• What foods do bears eat?  Which nutrients to these foods provide?
  • Which types of habitat are occupied by each species?
  • What preys on bears?
  • What other enemies do bears have?
  • Where do bears den?
  • When do bears den?
  • What is hibernation?
  • Why hibernate?  
  • Is the torpor of all hibernating animals equally deep”
  • What other differences occur between small- vs. large-bodied hibernators?
  • Why don’t bears store enough fat that they don’t need to hibernate?
Bear Ecology

  • Fruits (primarily sources of energy and vitamins)

  • Succulent herbs and sedges (primarily a source of protein, vitamins & minerals)

  • Buds and new leaves on shrubs and trees: (mainly protein)

  • Underground plant parts: bulbs, corms, tubers, roots, rhizomes (mainly energy in the form of carbohydrates)

  • Bamboo shoot and hearts:  These are normally eaten only by panda bears; and they are virtually the only thing that pandas
    eat.

Prey
Invertebrates:
  • On land:  ants, termites and other colonial insects; grubs, beetles, larvae, crayfish, worms, etc.  In fact colonial termites are
    the main food of sloth bears.  Their mouth is specially adapted for this.
  • On sea shores:  Crabs, barnacles, clams and certain other shellfish

Fish: Mainly those that occur in large schools in shallow water where they can be caught by a bear.  These are usually fish that
Amphibia, reptiles and birds: Rarely captured by a bear.

Small mammals:  mostly rodents (beaver, ground squirrel, marmot, etc.)

Large land mammals:  eaten by both black and grizzly bears
  • Mainly ungulates (i.e., hoofed animals such as moose, deer, caribou, elk or wild sheep):  young fawns/calves are found by
    searching; old animals may be ambushed or chased down.  Relatively few killed.

  • Other bears: Occasionally a member of the same species or a different species.

Large marine mammals: About the only thing that polar bears eat.  
  • Pinnipeds:  Their primary prey are ringed seals (named for the ring-shaped designs on their coats).  They are readily small
    and easily handled by a bear.  Polar bears also eat larger seals such as bearded seal, as well as walrus,

  • Toothed Whales:  beluga whales and narwhal.  

  • Baleen Whales:  Larger whales such as bowhead are eaten only after they die of other causes and wash up on ice or
    shore.  Polar bears rarely if ever catch live fish and eat very little plant material.

For further details on bear diets, read the appendix to
Beauty Within the Beast


Which types of habitat are occupied by each species?
Black bears: They frequent wetlands in spring to forage on succulent, protein rich greenery.  As that matures it is less nutritious, so
bears switch to colonial insects until fruits ripen during summer and fall.  Fall is also when most nuts ripen.

Black bears have been in North America over 100,000 years.  They once occupied virtually all forests in North America from
northern Canada sound into northern Mexico.  Their short, sharp claws enable them to climb trees  readily and thus to harvest nuts,
fruit, new leaves and buds growing there.  Trees also provide refuge from enemies.  They do little digging for underground plant
foods.

In general, the only non-forested habitat they occupy is in the far northeast of Nova Scotia or New Brunswick – habitat that they have
never had to share with any other kind of bear.

Grizzly bears:  did not immigrate to North America, across the Bering Land Bridge until roughly 12,000 to 14,000 years ago – about
the same time that humans did.  They arrived to find forests saturated with black bears.  They adapted to exploit underground plant
foods and rodents.  Their finger claws become longer and blunter, making them more efficient as shovels.  Their should muscles
became much larger to power those shovels.  To survive against enemies in habitat without trees to climb, grizzly bears had to be
much larger than black bears.  One side effect is that large body size and longer-straighter claws, reduced their ability to climb
trees – especially trees without branches to use like ladder steps.

Polar bears: Live mainly on sea ice.  There are three kinds of sea ice:
  • ice cap: ice surrounding he North Pole that stays in one solid mass year-around.  Size of the cap grows during winter and
    shrinks during summer.   Cap ice occurs over deep ocean waters and provides little opportunity for polar bears to catch
    prey.  

  • shore-fast ice = that which is frozen to the shore – often in a narrow band close to shore. It provides some opportunity to
    catch prey.

  • pack-ice:  broken plates of ice that move between shore-fast ice and cap ice, carried by ocean currents.  The many cracks
    (leads) between ice plates provide spaces where bears and their marine mammal prey can surface to get air, or climb onto
    the surface of the ice.  When a lead freezes over, the ice is initially thin.  Seals


What preys on bears?
  • Cubs: are preyed on by wolf, coyote, a wolverine, cougars, and sometimes fellow bears – which is why mother bears are so
    protective.  

  • Subadults: Are occasionally preyed on by a wolf pack or a larger bear.

  • Adults: are rarely preyed on by anything but a very large adult male bear.


What other enemies do bears have?
  • Large ungulates such as moose or elk may chase grizzly or black bears away as a precaution, even when the bear is not
    hunting them.

