The polar bear's title as the top Arctic predator may under challenge from a shark. The species is already threatened by a thaw of ice around the North Pole.
Scientists researching how far sharks hunt seals in the Arctic were stunned in June to find part of the jaw of a young polar bear in the stomach of a Greenland shark, a species that favours polar waters.
"We've never heard of this before. We don't know how it got there," Kit Kovacs, of the Norwegian Polar Institute, told Reuters of the 10 cm bone found in a shark off the Norwegian Arctic archipelago of Svalbard.
"We can't say whether or not the shark took a swimming young bear" or ate a carcass, she said. "We don't know how active these sharks are as predators."
Most shark experts contacted said it was likely the bear was dead before the shark found it. Even a young, two- or three-year-old bear would be a ferocious opponent for a Greenland shark, which can grow to up to 7 metres and weigh more than a tonne.
"It sounds like a scavenge," said Steve Campana, head of the Canadian shark research laboratory at the Department of Fisheries and Oceans.
He said he had not heard of a shark eating a bear before and it was a "million dollar question" for researchers as to whether Greenland sharks attack live bears.
Bits of animals including caribou have been found in Greenland shark stomachs in the past -- scavenged or attacked swimming.
Campana said there was even a myth that the sharks could leap out of the water and seize caribou standing on ice.
"There's no possibility a Greenland shark could predate a live adult white bear unless it was injured or seriously ill," said Jeffrey Gallant, co-director of a Canadian-based Greenland shark education and research group.
Sonja Fordham, deputy chair of the shark specialist group of the International Union for Conservation of Nature, said more research was needed into the Greenland shark's habits.
"Greenland sharks do seem quite sluggish ... but they have been known to move very quickly when they are eating," she said.
The United States this year listed polar bears as threatened under the U.S. Endangered Species Act because their sea ice habitat is shrinking, apparently due to global warming.
A thaw may mean that bears spend more time in the water.
But less chill waters are unlikely to lure other big sharks, except perhaps the porbeagle, to polar regions, Campana said.
Most sharks favour much warmer conditions. Killer whales, however, have been spotted further north in recent years.
"Both walruses and polar bears are powerful in the water. Both could handle most potential predators, but not killer whales," Kovacs said.
Gallant said warming was unlikely to help the Greenland shark catch bears. "The Greenland shark simply cannot afford the risk of injury nor the expenditure of energy required to kill such a large and dangerous animal, with or without the help of global warming," he said.
"There is far easier prey to be found." Kovacs also said: "For polar bears the greater risk is a loss of habitat. These other things will be ancillary."