Polar bears have captivated humans since the beginning of recorded time. Writings dating back to 57 A.D. in ancient Rome describe flooded amphitheatres where polar bears and seals were pitted against each other (not much of a fight, as it turns out). Kings of ancient Egypt and early Norway were the first to keep polar bears captive, and the fascination with them has held through the centuries [source: Polar Bears International]. A terrifying version of the animal inexplicably shows up on a tropical island in the TV series "Lost." Ad campaigns all over the world feature polar bears to sell everything from bottled water to transportation systems to soda.
Polar bears' shocking whiteness, ferocity and sheer size make them icons of purity and power. Males can grow up to 10 feet tall and weigh up to 1,700 pounds. A single bear can take on a 2,500-pound walrus, and hang onto the massive catch with 2-inch claws. They walk hundreds of miles across ice sheets every winter, following their prey.
In the Arctic, polar bears are at the top of the food chain, and it's not just because of their size. They're so well-adapted to their environment that they experience no change in body temperature at -34 degrees Fahrenheit (-37 Celsius). However, the incredible survival mechanisms that let them thrive in frigid winters may prove to be their undoing. Because of global warming, winters are getting less cold and polar bears are dying. Scientists project that if climate trends continue, polar bears could be extinct by 2050 [source: Polar Bears International].
What's life like for an animal that needs such extreme cold to survive? Let's head to the Arctic and see how polar bears live on the ice.
Polar Bear Habitat: Life on the Ice
The polar bear, or Ursus maritimus (sea bear), probably evolved about 200,000 years ago from brown bears [source: Polar Bears International]. A polar bear can actually mate successfully with a brown bear and the resulting offspring is fertile. There are a lot more brown bears out there than polar bears, though. Brown bears number a couple hundred thousand worldwide [source: WWF]. Polar bears only number about 25,000.
Polar bears live only in the Northern Hemisphere -- you won't find them at the South Pole. The 25,000 live in 19 separate populations throughout the Arctic, in only five countries: the United States (Alaska), Canada, Russia, Greenland and Norway. About 60 percent of the population lives in Canada.
Life in the Arctic is harsh: The bears live in total darkness between October and February, and the temperature can drop as low as -50 F (-45 C) in winter. And that's exactly how they like it.
Polar bears are built for extreme cold. They experience almost no heat loss: Two layers of fur and a blubber layer up to 4.5 inches (11.5 centimeters) thick keep them so well insulated, they'll overheat if they run. The areas that lack this insulation -- ears, tail and muzzle -- are especially small, minimizing non-insulated surface area.
Polar bears mostly walk slowly, following their favorite prey, the seal, from ice sheet to ice sheet. They need the ice to hunt. In warmer months, when ice sheets get smaller, the bears will walk hundreds of miles to find solid spreads of ice.
Polar bears can walk up to 20 miles (30 kilometers) per day, for several days in a row, relying on tiny bumps on the bottoms of their feet to keep them from slipping on the ice. They'll swim, too, both to cool off after a meal and to bridge the gap between ice sheets when they're following seals. Polar bears use their front paws to paddle and their hind legs to steer (imagine the most powerful doggie paddling ever). They'll go only slightly under the water when they swim, and their nostrils close up when they're submerged. As much as they thrive on the ice, they're strong swimmers. Polar bears have been tracked swimming up to 60 miles (100 kilometers) at a time, and at up to 6 miles per hour (10 kilometers per hour) [source: SeaWorld].
Aside from their dependence on staggering cold, one of the biggest differences between polar bears and other bears is that polar bears don't hibernate. Females go into a sort of semi-hibernation toward end of their pregnancy, but they don't experience the drop in heart rate and body temperature that characterizes real hibernation. They mostly just rest and sleep a lot in the months immediately before and after they give birth.
However, births are declining. Of the 19 polar bear populations, at least five are known to be shrinking dramatically. One population in Canada has decreased more than 20 percent in last two decades [source: Polar Bears International].
