By Ella Davies
A polar bear swam continuously for over nine days, covering 687km (426 miles), a new study has revealed.
Scientists studying bears around the Beaufort sea, north of Alaska, claim this endurance feat could be a result of climate change.
Polar bears are known to swim between land and sea ice floes to hunt seals.
But the researchers say that increased sea ice melts push polar bears to swim greater distances, risking their own health and future generations.
In their findings, published in Polar Biology, researchers from the US Geological Survey reveal the first evidence of long distance swimming by polar bears (Ursus maritimus).
"This bear swam continuously for 232 hours and 687 km and through waters that were 2-6 degrees C," says research zoologist George M. Durner.
"We are in awe that an animal that spends most of its time on the surface of sea ice could swim constantly for so long in water so cold. It is truly an amazing feat."
Although bears have been observed in open water in the past, this is the first time one's entire journey has been followed.
By fitting a GPS collar to a female bear, researchers were able to accurately plot its movements for two months as it sought out hunting grounds.
The scientists were able to determine when the bear was in the water by the collar data and a temperature logger implanted beneath the bear's skin.
The study shows that this epic journey came at a very high cost to the bear.
"This individual lost 22% of her body fat in two months and her yearling cub," says Mr Durner.
"It was simply more energetically costly for the yearling than the adult to make this long distance swim," he explains.
Swimming long distances puts cubs at risk
Mr Durner tells the BBC that conditions in the Beaufort sea have become increasingly difficult for polar bears.
"In prior decades, before 1995, low-concentration sea ice persisted during summers over the continental shelf in the Beaufort Sea."
 | POLAR BEAR FACTS  Polar bears are the world's largest land carnivores They have black skin and transparent hairs but appear white, turning yellow with age On land, they can reach up to 40 kph (25 mph) when sprinting short distances to catch prey  |
"This means that the distances, and costs to bears, to swim between isolated ice floes or between sea ice and land was relatively small."
"The extensive summer melt that appears to be typical now in the Beaufort Sea has likely increased the cost of swimming by polar bears."
Polar bears live within the Arctic circle and eat a calorie-rich diet of ringed seals (Pusa hispida) to survive the frozen conditions.
The bears hunt their prey on frozen sea ice: a habitat that changes according to temperature.
"This dependency on sea ice potentially makes polar bears one of the most at-risk large mammals to climate change," says Mr Durner.
The IUCN red list identifies polar bears as a vulnerable species, citing global climate change as a "substantial threat" to their habitat.
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