  • Walrus may likewise chase away polar bears – although not very far

  • Humans are the primary enemy of bears.


Where do bears hibernate, sleep or rest?
Hibernation Dens
Dug dens  
Black and grizzly bears
  • They commonly dig holes in the side of a hill, especially at the base of a large tree whose roots will hold the soil in place
    and keep it from collapsing on the bear.  Collapse is most likely during spring when snow melts and water seeps into the
    soil.  

  • Another kind of site favored by bears is at the base of a cliff, at the top of a talus slope.  These sites are remote enough that
    a den is not likely to be discovered by wolves that could dig in and kill the bear to eat it.  These dens are also above any
    avalanche that otherwise sweep a bear away or bury it.

Pregnant polar bears dig dens in a snowbank, for instance on a steep slope or in a gully.  Their dens seldom penetrate into the
underyling soil.

Cave dens:  Some grizzly bears hibernate in a pre-existing cave; black bears may be less likely to do so.

Tree dens:  Black bears sometimes den inside a hollow tree, even if they have to climb over 50 feet up the trunk to find an
opening into the tree – usually a hole where a branch had formerly grown out, but later broke off.  Grizzly bears are not known to den
inside trees, if only because trees large enough for a grizzly bear are virtually unavailable on grizzly bear habitat.  

Ground dens: Black bears are much more likely than grizzly bears to den under a pile of brush or log jam.  Occasionally, an adult
male black bear will simply lie down in some sheltered spot and let itself be buried in snow.  This is most likely in habitat with mild
winters and where wolves have long been absent.

Sleeping/resting sites  
Storm shelter:  During blizzards, polar bears may take shelter by digging into a snowbank.
Seldom do black or grizzly bears seek sheltered in a cave even during a bad rainstorm.

Normal ground-level day beds:  Any place but a “cave” where a bear rests or sleeps on the ground is known as a day-bed.
When the ground is free of snow, bears may simply lie down on the ground as it is; or they might rake away rocks and sticks, or  
pile up leaves and grass to make the spot more comfortable.  If the bear's belly is bulging with food, it sometimes keeps weight on
its belly to a minimum by digging a belly hole into which its belly can sag while the bear lies with its chest on the ground.

Trees:  
  • Grizzly bears almost never sleep in a tree, even as cubs.  

  • Black bears – especially cubs and subadults – often rest and sleep in trees, where they are relatively safe from enemies.  
    Mom often sleeps at the base of a tree containing her cubs.  When bears sleep in trees, they usually rest on one or more
    branches.  Black bears almost never build a nest in a tree – by contrast to sun bears in Malaysia or spectacled bears in the
    Andes.


When do bears den?
  • Snow cover & seasonal famine:  Bears that den do so mainly when the ground is covered with snow and food is
    unavailable.

  • Latitude & altitude:   In northern Alaska grizzly and black bears enter may enter dens in September and remain until May or
    June.  In central Alaska, they likely delay until October and emerge in April. Farther south, in climates without winter snow,
    the only bears that den may be pregnant females. Other bears may just lie around lethargically until food supply improves.

  • Motherhood:  As a general rule, pregnant females are the first to enter dens in autumn; then, after they have given birth, they
    are the last to emerge in spring – allowing plenty of time for their cubs to mature enough to cope with life outside the den.

  • Adult males tend to be the last to enter dens in autumn and the first to emerge in spring – whereupon they search for
    carrion and early growth of green vegetation.  They need to fatten up as much as possible in preparation for mating season,
    when they eat very little for weeks on end, despite spending great amounts of energy searching for mates, competing
    against other males for mates, and then courting the females.


What is hibernation?
Hibernation involves slowing metabolic rate much farther than happens during sleep.  Hibernating animals achieve
correspondingly deeper levels of unconsciousness.  That is, it takes greater degrees of disturbance to wake them up.  This
profound slowing of metabolic rate and deep unconsciousness is called “torpor.”


Why hibernate?  To avoid starvation  
Winter famine:  During winter time when snow covers the ground, nutritious plant foods such as hard and soft mast, succulent
vegetation, and underground plant foods can’t be harvested.  In general, the only plant foods available are woody tissue that only a
small range of species can subsist on because their guts house microbes that can break down the woody material into sugar.  

Bears that don’t hibernate:  Subadult and adult male polar bears and females with cubs at least one year old do not hibernate.  For
winter is the season when their main prey are most readily available.  That’s the season when polar bears fatten up.  In habitats
where salmon, marine carrion or other foods remain available all winter, grizzly and black bears may also not hibernate.  

Bears that do:  However, even in habitats with abundant winter food, pregnant female bears must den up to give birth and rear cubs
until the youngsters are mature enough to cope with life outside the den.  So during pregnancy and the first months after birth, polar
bear mothers do hibernate, much as do black and grizzly bear mothers.  During periods without sufficient food, all bears hibernate
or risk severe weight loss and starvation.

