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The polar bear is the largest member of the family Ursidae. Polar bears have a heavy build overall, large feet, and a longer neck relative to their body size than other species of bears. The fur is white, but the shade may vary among white, yellow, grey, or almost brown, depending on the time of year or light conditions. The vitamin A content of the liver ranges between 15,000 and 30,000 units per gram and is toxic to humans if consumed in any quantity. Polar bears are distributed throughout the circumpolar Arctic. In North America, their range extends from the Canadian Arctic Islands and the permanent multiyear pack ice of the Arctic Ocean to the Labrador coast and southern James Bay. The southern limit of their distribution in open ocean areas such as the Bering Sea or Davis Strait varies depending on how far south seasonal pack ice moves during the winter. Polar bears feed on ringed seals and to a lesser degree on bearded seals. About half of the ringed seals killed during the spring and early summer are the young of the year. These young seals are up to 50% fat by weight and are probably easy to catch because they are vulnerable and inexperienced. Polar bears are curious animals and will investigate human settlements and garbage. They have been observed to ingest a wide range of indigestible and hazardous materials, such as plastic bags, styrofoam, car batteries, ethylene glycol, and hydraulic fluid.