Polar Bear
Reproduction
Female
polar bears first start to breed between 3 and 5 years of age.
Mating takes place from April until early June with the time of oestrus
for individual bears lasting about 3 weeks.
Polar bears have a characteristic known as delayed implantation.
This means that the fertilized egg does not immediately implant in the
wall of the uterus and embryonic development does not begin until late October.
Denning
usually begins about mid-November or early December.
During the winter any polar bear may dig a den and use it for a few days
during a storm. Pregnant females,
however, den for extended periods and their cubs are born in the protected den.
Dens are
often situated in snowbanks on south-facing hill slopes near the coast.
The largest dens are excavated by females with cubs over a year old, and
may be as roomy as 1 metre high, 2.5 metres wide and 3 metres long.
Temporary dens occupied by single bears usually have just enough room for
turning space. A variety of features such as porches, lairs, sills, alcoves
and ventilation holes may be built into the den.
Temperatures inside the den vary according to the depth and density of
surrounding snow, but are always warmer than the outside air.
 Sometime
in early January, the female bear gives birth to a litter of cubs.
Twins are most common, but sometimes, especially in first pregnancies,
only a single cub is born. Occasionally
there are triplets. The cubs are
about 40 cm long and weigh no more than 0.7 kg.
They are blind and deaf and completely dependent on their mothers. The first few weeks after birth are spent in the den
suckling, sleeping, and gaining strength and weight.
By
mid-March the cubs weigh about 9 kg and are ready to leave the den.
For several days after emerging, the family stays near the den while the
cubs play and acclimatize to the outside. They
then begin the journey to the sea ice, the tiny cubs bravely following their
mother in single file. Inuit call
the cubs "ah tik tok" which means "those that go down to the
sea".
The
mother bear shows great concern for the cubs, stopping frequently to allow them
to rest or suckle. She rarely
leaves them alone for even a short time as the danger of wolves or large male
bears killing the small cubs is very great.
A
family group cannot travel far. It
is fortunate that den emergence and seal pupping occur at about the same time.
The mother bear scoops the seal out of its den, kills it and gulps down
fat and skin stripped from the carcass. The
cubs are then nourished indirectly through her milk.
By July, the cubs have acquired a taste for seal blood and fat and they
begin hunting lessons during their first summer.
They eat mostly fat and meat after their first year although they
continue to nurse until nearly 2 years old.
By
August the cubs weigh over 45 kg and are larger than a German shepherd dog.
However, they are still dependent on their mother and den with her again
that winter. A family stays
together until the spring when the cubs are approaching 3 years of age.
When
the family ties are broken, usually as the mother bear is coming into oestrus in
spring, the half-grown cubs are abandoned.
They may stay together for a short time, but eventually each goes its own
way. The inexperienced young bears
face many dangers such as starvation, human hunters and old male bears.
Other causes of mortality of polar bears of all ages are injuries,
disease, occasional wolf predation, death from encounters with adult male
walruses, and old age.
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