Shortly before the puppies are born, the pregnant she-wolf looks for a cave in which she gives birth to her offspring and houses them for protection. A wide variety of loopholes are useful for this, such as deep riverbank hollows, cavities under fallen trees or rock overhangs, hollow tree trunks, a gap between rocks and so on. Sometimes wolves also use abandoned burrows of other animals such as bear caves or beaver dams.
Often caves have been used by wolves for generations. A cave in Jasper National Park was used eight times in 15 years. Another on Ellesmere Island may be even older. Bones up to 783 years old have been found in it.
In an ordinary wolf cave, the birth chamber is at the end of a tunnel that can be up to 5 meters long in soft earth. The cave itself is often slightly elevated from the rest of the tunnel. The birth chamber has a diameter of about 90cm and is about 60cm high.
The cave entrance has a width of about 50cm to 70cm and a height of about 38cm to 50cm. Often the cave is located near a water source and at an elevated place so that the wolf can spot approaching enemies early.
The alpha male stays outside the cave for protection and often plays the decoy to lure approaching predators such as bears and eagles away from the cave, whose prey can be a very young puppy. Although wolves often defend their den against wild predators, they will usually run away when humans approach them.
Over the years, wolves change their burrows and relocate with their offspring, covering a distance of more than 1.6km. This happens either due to a parasitic infection of the cave or disturbance from humans or other animals.