Grizzly Bear Behaviour:  Grizzlies in Action

Why Hibernate?      Home Range      Feeding Habits      Daily Patterns     Social Behaviour

Source: Webshots

Winter Lethargy / Winter Dormancy

•  The Grizzly is less active after the breeding season and grows fatter eating 
   all the food available during the summer, which helps it to survive in its den 
   throughout the winter months 

  • The word hibernate comes from the Latin for "winter home" (the latin root is hibernus - of winter).

  • Grizzlies are not true "hibernators" 

  • True hibernation requires a significant drop in body temperature and breathing rate 

  • But the Grizzly's body temperature drops only a few degrees and its respiration rate is only slightly below normal

    According to the Natural History Museum 

    • Grizzlies live in climates with cold winters when food sources are limited 
      and travel in deep snow is difficult 

    • So grizzly bears spend the winter in a dormant or sleeping state 

    • This "sleeping period" is often referred to as "hibernation", 
      but it differs from true hibernation in several ways 
      –  In true hibernation, an animal's body temperature drops to a few degrees above freezing and if disturbed it takes a couple of hours to wake up 
      –  Although a sleeping Grizzly Bear's body temperature is lower than normal, it is not as low as true hibernators 

    • During winter sleep, bears are alert and easily aroused 

Why Do Grizzly Bears "Hibernate"?

Picture Source: Webshots

This allows them to survive in northern regions where their main foods – green vegetation, berries, insects, and fish – are not available in the winter 

•  When? 

  • Female Grizzlies usually enter their dens first, sometime around mid-November, depending on the weather and their condition 

    • Males Grizzlies start to get sleepy and are likely to stay outside the den until late November or early December and emerge from it as early as March 

    • Sows especially those with cubs (Mamma Bears) tend to stay in their dens until their offspring are fairly well grown in late April or early May 

•  Where? 

  • The onset of cold temperatures and falling snow signals cause Grizzly Bears to enter their dens 

    • Dens are usually located at up high in areas of deep snowfall and where snow is slow to melt  

    • Dens often have a porch, an entrance tunnel, and a "den chamber" like a bedroom 

    • The bed will often be lined with sticks, moss, leaves, and other vegetation 

    • Dens may be located in natural caves, hollows under the roots of trees, or they may be excavated under trees or into the banks of steep slopes

    According to the BC Government: 

    • Grizzly Bears are fussy about the location of their  winter dens 

    • Most dens are used only once – Grizzlies usually den in the same area each year but dig a new den each winter 

    • Sometimes they dig more than one before they are satisfied and sometimes they move to a new site during the winter

    • Dens may be up to 4 m long They have an entrance tunnel about 0.7 m in diameter, and a chamber that is 1 to 2 m wide and about 1 m high 

    • Dens are built horizontally into the ground on steep slopes (20 - 40º)

     According to the Grizzly/Brown Bear Facts web-site: 

    • Dens must provide protection and security during the winter months 

    • Dens are dug in dry, stable soil where winter temperatures will remain above freezing 

    • As snow falls it covers and helps to insulate the den (keep the den warm) 

    • The entrance to the den leads to a tunnel that slopes downward to the actual sleeping chamber 
      – this sloping tunnel allows stale air to escape 

•  How Long? 

  • In coastal BC, bears hibernate from about early November to mid-April 

  • In the interior, hibernation lasts from about October to May 

•  What Happens? 

• During their winter sleep, a Grizzly Bear’s body temperature drops by 4 to 5ºC, oxygen consumption declines by up to 50 percent, the heart rate falls to only 8 to 12 beats per minute, and the bears do not urinate or defecate 

• These physiological changes (changes to their bodies) allow the bears to live on stored fat for several months, although they usually lose a lot of weight 

Source:  Bears In BC - Hinterland - BC Government - Grizzly Bears In BC

Find out more... Grizzly/Brow Bear Facts 

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Grizzly Bear Introduction
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What's Under All That Fur?
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