Birth of a Brown Bear

January 22, 2014 Posted by: Michael Fitz
illustration of a newborn bear cub
You’d never know it trekking across Katmai’s frozen landscape, but even in the depths of winter, bears occasionally stir. Katmai’s brown bears hibernate for 5 to 6 months or more. Remarkably though, some of these bears are giving birth in January and February.

In placental mammals, larger species generally give birth to larger offspring (picture the size of cattle, elephants, and whales when they are born). This is not the case with bears. Brown bears are extremely small, averaging about 1 pound (.45 kg), at birth. This is tiny, especially compared to the mother bear which may weigh easily 400 pounds. Brown bear cubs are from 1/3 to 1/10 of that predicted for female mammals of comparable size.They are also blind and helpless at birth. Why would brown bears give birth to such small and vulnerable offspring? Like many natural phenomenon, no one knows for sure but biologists have some ideas.

During hibernation, bears neither eat nor drink. They are surviving solely on stored body fat. In a healthy hibernating bear, fat is in excess but protein and carbohydrates are limited. Developing embryos in humans as well as in bears can’t grow on fat alone. They need protein and carbohydrates in order to grow and develop.

Mother bear can only provide a limited amount of protein during the 50 day gestation period of her cubs. She needs to conserve almost all protein for herself in order to maintain muscle mass. Perhaps, mother bear can’t “afford” to provide protein to the growing cub. What can a fat, sleeping bear provide then? Milk fat.

Once the cub is born, it soon begins to nurse. A newborn cub’s physiology changes from one that couldn’t survive on fat in the womb to a system that can better metabolize fat. Mother provides all the nourishment that the cub needs and the den offers warmth and protection. Even though the bear has been born, for the next several months the den acts as a surrogate womb for the rapidly growing cub.

Inside of a bear den on Dumpling Mountain
Bear cubs in Katmai spend the first few months of their lives inside of the den their mother dug the previous fall.

Hypothetically, bears are faced with an evolutionary “choice” when it comes to the timing of birth. They could have a longer gestation period and deplete the mother’s protein stores which may reduce her chances for survival. Or, they can give birth to very small, vulnerable offspring and nurse them as they continue to grow in the protective environment of the den. Bears have chosen the latter option.

Plants and animals have countless ways to deal with winter’s threats and opportunities. How and when bears give birth is another example of this. The timing and physiology behind the birth of a brown bear provides another amazing example of bears’ ability to adapt and survive.

bear, cub, birth, survival, adaptation, evolution

25 Comments Comments icon

  1. Monty
    October 07, 2020 at 01:08

    While your answers are interesting, they deny the fact that the more science learns about man, animals, plants, ocean life, and all natural phenomena, it It all leads more and more to the concept of a designer. Most of your unclear answers can all be settled with one: God, the Creator. Lineage speciation occurs of course, but "evolution" is a theory even Darwin saw as flawed.

  2. molly
    March 03, 2020 at 10:46


  3. Alisha Elbrolosy
    April 05, 2019 at 04:32

    That is so cute

  4. December 04, 2018 at 02:00

    why is it naked

  5. Jim
    August 06, 2018 at 06:58

    Hi R.Mike, Today 8/6/18 on the Brooks Falls cam was a mother and 4 (!) adorable cubs. The youngsters appeared to have quite a range in size. How far apart might they have been born?

  6. Ashley
    March 31, 2018 at 04:15

    how are brown bears born?

  7. Sarah
    July 01, 2017 at 04:58

    Bears haven't chosen the latter option to give birth to small young. Evolution has. Just being picky, sorry.

  8. February 02, 2016 at 01:30

    @Louise: As far I as know, bears do wake up when they give birth. Afterwards, the mother goes back to sleep. Newborn cubs nurse while their mother hibernates for the rest of the winter.

  9. February 01, 2016 at 03:23

    Does the Mother wake up then or continue to sleep?

  10. February 01, 2016 at 01:22

    So interesting to read. Thanks!

  11. December 23, 2015 at 01:11

    This is nice to knew.Thanks for sharing.

  12. May 30, 2015 at 07:14

    Thanks Michael Fitz for your nice interesting post on Bears.Thanks for sharing your experience and also brief description on Bears life lead.Need to know more ,please keep sharing.

