Fat may be our undoing, but other animals thrive on it. The rump of a healthy adult polar bear is covered in a layer of the stuff up to 10 centimetres thick. The animal's diet consists mainly of seals, especially the thick blubber of adults. Its taste for fat is so strong that it will often go to all the trouble of killing a seal pup only to reject the lean body, eating nothing but the lipid- rich brain. Polar bears may look like bleached variations on a theme, but in adapting to their Arctic environment they have evolved a lifestyle that is very different from any other bear's.
Obesity is the key to their survival.
These are evolutionary upstarts. Their ancestors would have been omnivorous, like most modern bears, eating leaves, fruit and roots, together with a little fish, small prey and carrion. But in the high Arctic it is too cold, and for several months of the year, too dark, to sustain plants and the animals that feed on them. The sea is the only larder for a large, active mammal, and not a very accessible one for a doggy-paddling bear. By concentrating their hunting efforts on ringed seals, polar bears make the most of their talents.