Polar Bear Fact Sheet:
Polar
bears are closely related to the grizzly or brown bears of
Eurasia and North America. They are the only terrestrial
mammals to have adapted completely to life on the arctic
sea ice. Where there is Arctic sea ice there are polar
bears. Where sea ice is completely absent year round, there
are no polar bears.
Polar bears are sexually dimorphic with males weighing
substantially more than females. An adult female is
typically about 2 metres long and weighs 150 kg. An adult
male may exceed 2.4 metres and 800 kg.
They are solitary species in general with the most stable
social group being adult females with their current litter
of cubs. Cubs stay with their mothers for about 2.3 years,
although some may be weaned at 1.3 years and others stay as
long as 3.3 years.
Groups of adult males are sometimes seen on land in summer
along the southern limits of their range where the ice has
seasonally melted completely and the animals are awaiting
autumn and the re-freezing of the local ocean. In the later
part of the ice-free period, sub-adult males can
occasionally be observed play fighting.
Breeding occurs in spring (April/May) with cubs born early
the following January. Soon after conception, the
developing embryo enters a quiescent stage and further
development ceases for 3 to 4 months. In autumn the embryo
begins development again, develops a placental connection
in the uterus and proceeds with gestation. Cubs are born
around January 1st after a short gestation period. At birth
the young weigh about 450 g. The mother nurses them in her
den until emergence in March or April. By that time the
cubs have grown to about 11 kg. The most common litter size
is two with a few females having three cubs and some only
one.
Polar bears are the most carnivorous of the bear family,
feeding almost exclusively on ringed seals and to a lesser
extent on bearded seals. Individual polar bears can move
over an area the size of Manitoba (or Great Britain) over
the course of a year as they search for prey and other
resources. However, they can return precisely to the same
spot on land year after year. Despite the large distances
moved, individual polar bears do not roam over the entire
Arctic and, instead, are members of distinct
sub-populations.
The heaviest feeding period is in spring, soon after seal
pups are weaned. The bears feed heavily at that season and
some may restrict themselves primarily to eating the
blubber (or fat) of the seals and abandon the muscle mass
and most of the carcass. Polar bears likely have the
highest fat intake in their diet of any mammal.
At other times of the year the bears may feed little. Only
pregnant females obligately enter a den in summer or autumn
and remain within it until the following spring after the
cubs are born. These bears will fast for more than 8
months, during which they undertake all of active gestation
and early lactation. Other age and reproductive classes of
polar bears remain active throughout the winter, although
they may enter temporary shelters during the most inclement
portions of the winter.
To survive such lengthy fasts the bears become massively
obese during the peak feeding periods. A pregnant female
can increase her weight by more than a factor of 4. There
is no evidence that the bears suffer any health
consequences from their extreme feed/fast eating cycle.
As the top predator of the arctic marine food web, they are
exposed to relatively high loads of environmental
contaminants. These can include various organochlorines
(e.g., DDT and PCBs), radionucleides, and heavy metals
(e.g., Pb and Hg). Their high-fat diets make them
especially susceptible to the lipid-soluble contaminants.
At this time, however, there is no firm evidence of any
deleterious effects in polar bears from their contaminant
loads.
Provided by:
Dr. Malcolm Ramsay
Department of Biology
University of Saskatchewan
Saskatoon, SK
Canada