Polar Bears prey mostly on Ringed Seals. In the spring, the pups are easy prey. They also hunt Bearded and Harp Seals, Walruses, Beluga, Narwhals, and sea ducks, and very occasionally eat small amounts of plant matter. They travel long distances to stay with pack ice, and fast for long periods of time if food is not available. In Hudson Bay, the ice melts by late July and the bears fast until November, when it freezes again. They do not drink water or urinate the whole time. Pregnant females, who enter dens to give birth, continue fasting for another four months, living only on stored fat for eight months. Cubs are born in December, and are ready to follow their mother out of the den and learn to hunt by March or April. They are independent when they are about two-and-a-half years old, at which point the female mates again.

Links:
Mammal Species of the World
Click here for The American Society of Mammalogists species account
  More images, video and sound