Global warming may not be the only threat to the polar bear. Scientists are puzzling over the discovery of the jawbone of a young polar bear in the stomach of a Greenland shark, a species that thrives in the cold waters of the far north.

The find suggests that the polar bear may have a serious challenger to its place at the top of the Arctic food chain. Until now, only killer whales were thought to offer a threat to Ursus maritimus as the Arctic's top predator.