ECOLOGY AND BIOLOGY OF THE PACIFIC WALRUS 



165 



Calhoun 1966), and the powerful cephalo-cervical musculature of the walrus, 

 are unlike those of any other pinniped. Each points to frequent use of the snout as 

 a digging organ. 



Some signs of digging by walruses were found during submarine exploration of 

 the St. Lawrence concentration area in March 1972 by Ray (1973) and S. W. 

 Stoker (personal communication). Many broad furrows and pits, about 25 to 

 30 cm wide and 15 to 20 cm deep, were sighted by them in the sandy-mud 

 bottom where several hundred walruses had been feeding not long before. 

 Identification of those features as having been made by feeding walruses was 

 circumstantial, since none of the animals actually was seen creating the furrows 

 and pits. However, all other possible causes, such as ice scouring, were reliably 

 excluded. 



Some of the walrus' prey that reside deep in the sediments may be obtained 

 without excavation. Those such as Spisula sp. and, especially Mya spp., extend 

 their siphon several centimeters above the sediment and are slow to react to 

 touch. In such instances, the walrus probably grasps the siphon in its mouth and 

 simply pulls, tearing the siphon away and leaving the body of the clam still 

 buried in place. When Mya is found in walrus stomachs, it ordinarily is repre- 

 sented by the siphon alone, occasionally with other bits of the mantle attached. 

 Usually, Spisula is represented only by the foot, but other parts (siphon, gills, 

 stomach) occasionally are present as well, suggesting that the whole clam was un- 

 earthed before the soft parts were removed. 



Selection of Prey 



Preliminary sorting of benthic materials and selection of prey from among 

 them probably is done mainly with the mystacial vibrissae. Each of these stout 

 hairs is provided at its base with a large sensory nerve ending and musculature 

 that permits extraordinary maneuverability, as noted earlier. According to 

 Murie (1871) the musculature of the snout and vibrissae is more highly developed 

 in the walrus than in any other carnivore. 



When captive walruses are feeding or exploring the bottom of their tank, their 

 vibrissae appear to have nicely coordinated tactile and motor functions. They 

 perform in much the same way as a pair of hands or two batteries of cilia, 

 continuously examining, sorting, and selecting the materials to be ingested or 

 rejected. The remarkable ability of captive walruses to sense and manipulate 

 food and other materials with the vibrissae has been mentioned many times (see 

 Howell 1930:77; Coates and Atz 1958), and I presume that free-living walruses 

 are no less remarkable in this respect. 



Ingestion of Prey 



More than half of the genera of prey that are eaten by the Pacific walrus are 

 soft-bodied or are taken when in a soft condition (e.g., molting crabs). These 

 usually are swallowed whole, though they often show signs of having been 

 pinched or compressed. They usually are not cut, torn, or crushed in a manner 

 indicative of mastication by the teeth. 



The genera that make up the greatest proportion by volume of the food are 

 mollusks, which usually are not ingested whole unless they are rather small (<3 

 cm in diameter). Apparentiy, the reason for this is that the walrus is unable to 