This serious drop in population is due to climate change, and it has a lot to do with the way polar bears hunt. In the next section, we'll find out why.
Polar Bear Diet: Bears on the Hunt
In the Arctic, polar bears top the food chain. Both males and females hunt, and their favorite meal is seal, especially a ringed or bearded seal. When hunting's especially good, they'll just eat the seal's fat -- which helps build up insulation -- and leave the rest behind. The biggest bears can eat 100 pounds of blubber in one meal [source: Polar Bears International].
While polar bears prefer seals, they're opportunists. They'll eat anything from beluga whales to walruses [source: Polar Bears International]. The bears can find food on land if they have to, attacking reindeer and raiding birds' nests (bird eggs are a favorite snack), but they aren't well-adapted to hunting on land.
Instead, their strength is on the ice. Polar bears can't swim fast enough to catch a seal in the water and need an ice platform to use their full strength to grab one. But with global temperatures rising, Arctic ice floes melt earlier in the year than they used to, reducing the surface area on which bears can hunt. It also means that when polar bears wander onto land, they can end up stranded because the distance between land and the nearest ice sheet is too far to swim in one shot.
A hunt starts with a scent. Polar bears can smell seals up to 20 miles (30 kilometers) away, often by the scent left on their breathing holes [source: SeaWorld]. In the fall, when the ice is softer, seals cut holes in the ice so they can come up for air when they need to. Polar bears find such breathing holes and wait, sometimes for several days, until a seal comes up for a breath.
When a polar bear spots a seal coming up for air, it gets down on all fours, delicately putting each paw on the ice to keep silent. The bear then makes a shallow dive through the hole to grab the seal with its claws. Those sharp, 2-inch claws grip the seal extremely well. Still, seals sometimes get away. Polar bears have been known to get upset when they lose their prey, pounding the ice or throwing blocks of it in a sort of tantrum. When the hunt is successful, a bear will share a kill with others as long as they beg properly: keeping low, circling the kill and occasionally nudging the hunter with their noses.
After eating, polar bears always clean up, which is why their fur stays so white. They need to stay dry and fluffy to maintain their body heat. They bathe in water during the summer and snow during the winter, using their tongues to clean their paws and cubs. After a good meal and a bath, they'll often take a nap, for much the same reason: Their survival depends on energy conservation. After any kind of exertion, they'll often lie down and go to sleep for an hour or so. Like humans, a polar bear's full "night's rest" lasts for about eight hours, although it's as likely to take place during the day as at night. The bears spend most of their time in 24-hour darkness and 24-hour light depending on the season, so time of day is largely irrelevant
Life on the ice is pretty routine for a polar bear, unless it's a pregnant one. Next, we'll see how a polar bear prepares to give birth and how she cares for her young.
Polar Bear Reproduction
As with most animals, polar bears mate in the spring. Scientists aren't sure how sexually mature males find females who are ready to mate. Brown bear females seem to leave a scent trail for males to follow. This could also be the case with polar bears.
Females usually mate successfully for the first time between the ages of six and eight. They only have about five litters in their lifespan, which is one of the lowest reproduction rates among mammals. Females only mate every few years, so males must compete fiercely for a female to mate with. When male bears fight over a female, the challenger will lower his head, put his ears back and open his mouth wide, roaring and bearing teeth. Polar bears seldom fight to the death for mating rights; the weaker bear usually submits after an injury.
Female polar bears don't really go into heat the way some other mammals do. They're induced ovulators, which means that it's intercourse itself that causes their ovaries to release an egg. Ovulation doesn't happen immediately, and it may take several tries before a successful mating occurs. Once mated, the couple stays together for about a week before separating. Polar bears aren't monogamous animals -- a strong male might impregnate several females in one season.
Like most aspects of polar-bear life, the pregnancy process is about energy conservation. A pregnant female feeds heavily in the spring to build up her fat reserves and prepare for a maternity rest in the fall. In the maternity den, only the cubs eat, consuming their mother's high-fat breast milk for their first months of life.
