Swimming polar bear mother with two cubs, Svalbard. Polar bears are technically marine mammals. They spend most of their lives on the frozen surface of the sea, where they hunt for the seals which they feed on almost exclusively. In summer, when the ice breaks up into smaller floes or melts away entirely the bears are obliged to swim to the next ice floe or to dry land. They are good swimmers and have always been known to do this. However, now the ice is breaking up earlier and re-freezing later, bears are both having to swim more and go for longer periods without feeding. This hits cubs particularly hard and scientists have found a clear link with poor sea ice cover and poor cub survival

Photograph: BBC