The polar bear is the world's largest meat-eating land animal. Polar bears are well adapted to hunt seals out on the ice. They are excellent swimmers, and the coldest water does not bother them. They wait by breathing holes if they pick up the scent of a seal. They can also break through the ice at thin areas. Sometimes they can sneak up on a sleeping seal. They have a very strong sense of smell, which they use to find prey out on the ice. Polar bears get most of their food when there is pack ice. They must eat and store fat before the ice is gone: they cannot catch seals in open water. After mating, females dig themselves into a den of snow on land for the rest of the winter. There, they have their cubs, usually twins. Bears usually eat fish, birds' eggs, seaweed, and dead whales, but they would rather have a meal of seal or walrus.