ECOLOGY AND BIOLOGY OF THE PACIFIC WALRUS 



145 



of the demands of thermoregulation in a cold environment (Fay and Ray 1968; 

 Iversen and Krog 1973). 



After the calves begin to take solid foods, their daily rate of intake of milk/kg 

 TBW probably declines, but the volume ingested per day may increase for 

 several months, as their body weight increases. The nutritional demands on the 

 mother, during that period, undoubtedly are much higher than they would be 

 for her own maintenance. In other mammals, lactating females may consume 

 one to two times more food per day than do non-lactating animals of comparable 

 size (Brody 1945; Kleiber 1961). ' 



Diet After Weaning 



The principal foods eaten by walruses (other than by suckling calves) are 

 mollusks, mainly the bivalves Mya truncata, Astarte horealis. Serripes groen- 

 landicus ( = Cardiiim groenlandicum), and Hiatella arctica ( = Saxicava arctica) 

 (Johansen 1912; Tsalkin 1937; Nikulin 1941; Vibe 1950; Brooks 1954; Mansfield 

 1958a; Loughrey 1959). In the past there has been some debate over the 

 presumed herbivorous habits of walruses (Allen 1880). More recent observations 

 by several investigators have established that marine vegetation is not an 

 important part of the diet, though it may be ingested incidentally in small 

 amounts. Many kinds of benthic invertebrates and even a few vertebrates are 

 eaten. None other than mollusks appear to be of more than secondary or tertiary 

 importance by volume, but some prey that are taken in small quantities may be 

 of critical importance because they contain needed trace elements or other 

 nutrients that are scarce or absent in mollusks. 



Kinds of Prey 



More than 60 genera of marine organisms, representing 10 phyla, have been 

 identified as prey of the Pacific walrus (Table 23). A few examples from each 

 phylum will be discussed here, in regard to their distribution, habitat, and 

 relative abundance, based on Ricketts and Calvin (1948), MacGinitie and Mac- 

 Ginitie (1949), Pavlovskii (1955), Filatova (1957), MacGinitie (1959), Savilov 

 (1961), Sparks and Pereyra (1966), and Petersen (1978). The purpose of this 

 description is to illustrate the wide variety of life forms eaten by the Pacific 

 walrus and to provide a basis for consideration of the kind of effort required of 

 the walrus to obtain them. 



Coelenterata. — The soft coral Eunephthya rubiformis (Fig. 96a) is a tough, 

 sessile colony of rosy to translucent polyps, usually 3 to 7 cm in diameter when 

 retracted. It attaches to stones or molluscan shells on the sea bottom at depths 

 from less than 10 to more than 200 m and is widely distributed in the Bering and 

 Chukchi seas. I have found it frequently in beach windrows on western St. 

 Lawrence Island, which suggests that it is abundant there, near shore. E. 

 rubiformis was present in only 1 of 51 walrus stomachs that I examined from that 

 area, but I found fragments of it in feces from several other individuals in the 

 same locality. 



Annelida. — At least five genera of polychaetes have been identified as prey of 

 the Pacific walrus. These include both predaceous and detritophagus forms that 

 reside in tubes or burrows in the surface layers of sediments at water depths from 



