Polar bears depend on sea ice.
Polar bear . Although it is the sister species of the brown bear , it has evolved to occupy a narrower ecological niche , with many body characteristics adapted for cold temperatures , for moving across snow , ice and open water , and for hunting seals , which make up most of its diet . Although most polar bears are born on land , they spend most of their time on the sea ice . Polar bears hunt their preferred food of seals from the edge of sea ice , often living off fat reserves when no sea ice is present . Because of their dependence on the sea ice , polar bears are classified as marine mammals ; an alternative basis for classification as marine mammals is that they depend on the ocean as their main food source .