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Due to its great size and tusks, the walrus has only two natural predators: the killer whale (orca) and the polar bear. The walrus does not, however, comprise a significant component of either of these predators' diets. Both the killer whale and the polar bear are also most likely to prey on walrus calves. The polar bear often hunts the walrus by rushing at beached aggregations and consuming the individuals crushed or wounded in the sudden exodus, typically younger or infirm animals. The bears also isolate walruses when they overwinter and are unable to escape a charging bear due to inaccessible diving holes in the ice. However, even an injured walrus is a formidable opponent for a polar bear, and direct attacks are rare. Armed with its ivory tusks, walruses have been known to fatally injure polar bears in battles if the latter follows the other into the water, where the bear is at a disadvantage. Polar bear–walrus battles are often extremely protracted and exhausting, and bears have been known to break away from the attack after injuring a walrus. Killer whales regularly attack walruses, although walruses are believed to have successfully defended themselves via counterattack against the larger cetacean. However, killer whales have been observed successfully attacking walruses with few or no injuries.