Black bears looking for winter dens

By Amy Renfranz (

Article Published: Nov. 29, 2012 | Modified: Nov. 29, 2012
Black bears looking for winter dens

A bear cub reaches new heights.

Photo submitted

Female black bears of the High Country are currently looking for a good den site for the winter. It is there that they will rest for the coldest of the winter months.

The winter den is also where they will have their cubs, though the process is quite different from the human reproductive cycle.

Female black bears are pregnant for just two months. However, they mated in July and will not give birth until January. Scratching your head yet?

These bears have the amazing adaptation of delayed implantation.

After mating in the summer, the fertilized egg develops, divides a few times, and develops into a ball of cells called a “blastocyst.” After this, development of the egg comes to a stop. It floats around unattached to the uterine wall until late November.

Sows typically need to weigh 170 pounds in November to successfully reproduce and provide nourishment for growing cubs. If a bear was not able to gain the right amount of weight in the summer and fall months, then her body will typically absorb the fertilized eggs. Cubs born to a malnourished sow usually do not survive.

If you have ever wondered why we see more bears in late autumn it is because they are busily trying to fatten up for the winter. On average, a bear will need to eat 10,000 to 12,000 calories a day.
To put it in a completely unhealthy perspective, this would mean eating about 60 slices of pepperoni pizza every day for a few months. I think I could do that.

Once our properly fattened sow finds a cozy place to rest for the winter, the fertilized egg implants itself in the uterine wall. It takes just six weeks for the embryo to turn into a bear cub.

Black bear cubs are born in January, while the mother is still in her den. This is convenient because they are born blind, hairless and weighing just a half a pound. The time in the den gives them time to drink their mother’s milk and grow.

After nursing for three months, the mother and cubs, now weighing up to 10 pounds emerge from the den. They will stick with her until they are 1½ years old and set out on their own.

Other mammals that carry out delayed implantation are weasels, seals, otters, bats, armadillos, kangaroos and red pandas.

If you have a question concerning flora and fauna, please email ( All of your questions will be answered. One will be featured next week. See you on the trails!

Amy Renfranz is an interpretive park guide on the Blue Ridge Parkway. She is a certified naturalist through the Yellowstone Institute and a certified environmental educator in the state of North Carolina. Her comments are made independently and do not reflect the views of the National Park Service.

Additional Images

A bear cub reaches new heights.
Photo submitted

Mother black bears will respond to every cry from their babies and keep them safe, dry, warm and fed throughout the winter months.
Photo courtesy of the North American Bear Center

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