not held firmly in hand. The walrus took each clam in his lips, promptly 

 removed the siphons by suction, and ejected the broken shells (Fig. 105). The 

 shells seemed to have been broken during manipulation of the clams with the 

 snout and lips. Again, the animal demonstrated ver\- clearly that the siphons 

 could be removed by suction alone and, further, that the whole clams were not 

 taken into the mouth beyond the incisive area. 



• The third test was conducted by E. D. Asper and me at Marineland of the 

 Pacific in 1970, using an adult female Pacific walrus and her mate. Both animals 

 were about 9 years old and had been in captivity since infancy. Much of their 

 normal diet was made up of small, whole herring and bonito. which they 

 ingested by sucking them in, one by one. We attached a stiff, plastic tube to a 

 vacuum gauge capable of registering one negative atmosphere (-30 in. of 

 mercury) and inserted it into one of the herring, which the female took readily 

 into her mouth. The gauge read -10 in. Hg. Repeating this process several times 

 with each of the animals, we obtained readings on the gauge of up to -15 in. Hg 

 (0.5 negative atmosphere) but observed that the animals also were sucking in a 

 great amount of air around the tube. It was clear that the vacuum was stronger 

 than our measurements had indicated. Asper continued the trials intermittently 

 during the following year and reported (personal communication) that he had 



