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Originally published: 27 January 2004
Original link:
http://www.nytimes.com/2004/01/27/science/27POLA.html
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Polar bears play-fight, a phenomenon that
puzzles scientists. |
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A tiny cub is protected by his hulking
mother, another mystery. |
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A cub at a zoo demonstrates, for doubters,
why polar bears are marine mammals |
By NATALIE ANGIER
To a hairless, thin-skinned tropical primate, the grim winter morning at
the Baltimore Zoo had all the appeal of a pair of wet pajamas.
Yet for Magnet and Alaska, two fine, floppy polar bears with a new $7
million, sensorially enriched ''Arctic habitat'' at their disposal, the
hypodermic cold, the pistol-gray skies and the tap-dancing sleet were
obviously a treat, an ursine month of sundaes with a cherry on top.
Magnet, a 15-year-old, half-ton male, gamboled clumsily across the
boulders before diving into his beloved Nordic watering hole, where he
breast stroked and backstroked and otherwise revealed why the polar bear,
or Ursus maritimus, is considered a marine mammal. Alaska, a 10-year-old,
578-pound deaf female only recently rescued from a Mexican circus,
continued her project, an effort to dig out an acceptable den where she
might someday have cubs.
''They're in seventh polar-bear heaven today,'' said Dr. Chris Bartos, the
assistant mammal curator. ''Alaska spent most of her life in a small cage,
with very little experience of things. The first time she saw snow, she
was ecstatic.''
Of course, polar bears are built to bear much worse than any Baltimore
cold snap. ''They are exquisitely adapted for life in one of the harshest
environments on earth,'' Dr. Bartos said. ''They are super-specialized
members of the bear family, a magnificent example of natural selection at
work.''
Scientists have long appreciated a number of the polar bear's adaptations,
which allow it to survive two decades or more on the glacial ice of the
Arctic Circle, where temperatures reach minus 50 degrees Fahrenheit, all
the while defying standard bear omnivorousness to subsist almost
exclusively on seal. The polar bear comes specially equipped with a double
layer of fur, undergirded by four inches of blubber, that almost
completely prevents heat loss; broad, fluffy paws that act as snowshoes,
and short, solid claws that grip the ice; an elongated snout for poking
into ice holes and pulling out seals; and the ability of that snout to
smell prey from a distance of 20 miles.
''It's just incredible,'' said Dr. Scott L. Schliebe, head of the Polar
Bear Project at the federal Fish and Wildlife Service in Anchorage. ''If a
carcass washes up on the beach, it's like a dinner bell was rung, and
polar bears seem to materialize out of thin air.''
Yet as a handful of hardy researchers continue to study the biology and
behavior of the polar bear, they are unearthing ever more impressive and
sometimes mystifying details about the great blanched beast. They have
discovered that full-grown male bears play with each other for hours on
end, an extremely rare behavior among adult animals. Moreover, they play
at the most improbable time of year: after the long summer fast, when they
are gaunt and famished and by any ordinary calculation should be
conserving calories rather than frittering them away on sports.
Researchers have also learned that the bears can switch back and forth
rapidly between a normal physiological state and one akin to hibernation.
During the summer months, when the Arctic ice retreats and polar bears
have no base for hunting seals, they migrate onto land, eat almost
nothing, and lapse into a state of what is called ''walking hibernation'':
the heart rate slows, the body temperature falls, they cease urinating or
defecating, and they recycle nitrogen.
But whereas a black bear adheres to a strict seasonal schedule and
requires weeks to switch between the distinctive physiologies of denning
versus dining, a polar bear needs little more than happy happenstance to
radically adjust its internal rheostat. ''A polar bear can be in a state
of walking hibernation, but if it finds a carcass, the bear's metabolism
flips right back into high gear,'' said Dr. Ian Stirling, a research
scientist with the Canadian Wildlife Service in Edmonton and the author of
''Polar Bears'' (1998). ''That's a brilliant adaptation, to be able to
switch your physiology, depending on need and luck, rather than to be at
the mercy of an annual cycle.''
The supple metabolism, he said, offers more proof that extreme
circumstances demand extraordinary aptitudes, both physical and mental.
Dr. Stirling, who has studied polar bears for more than 30 years, never
fails to be amazed at the bear's curiosity and intelligence. ''If you're
looking at caribou, they're like a bunch of sheep, and you can't pick out
their personalities,'' he said. ''But every polar bear is an individual.
They're investigative and continuously trying to solve problems.''
Dr. Kathryn A. Foat, curator of education at the Baltimore Zoo, recalls
watching polar bears from inside an all-terrain vehicle in Churchill,
Manitoba. A gust of wind blew the cap of a camera lens out of the vehicle,
and the bears immediately came over to investigate. One animal swatted the
cap across the ground with its paw, and soon all the bears were pouncing
on the disk and thwacking it around like a hockey puck.
