Over many tens of thousands of years, polar bears evolved to be uniquely well-adapted for a very narrow ecological niche. Their physical features provide superb protection from the cold: they have extremely dense fur and almost no exposed skin, their thick layer of fat provides excellent insulation, and their low surface-area-to-volume ratio helps retain heat. In addition, they are well-designed to walk on ice and snow, and swim in frigid waters. Huge feet function like snowshoes and make good paddles for swimming; bumpy foot pads provide traction on slippery surfaces; short, curved, sharp claws grab the ice effectively; and a blubber-wrapped body has excellent buoyancy.