Polar bears have been spotted carrying their cubs on their backs while they swim through icy waters, researchers said yesterday.

The phenomenon, revealed while tagging and tracking polar bears, is thought to be new and the result of the bears having to swim longer distances in the sea because of reductions in the Arctic ice in the summer.

The scientists say that in the face of the longer swims, travelling on the mother’s back could be vital for the survival of the cubs in waters surrounding scattered ice, which is prime seal-hunting territory for the animals.

Travelling on the mother’s back will mean the cub’s body will be in direct contact with the adult’s fur and a large part of the baby will be out of the icy water - reducing heat loss.

This is important because the young polar bears have not built up a sufficient layer of fat to stop them getting cold if they are swimming in the sea for a prolonged period of time.

The data from the WWF-Canon polar bear tracker programme, which has been running since 2007 in the Arctic, indicate that tagged bears in Alaska have swum around 350 to 400 miles in the past four years.

Radio collars, which can only be fitted on the females as the males’ necks are wider than their heads so the collars slide straight off, send signals via satellites monitoring the bears’ movements to track their behaviour and help determine how they are affected by climate change.

Polar bears, the largest land predators in the world, are excellent swimmers but they hunt and breed on top of the sea ice, which in the Arctic has been in decline in recent years.

Geoff York, polar bear specialist from conservation charity WWF, said: “As the Arctic ice continues to melt, it is likely that polar bears are increasingly going to have to swim longer distances.

“Data from tagged bears near Alaska has indicated swims of 350-400 miles in the past four years and if polar bear cubs are forced to cover these distances, then it is vital for them to behave in a way that minimises heat loss.