Scientists say they believed it to be a new behavior, possibly the result of bears having to swim longer distances in the ocean because of reduction in the amount of ice used by the bears as seal-hunting territory, Britain's Daily Telegraph reported.
During the longer swims, traveling on the mother's back could be vital for the survival of the cubs, scientists say, as being on the mother's back means the cub's body is in direct contact with the adult's fur and a large part of the baby is out of the icy water, thereby reducing heat loss.
Young polar bears have not built up a sufficient layer of fat to keep them warm if they are swimming in the sea for a prolonged period of time, researchers say.
Polar bears are excellent swimmers but they hunt and breed on top of the sea ice, which has been in decline in recent years, wildlife experts say.
"As the arctic ice continues to melt, it is likely that polar bears are increasingly going to have to swim longer distances," Geoff York, polar bear specialist from the World Wide Fund for Nature, said.
"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 minimizes heat loss," he said.
"This reported behavior, and anything else that helps cub survival in those circumstances, is good news."