Polar - and grizzly bears are rather similar, opportunist omnivore mammals and the largest land carnivores. However the polar bear gave up hibernating, and is forced south by the warming climate, which causes the vital ice to melt, and allows grizzlies to expand north. So now their diets and hunting grounds overlap, with each-other and with humans - they even roam in towns. Life has grown even harder for polar bears, especially in summer.