In late fall, the female digs a cave in a snowdrift, either on a mountainside very close to sea ice or on the sea ice itself. This den is protected from the wind, and provides a secure place to sleep. In early winter, the female gives birth after an approximate eight-month gestation period [source: SeaWorld]. However, it only takes four months for the unborn cub to actually develop. During the first four months of pregnancy, the embryo is stagnant in the uterus while the mother gains the weight (about 450 pounds or 200 kilograms) she'll need to ensure its development and proper feeding after birth.
There are typically two cubs per litter, and they're surprisingly small. A cub weighs about a pound at birth and measures about a foot long. It's also blind, toothless and lacking insulation, with only very short, thin fur. Polar bear cubs have no chance of survival without their mother. The family stays in the maternity den until early spring, and the mother doesn't drink, eat or defecate during that time. All she does is protect and feed her young. Even after they leave the den, cubs stay with their mom until they're about two years old, learning to hunt, clean themselves and survive in the harshest of habitats.
Like many mothers in the animal kingdom, a polar bear will kill anyone or anything that comes near her young. Polar bears are predators, pure and simple. So what happens when they get stuck on land, with people around? There might be trouble -- but as we'll find out, polar bears seldom go looking for a fight with a human.
When Polar Bears Attack
Modern zoos around the world keep polar bears, often trying to emulate their natural habitats as much as possible. A zoo is pretty much the only place where it's safe for a human to get anywhere near a polar bear.
With sea ice melting, polar bears are frequently forced to move onto land in the warmer months. More polar bears on land -- looking for sustenance where their natural prey is absent -- means more interactions with humans, and more attacks. Polar bears don't seek out confrontations, but if they're really hungry or feel threatened, they'll kill -- and eat -- a human without a second thought.
Still, attacks are extremely rare, and they're always by a bear that's either starving or provoked [source: Polar Bears International]. The town that hosts the most polar bears in the world, Churchill, Manitoba, has had two people killed by polar bears in almost 300 years [source: Polar Bears International]. The first attack took place after several teenagers came upon a bear and started throwing rocks at it. The second attack was on a man who had meat in his pockets and happened to come upon a hungry bear.
In the last 30 years, eight people have been killed by polar bears in United States and Canada. In all of recorded Russian history, polar bears have killed only 19 people [source: Polar Bears International]. In the town of Svalbard, Norway, which covers an area about the size of Maryland, the population consists of 2,300 people and 3,000 polar bears. The town posts polar-bear warning signs the way other areas post deer-crossing signs. In such close quarters, there have been just four fatal attacks in the last 35 years. When the bears venture too close to humans, residents try to scare them away, revving snowmobile engines, sending up helicopters or shooting flare guns. As a last resort, if the bears won't leave the area, they'll shoot them to avoid an attack. The people of Svalbard had to kill 24 bears between 1998 and 2005 [source: AFP].
With only 25,000 polar bears left, 24 lost in a single town in the interest of protecting humans is actually quite a few. As food and proper hunting grounds continue to dwindle, interactions between polar bears and humans will increase, necessitating more shootings. It's a vicious cycle that starts with climate change and could end with an extinct species in the next 40 years.
For more information on polar bears, including what's being done to try to save them, paw through the links on the next page.
Related HowStuffWorks Articles
More Great Links
- Animals in advertising. Bartolomeo Mecánico: Advertising studies. http://www.elve.net/panim/en/bear02.htm
- Polar Bears. Seaworld.org. http://www.seaworld.org/infobooks/PolarBears/home.html
- Polar Bear Facts. Kidzworld. http://www.kidzworld.com/article/430-wild-things-the-polar-bear
- Polar Bears International http://www.polarbearsinternational.org/
- Svalbard, where man and polar bears share the art of living. AFP. March 16, 2008. http://afp.google.com/article/ALeqM5h9kkj3wGCFTXRGNcUYbw8-VkEULw