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Who has never craved a cream cake, longed for hot buttered toast or fancied
French fries? The urge to eat fatty food is understandable, given our origins.
Instead of counting the calories in nuts and meat, our hominid forebears out on
the African savannah would have counted on them to supplement their diet of
roots and berries. Times have changed, though, and in today’s fast-food world,
the evolutionary legacy of that particular survival instinct is obesity and
heart disease.

Fat may be our undoing, but other animals thrive on it. The rump of a healthy
adult polar bear is covered in a layer of the stuff up to 10 centimetres thick.
The animal’s diet consists mainly of seals, especially the thick blubber of
adults. Its taste for fat is so strong that it will often go to all the trouble
of killing a seal pup only to reject the lean body, eating nothing but the
lipid-rich brain. Polar bears may look like bleached variations on a theme,
but in adapting to their Arctic environment they have evolved a lifestyle that
is very different from any other bear’s. Obesity is the key to their
survival.

These are evolutionary upstarts. They appeared only about 100 000 years ago,
during one of the long cold periods of the Pleistocene ice age when the climate
over much of Europe was similar to that of northern Canada today. Their
ancestors would have been omnivorous, like most modern bears, eating leaves,
fruit and roots, together with a little fish, small prey and carrion. But in the
high Arctic it is too cold, and for several months of the year, too dark, to
sustain plants and the animals that feed on them. The sea is the only larder for
a large, active mammal, and not a very accessible one for a doggy-paddling bear.
By concentrating their hunting efforts on ringed seals, polar bears make the
most of their talents.

Seals have to surface to breathe every few minutes, and their pups are born
and suckled on the ice. Therein lie the bears’ best chances. Where the sea is
frozen, the seals make their own breathing holes by gnawing or scraping against
the underside of the ice with their teeth and claws. A polar bear may stalk a
seal while it is resting on the ice, or wait quietly beside a breathing hole,
sometimes for hours. When a seal surfaces, the bear grabs it by the nose and
hauls it onto the ice, where it cannot move fast or put up much resistance. Seal
pups make even easier prey. Until they are weaned, they shelter from the severe
weather in holes that their mothers dig in snowdrifts. Bears, with their acute
sense of smell and powerful clawed forelegs, can locate and excavate a concealed
pup through several metres of snow.

Unless bears are growing, starving or lactating, they only need a little
protein. Too much can be poisonous. Converting the carbon and hydrogen in
protein into glucose for energy production, or lipids that can be stored for
later use, is a slow and inefficient process. Most of the nitrogen in the
protein must be excreted, and this takes energy. While the liver is struggling
to convert excess protein into glucose, high levels of amino acids—the
building blocks of protein—circulate in the blood, and may upset the
chemistry of nerve transmission and other important processes. Digesting and
metabolising fat poses far fewer problems, and polar bears can take advantage of
this because they have high levels of lipoproteins to chaperone fats around in
the bloodstream and plenty of adipose tissue to store it in.

Seal blubber is nutritious but its supply is irregular, so polar bears could
not survive without stashing fat away in their own adipose tissue to be used
during hard times. Moderate levels of obesity are essential to their way of
life. Every year, researchers in the Arctic find a few individuals—adults
as well as newly weaned juveniles—in a severely emaciated condition.
Sometimes injury is to blame, but many very thin bears have suffered nothing
more than a run of bad luck.

Waiting for winter

The main problem is the unpredictable nature of the food supply. If the ice
is too thick, the seals cannot make breathing holes so they go elsewhere.
Satellite tracking shows that bears travel huge distances to track the seals,
covering at least 30 kilometres a day for several weeks. As soon as the ice
breaks up in the spring, seals can breathe anywhere so the bears cannot catch
them. Summer is a lean time for polar bears.

Without a permanent food source, males are unable to carve out territories
for themselves and their families as most bears do. Instead of a stable family
life, with each mature male defending his own patch and protecting his females
and cubs from intruders, polar bears go their separate ways after mating. Most
of the families that an itinerant father later encounters do not share his
genes, and he sees the cubs as just another source of food. The danger of
infanticide turns breeding females into refugees.

Pregnant polar bears living around Hudson Bay and the islands off the
Canadian coast leave the hunting grounds during November. After fattening
themselves up to as much as twice their non-pregnant body mass, they travel
inland, sometimes more than 100 kilometres, and dig a den in a snowdrift where
the cubs—usually twins—are born. A new mother remains with her cubs
for up to four months, suckling them on creamy milk produced entirely from her
own body reserves. She will have nothing to eat until March or April, when they
walk back to the coast.

Without her rolls of fat, the female polar bear would struggle to bring new
cubs into this harsh environment. Obesity, which for humans is a modern scourge,
is what keeps polar bears going. It is the key adaptation that has allowed the
largest living species of the order Carnivora to flourish against the odds.