Body fat or stored food?  During weeks or months when animals can’t obtain enough food to replace the nutrients and energy that
they expend, they must make up the difference with energy and nutrients that they have stored in a cache (e.g., food stored by picas
or by squirrels, beaver, and some other rodents) or on their bodies, for instance as fat.

Metabolic rate:  The faster an animal’s metabolic rate, the more energy and nutrients it burns per hour.  At high metabolic rate,
stored food and fat are used up quickly.  To avoid running out before spring, animals show their metabolic rate, making them more
lethargic.


Is the torpor of all hibernating animals equally deep?
Body size:  No.  
  • The smaller an animal, the deeper its torpor.

  • The smaller an animal, the higher its metabolic rate – i.e., the more calories of energy it burns per pound of body weight.  
    So 200 one-pound ground squirrels burn up about 4X as much energy per hour as one 200-pound bear.

  • The number of calories needed to support one 200-pound bear could support 200 one-pound squirrels only if the squirrels
    lowered their metabolic rate to ¼ that of a 200-pound bear.  Whereas a hibernating bear might be woken up by the noise of
    an enemy outside its den, a hibernating squirrel may remain sleeping even if picked up in the jaws of a predator.

  • In other words, a bear hibernating in a den can remain alert enough to protect itself from an invading predator.  If the
    weather is above zero, the bear can wake up fairly easily.  At 40 degrees F below zero, however, even a grizzly tends to sleep
    fairly soundly.  That's why Alaska Natives traditionally hunted grizzlies only at such cold temperatures.  They might discover a
    bear's den during warmer weather, but they'd wait for intense cold to hunt the bear.

  • In relatively warm climates a hibernating black or grizzly bear's metabolism may be little slower than while sleeping.  In
    some cases wintering bears might seem just a bit groggy unless disturbed.  

  • Recall that among polar bears, the only individuals that hibernate during winter are females which were pregnant when they
    entered the den and that will give birth in late December.  However, any polar bear that can't find food for weeks or months
    on end, tends to undergo a lesser slowing of metabolic rate called "walking hibernation," because it can still walk around,
    forage on plants, or even play.  This usually occurs during summer periods when bears a can't reach pack ice where they
    could catch seals and  they can't find marine mammal carcasses on shore.


Winter diet
  • Rumen:  Moose, deer, elk and other hooved mammals that inhabit northern lands have a 4-chambered stomach, one of
    which – the rumen -- houses microbes that can digest woody vegetation that is available even during winter.  However, the
    energy cost of traveling to those foods and consuming them sometimes exceeds the energy gain from those foods.  When
    that happens the animals must live off fat reserves.  To make that fat last as long as possible, these animals also undergo
    walking hibernation.”

  • Caecum:  Horses, rabbits, hares, and pikas have a caecum – which can be thought of as an enlarged appendix -- that
    houses digestive microbes.  

  • Bears and other carnivores don’t have either a rumen or a caecum.


What other differences occur between small- vs. large-bodied hibernators?
Winter nutrient supply:  A bear can store enough fat on its body to last it all winter – even if winter lasts up to 8 or 9 months.  
Rodents can only store enough fat to last a matter of weeks or days.  Even those that hibernate (e.g., ground squirrels) have to
wake up and eat food they have stored.  Because they eat, they must also urinate and defecate during winter.

Body fat:  Hibernating bears live off body fat.  As fat is burned, the waste products produced are just water and carbon dioxide; both
carbon dioxide and water vapor are excreted through lungs as the bear exhales.  The protein that breaks down in the bear’s body is
regenerated, so that uric acid wastes don’t accumulate and cause acidosis.  Calcium and other minerals lost from bones are
recycled and re-deposited in bone so that bones stay strong – unlike what happens with bed-ridden humans.


Why don’t bears store enough fat that they don’t need to hibernate?
  • Even in habitats where snowy winter lasts only a month, they couldn’t store enough food to support themselves at the same
    metabolic rate that they have during rest of the year.  In habitats where snowy winter lasts 8 or 9 months, a bear would have
    to store a mountain of food.  

  • They have no way to gather even a small mountain of food

  • They generally have no cave where enough food could be stored.  

  • The few kinds of bear foods (e.g., nuts and tubers) that might store well would probably be vandalized by rodents.  

  • A bear carrying that much fat would be too slow and awkward to escape danger or vanquish enemies.
Index of  Bear  Webpages
Bear Viewing Association
To watch, to wonder, and to conserve
gobearviewing@hotmail.com
Ph/Fax (907) 260-9059 (Office)
39200 Alma Ave.    Soldotna, AK  99669
bear viewing Alaska, bear photography, bear safety, bear behavior



Answers to questions commonly asked by wildlife viewers on the topics listed below:
Index of  Bear  Webpages
Bear Viewing Association
To watch, to wonder, and to conserve
gobearviewing@hotmail.com
Ph/Fax (907) 260-9059 (Office)
39200 Alma Ave.    Soldotna, AK  99669
bear viewing Alaska, bear photography, bear safety, bear behavior