As Dr. Stirling observes, it takes keen intelligence to stalk seals,
fish-eating mammals that are themselves pretty clever. And even with their
cunning and attentiveness and willingness to sit in ambush by a seal
breathing hole for many hours at a stretch, polar bears are successful in
their hunts only about 5 percent of the time. Hence, they are awfully good
at conserving calories even during prime hunting season. A polar bear can
fall asleep anywhere, in the most comical positions, using a block of ice
as a pillow, snoozing as a snowstorm covers all but its black-tipped
snout.
In view of the polar bear's inherently precarious profession, biologists
are disturbed by recent evidence that it may be getting harder still to
survive as a result of global warming. Dr. Stirling and his colleagues
have determined that polar bears in the western Hudson Bay, off the coast
of Manitoba, are being forced by the precipitously early thawing of Arctic
ice in the spring to come on land and begin fasting two and a half to
three weeks earlier than they did 25 years ago. He and other researchers
fear that while the worldwide population of polar bears, estimated at
30,000, is not imminently threatened, subgroups like that of the western
Hudson Bay may well be wiped out if the climate continues to heat up.
''It would be a sad day,'' said Dr. Schliebe, ''if polar bears weren't
around for future generations to enjoy.''
Especially considering that the polar bear has barely begun to live.
Genetic and biochemical analyses reveal that polar bears diverged from
brown bears -- the group that includes grizzly and kodiak bears -- a mere
150,000 or so years ago, making the polar bear one of the most recently
evolved mammals on earth and roughly as youthful as modern Homo sapiens.
The bears are believed to have become isolated from other brown bears
during a glacial period, said Dr. David Paetkau, president of Wildlife
Genetics International in Nelson, British Columbia, and to have quickly
adapted to a glacially based life.
Today the bears are found throughout the circumpolar north, around Alaska,
Canada, Greenland, Norway and Russia. They do not live in the Antarctic,
and any depictions of polar bears with penguins is a Santa Claus fantasy.
Polar bears are renowned, and beloved, for their bright white fur,
although the fur in fact has no pigment. Each hair is a clear tube that
reflects all wavelengths of light and so appears white. Scientists have a
variety of theories to explain the coat's cast. By the most familiar one,
white fur evolved because it allowed the bears to blend in with the snow
and more easily sneak up on prey.
But as Dr. James D. Roth, a research assistant professor in biology at the
University of Central Florida in Orlando, points out, during the winter
when polar bears are hunting, it is almost always dark. Dr. Roth says some
experts have proposed that the white fur is instead a way of minimizing
heat intake, when the places where the bears summer can reach 90 degrees.
''Heat stress is such a problem for the animals,'' he said, ''and being
white could help.''
Polar bears share with all bears an extreme disparity between the size of
the mother, in the quarter-ton range, and that of a newborn cub -- about a
pound. ''It's dramatic trait in the bear family,'' Dr. Paetkau said.
''They're off the chart among placental mammals, and closer to marsupials
like the kangaroo.''
An expectant mother comes ashore in the summer, digs herself a den in
October, and two months later gives birth to one to three cubs. She then
nurses the larval-like offspring on milk that is among the fattiest in
mammaldom, until they are plump and fluffy and the darlings of calendar
photographers everywhere.
By the time a mother emerges with her young in April, she will not have
eaten for nine months and may have lost 75 percent of her body weight. But
at least she diets with aplomb, shedding blubber alone. When humans fast,
they begin metabolizing their own muscle tissue; polar bears get rid of
the adipose stores and leave their prized muscles in tact. The cubs stay
with the mother for up to three years, as she teaches them to hunt and
protects them against adult males, which have been known to cannibalize
cubs.
Yet young bears are not pussycats. ''I have seen a number of times when
two little bears, on being approached by a much bigger bear, will stand
shoulder-to-shoulder and make the big bear back off,'' said Dr. Jane
Waterman of the University of Central Florida.
Dr. Waterman and her husband, Dr. Roth, together study play behavior among
male polar bears. They have observed bears congregating in Churchill in
the fall, waiting for the sea ice to form, allowing them to get back to
the main glacial plain and begin hunting for seals and, later in the
season, mating. The females tend to keep quiet and off by themselves, but
not the males. Instead, the males play, in pairs. They rear up on their
hind legs and wrestle with each other. They lock mouths. They embrace.
They seem to dance. They flop down next to each other for breaks, get up
and start horsing around again. All the play is frisky, but gentle,
amicable, and in stark contrast to the violent clashes among males during
mating season, six months later.
''The big question is, Why are they doing it?'' said Dr. Roth. ''It takes
a lot of energy for large animals to play, and they've just been fasting
for months.''
The researchers are now testing several hypotheses: that the males are
establishing a dominance hierarchy; that they are measuring their personal
prowess to see how well they will be able to defend their hard-caught
seals against thieves; that they are practicing for the mating season;
that they are getting in shape after months of indolence; or that they are
building alliances, and making friends. Hard times lie ahead. Why not grin
and bear it together?
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