Shortly before giving birth, a female wolf will seek out a den for her pups. A den can be many things, a hollow log, upturned tree, rock crawl space, or cave. They also can be recycled from other animals, such as a bear, fox, or as shown, beaver den.
Dens are a meeting place for the pack as well as the primary place of care for young pups. The den, however, is not a perminent place, a pack usually has other dens for other seasons and prey migrations.