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ANCHORAGE - Musk ox kills by brown bears in the Arctic National Wildlife Refuge have dramatically increased over the past decade, according to studies by federal and state biologists.
A number of individual Arctic grizzlies have learned how to stalk and take down the shaggy animals, said ANWR ecologist Patricia Reynolds, with the U.S. Fish and Wildlife Service.
"I think the reason that bears are efficient predators is because they are adaptable, and they have the ability to switch to whatever is out there," Reynolds said.
In at least 10 instances, the bears killed two to seven musk oxen at once, possible examples of a relatively rare phenomenon biologists call "surplus killing," where a predator kills more prey than can be consumed immediately.
"This really surprised us," Reynolds told scientists at an Arctic science conference in Fairbanks last week, where she presented a paper called, in part, "A Search for Weapons of Muskox Destruction."
In an article published last year in the journal Ursus, Reynolds and two co-authors reported that 28 of 46 known musk ox deaths took place during multiple kills, with most occurring since 1999.
Bears don't usually engage in such surplus killing and musk oxen can be exceedingly dangerous prey, able to gore attacking bears with thrusts from their powerful heads and their sharp horns. At least one bear is thought to have died as a result of battling musk oxen on the tundra, and others have been wounded.
"It's a risk to them, and that's the interesting trade-off," Reynolds told the Anchorage Daily News.
The rise in bear predation comes as ANWR musk oxen have crashed in number, from a high of 386 in 1986 and an average of about 325 in the early 1990s to an estimated 50 this year.
Where bear kills once accounted for virtually no musk ox deaths, predation took an estimated 13 percent of the ANWR herd in 2003.