Polar bears are superb swimmers. But their hunting ground is the surface of the Arctic ice, where resting seals make easy and calorie-rich prey. In the summer though, a swim between ice floes to catch seals can turn into a marathon.



The researchers fitted the female bear with a radio collar, and tracked her as she swam continuously for nine days - the longest polar bear swim ever recorded. She covered almost 700 km in waters as cold as 2C. Then she hauled herself out of the water and travelled a further 1,800 km - intermittently swimming and walking on the surface of the ice.



When the team recaptured the bear, she had lost almost a quarter of her body weight and her year-old cub, which had been travelling alongside her, had disappeared.



The scientists say that the retreating Arctic ice could be forcing polar bears to take greater risks in search of food, embarking on ever more perilous long-distance journeys.



Victoria Gill, BBC News