Red foxes breed during February and March. Their breeding habits vary widely. They have been observed exhibiting monogamous breeding behavior, but males will also breed with multiple females.
The den is a hole in the earth, 15 to 20 feet long,
usually located on the side of a knoll. It may have
several entrances. Sometimes foxes dig their own dens.
More often, though, they appropriate and enlarge the
homesites of small burrowing animals, such as marmots.
They also will use abandoned wolf dens. Conversely, wolves
may enlarge and use a fox's den.
Within the den is a grass-lined nest where well-furred
but blind babies, called pups, are born after a gestation
of 51–54 days. A litter of four pups is common, though a
litter of 10 is not a rarity. At birth, pups weigh about
four ounces. Normally only one litter is born each year.
The pups' eyes open eight to 10 days after birth. The
young leave the den for the first time a month later. The
mother gradually weans them, and by the time the pups are
three months old, they are learning to hunt. Both parents
care for the young. The family unit endures until autumn,
when it breaks up and each animal is on its own.