ECOLOGY AND BIOLOGY OF THE PACIFIC WALRUS 



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• A female and calf on the ice were approached by a party of hunters, who 

 attempted to kill the female but succeeded only in wounding her. She dove into 

 the water, leaving the calf behind, and the latter was promptly captured alive by 

 the hunters. The wounded female then surfaced about 15 m away in an area of 

 gray ice about 15 cm thick. Sighting the calf and the hunters, she immediately 

 began chopping her way toward the group by vigorously thrusting her tusks 

 downward, breaking the ice before her. She continued until she reached the floe 

 where the calf was located. 



• A subadult female lay at the edge of a small floe on which several other 

 walruses were sleeping. She gazed first at the other animals, then at the water, 

 then began chopping at the ice near her with her tusks. After chopping away 

 some 3 to 5 kg of the edge of the floe, she rolled off into the water and swam 

 away. 



• An adult female surfaced in a round hole in ice that was about 20 cm thick. 

 The hole was only large enough for her head and neck. Immediately, she began 

 to abrade away the ice around her by pressing the shafts of her tusks against the 

 edges of the hole and swinging her head rapidly from side to side. This behavior 

 was similar to "ice-sawing" by Weddell seals (Leptonychotes weddelli) and 

 appeared to serve the same purpose. 



• On numerous occasions, I have seen walruses resting or sleeping in the water 

 with their tusks hooked over the edge of the ice, their body lying either 

 horizontally or vertically in the water. The tusks appeared to function both as a 

 prop for the head, keeping the mouth and nostrils out of the water, and as an 

 "anchor" preventing the animal from drifting away with the current. 



Clearly, the tusks are used in many ways in addition to their apparendy 

 primary function as social organs, but none of these as yet has been linked with 

 feeding. Their use in chopping and abrading ice has not been reported before, 

 although the Eskimos have been aware of this function for a long time. Use of the 

 tusks in these ways for creating and maintaining holes in the ice could be their 

 most important secondary function. Maintenance of access between air and 

 water is vital for survival, especially in wintering areas where leads and polynyas 

 often are scarce. 



Positive selective pressures and the potential for tusk development probably 

 have existed in all polygynous pinnipeds from the beginning. All but the late 

 Miocene to Recent odobenids, however, are (or were) piscivorous. Piscivory 

 requires a long gape that allows full use of the battery of pointed teeth for 

 capture of fast-moving prey. Obstruction of that gape by huge, laterally placed 

 tusks is disadvantageous in piscivory, hence evolutionary development of tusks in 

 piscivorous mammals probably tends to be prevented by strong negatively selec- 

 tive pressures. With benthic feeding, however, only the anterior end of the 

 mouth needs to be kept free from such obstructions; the integrity of the rest of the 

 gape becomes less important. This should tend to lessen the negative selection 

 and permit the positive pressures for tusk development to override them. The 

 odobenids' change from pelagic-piscivorous (much like present-day sea lions) to 

 benthic-molluscivorous feeding apparently took place in the Miocene epoch, 

 before walruses made the transit from Pacific to Atlantic. At the time of their 

 passage through the Central American Seaway in the Pliocene epoch, walruses 

 already had enlarged, tusk-like canines (Repenning 1976; Repenning and 



