The polar bear builds its warm home in the winter each year. The pregnant polar bear begins in November to construct the den and prepare it for the birth. The cubs are being born during the Arctic winter. The bear itself has a thick coat which would keep it warm even in the harshest Arctic winter. The cubs are born blind and with short thin hairs which definitely would not keep them from being frozen.

The last year's snow which is compact and dry during several months of settling will be the ideal location for digging a den. The bear digs an entrance of 1.8 m long with its huge paws and she chooses to do it in a snowdrift. Then she builds a raised threshold and a vaulted chamber.

The researchers also found dens with more rooms than one. The biggest they ever found measured 13 yd (12 m) long and with five chambers.

Through the warmth of the mother's body the temperature can be as much as 21oC (37oF) higher than the outside temperature of -30oC (-22oF). Some dens have ventilation holes but it is also thought that it could occur accidental because the bear scrapes snow off the ceiling to drink.

Between December and January the bear give birth to one or two cubs. She never leaves the den. Cubs are 10 in (25cm) long and weigh up to 2 lb (1 kg). The cubs open their eyes when they are about five weeks old. They start to walk when they are about seven weeks old.

The mother bear also suckles her cubs not only lying down but sitting up and holds them each in the bent of her forelegs. The polar bear has four nipples. Since she doesn't eat in all this time and lives of the built-up fat; she sleeps a lot while the youngster play together.

The babies get suckled for two and half years.

On observation it was noticed that the mother bear keeps her den spotlessly clean all the time. She herself does only urinate very little or faeces. She only drinks by scraping the snow from the ceiling. She also scrapes snow from the ceiling to cover the cubs' excreta.

In March the mother bear can see the light through the ceiling and realizes the dark winter is coming to end. She is also almost starved to death and therefore breaks through the snow and comes out of her den. The cubs follow and she leads them to the open sea and gets a long awaited seal for her meal.