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 The answer: "bear necessities," genetically speaking.
Scientists unveiled a thorough genetic analysis of the polar bear on
Thursday and compared it to its closest cousin, the brown bear. They
found that since diverging from brown bears less than 500,000 years
ago to become a new species, polar bears have undergone remarkable
genetic changes to permit the high-fat diet they need in the frigid
Arctic conditions they call home.
Multiple genes related to cardiovascular function and fatty acid
metabolism have changed radically through mutations to permit a
high-fat menu without high risk of heart disease, the researchers
said.
"For polar bears, being very fat is no problem," said Eline
Lorenzen, a molecular ecologist at the University of California,
Berkeley.
In the Arctic regions where they live, energy is in high demand and
polar bears have lots of fatty tissue to do the job. Up to half
their body weight can be fat and their source of fresh water is
metabolic water - water that is a by-product of the breakdown of fat
in the body, Lorenzen said.
 "They essentially live in a polar desert," Lorenzen added.
The researchers deciphered the genome - the genetic blueprint - of
the polar bear based on blood and tissue samples from 79 polar bears
from Greenland. They also used samples from 10 brown bears to study
the genome of that species.
Rasmus Nielsen, an evolutionary geneticist at the University of
California, Berkeley, said genetic data showed polar bears diverged
as a species from brown bears much more recently than previously
thought - perhaps 400,000 years ago. Some previous estimates had
placed the origin of polar bears as long as 5 million years ago.
"In this short amount of time, polar bears have adapted to the cold
environment of the Arctic and to a new diet. We see the footprints
of this adaptation in the genome of the polar bear," Nielsen said.
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The polar bear, with its white fur a symbol of the Arctic, is
Earth's biggest land carnivore and the largest of the eight bear
species. It occupies the top spot in the Arctic food chain and
spends a lot of its life on Arctic sea ice hunting for prey like
ringed seals and bearded seals. Adult males can weigh up to about
1,700 pounds (770 kg).
It is endangered - climate change is causing shrinking of the sea
ice on which it depends - with an estimated total population of
20,000 to 25,000.
Their diet allows for a body fat build-up for insulation and
buoyancy while swimming. But it comes at a cost - very high levels
of LDL cholesterol, known as the "bad cholesterol," and
triglycerides in the blood. They have levels that would cause
cardiovascular disease in humans, but their genetic changes seem to
have taken care of the problem.
One important example was a change in the gene APOB, which plays a
role in getting cholesterol out of the bloodstream and into cells,
thereby lowering heart disease risk.
"This all makes very good sense in a species which is completely
reliant on fat for survival," Lorenzen said.
The study was published on Thursday in the journal Cell.
(Reporting by Will Dunham; editing by Andrew Hay)
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