Adult male polar bears, especially during mating season or when competing for food, may kill and even eat cubs or smaller bears.
In some cases, killer whales or Orcas have been observed hunting and killing polar bears, particularly in the water where the marine animals have an advantage.
The most dangerous animal of all, humans, while not a natural predator, have been known to kill polar bears for various reasons, including self-defence and hunting. Habitat destruction due to human activities is another factor impacting the polar bear population.
While not a direct threat to adult polar bears, walruses have been known to injure or kill polar bear cubs if they feel threatened.