Arctic sea ice in December was at its second-lowest level since 1979, according to the National Snow & Ice Data Center.
In this region, where Arctic sea ice declines have been among the most rapid, the polar bear population has declined by about 40 percent over the past decade.
"It's quite fascinating to see how the bears are using the habitat and how the habitat is influencing their behaviour, activity patterns and their ability to catch seals, " said lead author Anthony Pagano, a wildlife biologist at the US Geological Service. "And it appears to be related to changes in sea ice that are occurring". It has been hard, however, for researchers to study the fundamental biology and behavior of polar bears in this very remote and harsh environment, Pagano said.
In 2015, after working with the accelerometer manufacturer, we managed once again to deploy 4 Global Positioning System video camera collars with triaxial accelerometers on polar bears. Four of the bears lost nearly 10 percent of their body mass - about 40 pounds (18 kilograms).
To measure the animals' energy expenditure, they dosed the bears with doubly labeled water - water molecules whose hydrogen and oxygen atoms had been replaced with heavier isotopes with extra neutrons.
The amount of carbon dioxide produced reveals how hard the animals are working. The study reinforces the understanding that polar bears are reliant on a diet of fat-rich seals to survive in the energetically-demanding Arctic.
But the scientists found the bear's metabolic rate was 1.6 times greater than previously thought - akin to that of other carnivores.
University of New Mexico biologist John Whiteman wrote a perspective piece on the study for Science. This means polar bears have to travel farther during spring - the key period when they do most of their hunting and put on most of their weight - to find the seals they prey upon.
Previously, polar bears were thought to expend relatively little energy during days where they often wait for hours beside holes in the ice, which seals emerge from in order to breathe. Getting stuck on land simply wouldn't keep these apex predators alive.
One bear lost close to 44 pounds, including her lean muscle, in 10 days. Even though the findings are based on only a few female polar bears, the result will be same for the polar bears of other areas.
The research was conducted during a time when the polar bears were supposed to be putting on weight so that they could later have cubs, feed those cubs, and survive the winter.
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Polar bears are starving and scientists believe that shrinking sea ice caused by climate change is to blame.
As the ice dwindles, "we are essentially pulling the rug out from underneath the polar bears", Durner said.
As a scientist, he stresses that we shouldn't go off of gut feelings, but rather reliable data - "and for polar bears, those (data) aren't there yet".
The new study also gave an idea how they hunt.
By comparing the bears' blood samples before and after, they could calculate the amount of carbon dioxide that the animal had produced, and thus its metabolic rate.
While four bears put on weight, five bears lost up to 10 per cent of their body weight - between 18 and 20 kg - in 10 days.
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From an evolutionary point of view, a loss in body weight in some animals doesn't necessarily mean that the total number of polar bears will decline, Fickel stresses.
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