National Geographic News
A polar bear swimming in the Beaufort Sea.
A polar bear swims in the Beaufort Sea (file picture).

Photograph by Steven Kazlowski, Alaska Stock Images/National Geographic

Anne Casselman

for National Geographic News

Published July 20, 2011

A female polar bear swam for a record-breaking nine days straight, traversing 426 miles (687 kilometers) of water—equivalent to the distance between Washington, D.C., and Boston, a new study says.

The predator made her epic journey in the Beaufort Sea (see map), where sea ice is shrinking due to global warming, forcing mother bears to swim greater and greater distances to reach land—to the peril of their cubs.

The cub of the record-setting bear, for instance, died at some point between starting the swim and when the researchers next observed the mother on land. She also lost 22 percent of her body weight.

"We're pretty sure that these animals didn't have to do these long swims before, because 687-kilometer stretches of open water didn't occur very often in the evolutionary history of the polar bear," said study co-author Steven Amstrup, chief scientist for the conservation group Polar Bears International. Amstrup is also the former project leader of polar bear research for the U.S. Geological Survey (USGS), which led the new study.

Another female bear in the study swam for more than 12 days, but appears to have found places to rest during her journey.

(Related "Polar Bears Turning to Goose Eggs to Survive Warming?")

Long Swims Deadly for Polar Bear Cubs

Biologists collared 68 female polar bears between 2004 and 2009 to study their movements. Thanks to what study co-author and WWF polar bear biologist Geoff York calls an "accident of technology and design," the researchers noticed data gaps in the bears' whereabouts. The researchers were later able to link the gaps to periods when the bears were at sea. (See polar bear pictures.)

The scientists examined GPS data for more than 50 female polar bears' long-distance swimming events, defined as swims longer than 30 miles (50 kilometers). This data was then correlated to rates of cub survival.

"Bears that engaged in long-distance swimming were more likely to experience cub loss," said study co-author George Durner, a USGS research zoologist in Anchorage, Alaska.

Five of the 11 mothers that had cubs before they began their lengthy swims lost their young by the time the researchers observed them again on land, according to the research, presented July 19 at the International Bear Association Conference in Ottawa, Canada. The study is not yet published in a journal.

Sea Ice Loss to Continue

Until 1995, summer sea ice usually remained over along the continental shelf of the Beaufort Sea, a critical habitat for polar bears due to its rich seal population. Now the sea ice in the Beaufort and Chukchi Seas is retreating from the coast by hundreds of kilometers, Durner said.

(Read "The Big Thaw" in National Geographic magazine.)

In 2010, Arctic sea ice extent was the third lowest on record, part of a long-term trend of ice loss that will continue for decades to come, according to the National Snow and Ice Data Center in Boulder, Colorado.

"So the sort of conditions that contribute to long-distance swimming are likely going to persist in the future, and if cub mortality is directly related to this, then it would have a negative impact on the population," Durner said.

It's unknown whether the cubs are drowning at sea or whether the metabolically costly act of swimming long distances in nearly freezing water kills them after they reach land.

5 comments
Krista Grieve
Krista Grieve

Polar bears live in the crisp, brisk Arctic region. Ursus maritimus, its scientific name, means sea bear, as they are the only marine mammals in the world. The polar bears that exist today evolved from brown bear ancestors over 600,000 years ago. However, unlike brown bears, polar bears live in Northern frozen areas with lots of snow, rather than in temperate climates.

Ursus maritimus are most common in Canada, home to over 60% of the world’s polar bear population. They are also known to live in parts of Alaska, Greenland, Russia, and Norway.

Male polar bears weigh between 775 and 1200 pounds, while females measure 330 to 650 pounds. However, female polar bears who are expecting cubs may weigh 600 pounds.

Polar bears usually have two cubs. Single cubs and triplets can occur, depending on the mother’s condition and health. After birth, cubs spend the first couple years of their life with their mothers, learning how to hunt and survive in the harsh environment.

The fur of a polar bear is not actually white; it appears white by reflecting visible light. When wandering on land, their heavy fur prevents almost all of their heat from escaping their body. In the frigid water, they depend on their fat to keep them warm. Due the lack of fat in cubs, mothers keep their cub’s fur dry for warmth and survival.

Polar bear paws can measure one foot across, making it easier for them to walk on thin ice. Small bumps on their paws, known as papillae, prevent the bear from slipping. Bears use their pointed claws and sharp teeth to catch and eat ringed seals, their favorite prey.

It is believed that polar bears are very intelligent animals, potentially even smarter than apes. Polar bears smash ice blocks to reach frozen fish lodged inside. Polar bears are very strategic about the techniques they use to hunt their prey. They have extraordinary senses: smelling, hearing, vision.

Polar bears communicate through vocalization and body language. Loud noises, made with their voice, are often used to exhibit anger. When a bear stands on its hind legs and shakes its head it is showing that it is playful.

Contrary to popular belief, only pregnant polar bears hibernate during the winter months. Males and non-pregnant females are active throughout the year. Bears that hibernate stay in their den until March or April.

Although the exact population of polar bears is unknown, it is estimated that the population is currently between 20,000 and 25,000 bears. The population is decreasing due to climate change melting the ice in the Arctic. Levels of greenhouse gas are rising because of human activities. People need to work together to reduce emissions so the polar bears can be saved before it is too late.

N deneb
N deneb

The poor bears !!. Their plight is extremely touching. Its matter of life and death and extinction.  I think we need to protect animals in general by keeping their environment safe.  Many are being killed by humans such as lions, tigers, elephants and other animals. Their  numbers were so great before but now there are  dwindling fast,  The whole Earth is  interdependent on the biodiversity including the rainforest. So  we should  be looking after  not only people but all the animals which all play and will play an important role in keeping the earth healthy for generations to come.  Animals take only what they need for their survival and not like humans sports  killing for trophy, body parts of the tigers and  elephants tusks.  And they grieve for their dead relative.   Who speaks for these animals?  We need to protect them. Humans have changed the environment and taken over the animals' lands and then they blame the animals.  Lets take care of all these magnificent creatures that are very sensitive and very intelligent and  have a lot to offer us and the planet.

Tess Nguyen
Tess Nguyen

Sory, now people are so poor, they just have enough ability to keep cool for themselves in a small room.

Ernst Binky
Ernst Binky

i can't believe the insensitivity of this "news". the bear is desperate, not "record-breaking". this isn't a vanity event like jumping off Mt Everest. it's a matter not only of life and death but not going extinct. 

Joan Churton
Joan Churton

I feel sad that polar bear cubs aredying because of global warming.  The population will definately decrease. Damn.

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