  13. February 06, 2015 at 06:45

    @Trisha: It’s probably accurate to assume that most, if not all single, adult females mated last year. Some very healthy looking female bears were single last year— such as 94, 409 Beadnose, 410, and others. This doesn’t mean they will all conceive and have cubs, but they possibility is there.

  14. February 05, 2015 at 09:40

    JUEGEN: YES-last year many new born cubs died. Heavy rains in JAN+Febr.meltet snow and bears lost dens. Hardly any new born cubs were observed during spring. Bear population is at low point. We are having another bad winter with very little snow.

  15. February 05, 2015 at 09:19

    Ranger Mike - thanks for your blog. Can you tell us which bears were known to have mated and from whom we can possibly see with cubs this summer?

  16. February 05, 2015 at 06:46

    @uclamom: Polar bears also undergo delayed implantation and give birth in dens. This greatly delays the amount of time female polar bears have to fatten up before the ice thaws and their favorite food (seals) becomes too hard to hunt. Since polar bears start to hunt seals regularly in the fall when ice forms, female bears aren’t going into dens to avoid famine like brown bears, just to give their cubs safe haven. Perhaps polar bears haven’t been able to shed that part of their shared evolutionary history with brown bears. Polar Bears International’s “About Polar Bears” page is a good source of info on those animals ( ).

  17. February 05, 2015 at 06:07

    @KathCK: The phenomenon you refer to is called delayed implantation. Seals and weasels are other groups of mammals that have this adaptation. Bears mate in late spring or early summer, but any fertilized eggs only divide a few times before going into a state of arrested development. At this stage the embryo consists of a few cells and is called a blastocyst. The blastocyst won’t implant until the mother begins hibernating. Only then, does gestation really begin. However, if a female “determines” that her body is not capable of supporting a pregnancy, then she will absorb the embryo and terminate the pregnancy. This is an unconscious decision by the female’s body. There is some evidence that suggests fatter females are more likely give birth to cubs. Body fat, therefore, not only allows bears to survive hibernation, but also affects their reproductive rate.

  18. February 05, 2015 at 06:05

    Do polar bear gestation etc have the same outcome? I wonder how much evolution has altered their whole cycle.

  19. February 05, 2015 at 05:50

    Thank you Ranger Mike for this interesting information.

  20. February 05, 2015 at 05:44

    I was startled by the 50 day gestation period you mentioned considering when mating went on last summer, but then I remembered that with bears there is a delay. What's that called? Can you explain it or refer me to a website with the information? Thanks, Ranger Mike.

  21. February 05, 2015 at 11:48

    I'm glad all our bears looked healthy before going to sleep.Thanks for the info.Maybe we will see Amelia with strong healthy babies this year.

  22. GK
    January 22, 2014 at 05:28

    Thanks, Ranger Mike, for another informative piece!

  23. January 22, 2014 at 02:56

    Thanks Ranger Mike, learning by reading is interesting, makes fun and is so easy....if you are the one who wrote the explanation. Great for us "Crazy Bear Watchers" to have you and also Ranger Roy at our side. And thanks to Juergen, good question. Best wishes

  24. January 22, 2014 at 01:40

    @Juergen: Warm winter weather may impact the bears, but not necessarily in a negative way. Bears may wake and even exit the den during warmer weather. In a four year denning ecology study on Kodiak Island, at least some bears were active every winter of the study, but Kodiak Island experiences milder weather than the Katmai region. What’s happening this winter in Katmai? I don’t know. I have not heard of any recent bear sightings, but it is certainly possible that some of Katmai bears have woken and stretched their legs a bit during some of the region’s warmer days this winter. If there is ample food available then a bear may not hibernate at all. At the very least, ample food will reduce the length of time a bear spends hibernating. Luckily, Katmai’s bears are not conditioned to eat human food, but evidently this is happening in the Lake Tahoe area: As usual with bears, their behavior can be explained by their appetite.

  25. January 22, 2014 at 12:41

    Hi Ranger Mike, did the warm weather conditions in Alaska affect the hibernation of the bear? Please answer this question at bear chat and G+ if possible Many thanks

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