| Polar
Bears of Churchill - Mothers and Cubs
The
reproductive process of a female polar bear is one of the most
incredible achievements of any animal on this planet. It begins
as receptive females enter estrous (reproductive state) and mate
out on the ice in late April or early May.
Mating
lasts for several days and, during this time, the male must fend
off several competitors, provided he has not steered the female
to their own private spot! This prolonged period of mating may
be an adaptation aimed at ensuring that the largest and most capable
male bears are most likely to mate. This would, thereby, improve
the likelihood of cub survival.
Testosterone
is running highest at this time of year. While the male is quite
gentle almost suave in his courtship, copulation is a different
story, occasionally resulting in a broken bacculum (the polar
bear’s penis bone). Ouch!
Of
course, if a male cannot fend off his competition for the duration
of the process, cubs born in the same litter, may not necessarily
have the same father.
While
the egg has been fertilized by mid-May, the pregnancy does not
begin for another four months. This is the result of an adaptation
called delayed implantation. Unless the female has maintained
a minimum weight of 300kg (660lbs) by September, the egg will
simply be reabsorbed.
She
needs these excess fat stores, as from mid-July through to the
following February, she will have little or no access to food.
During this time, she will move inland, prepare a maternity den,
give birth, nurse her young and, in late winter, lead them to
the coast. Once there, she must seek out seal birthing lairs and
break into them, sometimes through one metre (3’) of snow,
to both feed herself and her cubs. Only then, does she begin two
years of feeding, protecting and teaching her cubs.
Motherhood
starts in mid-September with pregnant females nestled into an
earthen maternity den and the three month pregnancy just beginning.
Near Churchill, cubs are born in early to mid-December once the
den has become covered and insulated by a layer of snow.
Less
than one kilogram (2.2 lbs) at birth, they are blind, lightly
furred and utterly helpless. While the den remains fairly comfortable,
close to 0C (32F), up to thirty or forty degrees warmer than temperatures
outside, she will also use her breath and body heat to keep her
cubs warm.
Inside
the den, the female does not eat or drink, her body absorbing
its waste. From a normal rate of 70 beats per minute, her heart
rate drops as low as eight beats per minute. It is in this state
that she gives birth and even nurses. The cubs grow quickly, thriving
on mother’s high fat, high protein milk. Weighing around
9-14kg (20-30 lbs), they emerge from the den between late February
and mid-March.
After spending a week or so acclimating to the temperatures and
exercising, they begin the journey to the sea ice. With most dens
30-50km (20-30 miles) inland, this is no small feat for a little
polar bear. Many do not make it. There seems to be less than a
50% chance of survival through the first year of life, overall.
Naturally, first time mothers (about five years of age) lose the
majority of their cubs while older, stronger, smarter females
are much more successful in raising their young. The chances of
survival for polar bear cubs increase with the age of the mother,
generally up to about fifteen years of age.
Spring
is a critical time of year for mothers and cubs. The females energy
is taxed by milk production and the fact that she has not eaten
for up to eight or nine months, one of the longest fasting periods
for any mammal.
In
this tender state, she must guide her cubs to the coast and the
seal birthing lairs. As well, the polar bears of Churchill are
one of the few populations with active predators, some cubs are
taken by the resident wolf population. The pack may work to separate
the mother from one or both of her cubs.
Young
mothers rarely succeed in this journey, some even abandon their
cubs along the way. Older, more experienced bears, however, have
a much better chance of success. They generally show an incredible
bond with their young, exhibiting patience and care. Through deep
snow or difficult terrain, a female may even let the cubs climb
on her back, to rest as she carries them to the ice.
The
incredible challenges associated with pregnancy and motherhood
means that this group of bears are most likely to be negatively
affected by the fluctuating weather and ice patterns associated
with climate change. Remember, females need to maintain at least
200kg (440lbs) of fat reserves for their pregnancy to initiate.
Without a good spring hunt, the egg will simply be reabsorbed.
If this hunting season is reduced significantly, it could mean
that overall cub production will be reduced to the point where
it does not offset the mortality rate.
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