A baby black bear. Photo via Flickr.

Black bear cubs emerge from their winter dens in the spring in the Rocky Mountains.  Have you ever seen this special event happen?

I don’t know about you, but I definitely remember seeing a video of black bear cubs crawling out of their den in the spring.  First the mom emerges onto the snow and then a couple of cute babies crawl out after her.  That image captured my imagination and is ingrained in my memory because it is so amazing.  I always wondered how could that happen that the babies emerge in spring?  Read on to learn all about how black bears give birth to their young during winter inside their den.

Black bears mating. Photo via Flickr.

It All Starts With Mating In The Spring/Early Summer

Similar to all mammal reproduction, it all begins with the male and female mating.  For black bears, this happens, surprisingly, in the spring or early summer.  I say surprisingly because we all know that the black bear cubs emerge in the spring from their winter den.  They mate in the spring, but the babies aren’t born until sometime during winter.

Black bears mating season mainly happens around May and June.

Then In Fall The Mother Bear Begins To Grow The Babies, Or Not – Delayed Implantation

Now comes the amazing part of how black bears make babies!  Despite the fact that the female mated in the spring, she does not actually begin to grow the baby inside of her until the fall!

Mammal Reproduction – To make sense of this I need to quickly explain how mammal reproduction works.  A male and female mate, which means that the male fertilizes the female’s egg inside of the female’s body with his sperm.  Once that happens a baby begins to grow in an egg inside of the female’s body.

For black bears, the male’s sperm fertilizes the female’s egg in the spring.  However, the egg does not begin to grow into a baby yet.  Female black bears have a special ability to not start growing new babies from that fertilized egg yet.  The female goes about her usual business the rest of the summer and then tries to fatten up for winter hibernation.

If she puts on enough fat stores to safely have a baby and nurse it over winter the egg will then begin to grow into a baby.  But, if she did not eat a lot and put on enough fat her body will simply not allow the fertilized egg to grow into a new baby.  Her body does this because if she had a baby there would be a good chance both her and the baby would not survive the winter.

In summary, if the female puts on a lot of fat in the fall she will become pregnant.  If, however she did not eat enough she will simply not become pregnant.

Black bear cubs climbing a tree. Photo via Flickr.

Black Bear Cubs Are Born In Winter

The baby bears grow inside of the mom for two to three months.  After that the black bear cubs are born in the middle of winter while the female is in her winter den.  Newly born cubs weigh less than one pound, can’t see, and have no fur.  Females give birth to between one and five babies.

The mom and her new cubs remain in the winter den until spring time.  The babies nurse from the mom during this time, but the mom remains resting.  She will move around from time to time to ensure the cubs are comfortable.  Then in the spring they will come out of their winter den and greet the world.