As many as five dozen walruses fell to their deaths this week from high bluffs in a southwestern Alaska wildlife refuge, leaving biologists wondering why.

The mysterious behavior appears to be unique to a single part of the 4.3 million-acre Togiak National Wildlife Refuge, where the walruses have been plunging to their deaths for three years.

"We're still trying to figure out why this is happening," refuge manager Aaron Archibeque said Thursday.

Between 41 and 60 walruses died Tuesday after dropping off cliffs up to 100 feet high above a haulout, or beach area, at Cape Peirce, the U.S. Fish and Wildlife Service said. The animals were part of a huge herd of non-mating walruses that gather at the haulout each summer to eat and sun themselves.

Cape Peirce, about 460 miles southwest of Anchorage, is the busiest mainland haulout in North America, with as many as 12,500 walruses visiting at one time.

Late Tuesday morning, biologists monitoring the herd said they spotted 225 walruses making their way up a slope leading from the beach to the cliffs. The workers were able to turn back 155 of the giant tusked pinnipeds, but about 70 had already reached the cliff's edge and were left alone for fear of scaring them.

In the next 5 1/2 hours, 45 walruses reportedly fell off the cliffs and either died or suffered serious injuries.

"They tried to take a shortcut back to the haulout and ended up tumbling off the cliff," said Joel Miller, a Fish and Wildlife Service biologist in Anchorage. By Wednesday morning, even more walruses had fallen.

One of the federal workers videotaped several walruses sliding helplessly down a steep grassy slope before dropping about 15 feet onto other walruses, rocks or sand. Two others managed to stop their momentum before the edge and inch their way back up the slope.

Archibeque said that because of their weight, which can exceed 2,000 pounds, the walruses can die or suffer massive internal injuries in even a relatively short fall.

At least 42 walruses fell off the same cliffs in 1994 and at least 17 more last year, Archibeque said.

Based on the first two years, biologists theorized that the walruses headed to higher ground to get more protection from high winds and stormy weather. But this year, Archibeque said, the weather was warm and clear with calm winds.

Archibeque said a high sand dune that discouraged movement up to the cliffs has blown away in recent years, and that walrus traffic has created something of a path that may be attracting others.

He said this is the first time this kind of behavior has been documented, but given the small numbers involved and the healthy walrus population in Alaska waters, federal managers aren't concerned from a biological standpoint. "It's more that from the human side that people are concerned about walruses dying," he said. CAPTION: U.S. Fish and Wildlife Service rangers observe walruses on a beach in the Togiak National Wildlife Refuge at Cape Peirce, where dozens of the pinnipeds have perished this week.