A den is a small cave or a hole dug out of the ground. The den must be big enough for the mother and her pups. It shelters them from the weather and protects the pups from other animals that may want to hurt them. Pups grow inside their mother for about sixty-three days. At birth they weigh about one pound and their eyes are closed.
Pups grow fast.
About twelve to fifteen days after they are born they open their eyes for the first time. By two to three weeks of age they can walk, and about a week after that they come out of the den for the first time. At first they live only on the milk from their mother. In a few weeks they eat more and more meat. This prey is brought to them in the stomachs of adult wolves.