Walrus ethology. II. Herd structure and activity budgets of summering males

Abstract

Male walruses, Odobenus rosmarus (L.), summering (outside the breeding season) at an Alaskan hauling ground, use all available kinds of beach habitat: cobble and boulder beaches, rock benches, and large boulders. Formation and dissolution of large herds on land can occur rapidly. Walruses are very gregarious and positively thigmotactic. In cool weather about 98% of walruses on land lie in passive body contact with other walruses. Dominant walruses (large, with long unbroken tusks) are most successful in entering herds on land, and in keeping positions in them. This results in overrepresentation of subordinates in the periphery of herds. Agonistic interactions occupy 5–10% of the time of walruses in large herds on land, and cause local disturbances that lead to agonistic involvement of up to 20 animals. Fewer kinds of social activity occur on land than in water. On land and in shallow water, small walruses are generally more active than large ones. Extensive body contact while they are hauled out is chiefly an adaptation for heat conservation and may also facilitate molting. The extreme gregariousness of walruses may have evolved because individuals joining large herds have a greater probability of achieving extensive body contact than have those joining small herds.