Hibernation in grizzly bears


        Bear hibernation is truly a miracle of nature.  During the 5-7 months of hibernation a bear lives completely off of its stored fat reserves (burning up to 8000 calories a day and hence its need to fatten up in the fall) and at no time does it eat, drink, defecate, or even urinate while in hibernation.  During hibernation the bear body essentially enters a mode of conservation, efficiency and recycling.  For months on end the bear doesn't consume any fluids all the while never becoming dehydrated (also a bear is actually losing considerable water via the moisture in its exhaled breath) which would obviously normally kill any animal.  However, the bear hibernation physiology allows it to get all the water it needs from the metabolization of fat.  Water is a byproduct of fat metabolization and the bear is able to use this water to supply all of its fluid requirements.  

        The treatment of wastes is even more amazing because although the bear isn't consuming any food or water most mammals still must regularly urinate in order to rid the body of urea (the same is obviously true with humans). Urea is a nitrogenous compound that is a byproduct of protein metabolization (a process that is required to regulate blood glucose levels since fat metabolization doesn't generate enough glucose on its own).  Urea is a poison that must be removed from the body via urine.  Obviously, bears have found a way around this dilemma.  Although not fully understood a bear is somehow capable of converting the urea back into protein which begins the cycle again.  So essentially the bear is recycling the poisonous urea into useful protein that it can reuse!  Modern medicine would love to be able to repeat this feat for humans suffering kidney failure and bears are the only animal able to do this trick.

        During hibernation a bear's body temperature drops from a normal temperature of around 100 degrees (this varies somewhat with activity) down to about 90 degrees depending on the den temperature.  Other hibernating mammals such as ground squirrels have a body temperature that drops nearly to the ambient temperature while hibernating and thus can't be awakened while hibernating.  However, a bear can be easily awakened during hibernation (so don't try and sneak into a bear den thinking it's safe!).  Also the heart rate drops from around 50 beats per minute to about 10 bpm in an effort to conserve energy as much as possible.  



2008 John Teel