• Large male brown bear at Brooks Falls


    National Park & Preserve Alaska

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

4 Comments Comments Icon

  1. GK - Washington DC
    January 22, 2014 at 05:28

    Thanks, Ranger Mike, for another informative piece!

  2. Martina - Germany
    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

  3. Michael Fitz - Katmai National Park and Preserve, Alaska
    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: http://www.nbcnews.com/science/hooked-garbage-nevada-bears-quit-hibernation-2D11926466. As usual with bears, their behavior can be explained by their appetite.

  4. Juergen - Ostfildern, Germany
    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 explore.org and G+ if possible Many thanks


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