Polar Bears Require More Food to Survive Than Thought

Polar bears' bodies work 60% harder than thought — which makes surviving climate change even tougher

Scientists were given an insight into the bears' weight problems by fitting nine females with satellite collars fixed with video cameras over the past three springs in Alaska's Prudhoe Bay.

They used tracking collars attached to the bears to record their activity, and analysed their blood and urine and the beginning and end of an eight to 11 day period to determine changes in metabolic rates.

"The high energy requirements of polar bears corroborate previous hypotheses that most terrestrial Arctic habitats, lacking prey as energy-rich as marine mammals, can not provide enough food for polar bears driven to shore by loss of sea ice", Whiteman wrote in a commentary on the paper. If Arctic sea ice continues to decrease at the current rate, the world's polar bear population is projected to decline by up to 30 percent in the next 30 years.

Pinning down that figure is an important step in understanding what is happening to polar bears now and in the future with climate change, said polar ecologist John P Whiteman of the University of New Mexico.

"Ice loss, if unabated, will eventually cause the extinction of polar bears in the wild", Whiteman said, "but continued research is needed to understand the climate-related pressures that polar bears face".

USGS researchers' most recent estimate of the polar bear population indicates their numbers have declined by 40% over the past decade, but a common problem for researchers is that it has been hard to study polar bear behavior in these harsh environments, Pagano said in a statement. Because of the video footage, the researchers know that these bears gained weight because they caught seals. In fact, five of the bears lost body mass during the time they were studied.

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Anthony Pagano, a research wildlife biologist with the U.S. Geological Survey and the lead author of the study, says this is the first time a group of polar bears has been studied this intensely.

The researchers hypothesize that the bears' lazy hunting style - when it works - allows them to conserve energy, helping them survive through the summer months when food is scarce. For example, since polar bears are primarily "sit and wait" hunters, it was thought this would minimize their energy expenditure during hunting. One bear lost 51 pounds (23 kilograms) in just nine days. Polar bears rely on seals for their enormous caloric needs-the four bears that lost 10% of their weight didn't catch a single seal during the study and relied on scavenging-and sea ice is pretty much a requirement for polar bears to catch seals.

"We've been documenting declines in polar bear survival rates, body condition, and population numbers over the past decade", Mr Pagano said.

Although it's just a 10-day snapshot, the study confirms that polar bears aren't made for walking, said Andrew Derocher, Canada's leading polar bear expert and a professor at the University of Alberta.

In a paper published Thursday in the journal Science, the researchers report that the polar bears have significantly higher metabolic rates than previously thought - meaning they consume more energy. During that time the bear crossed into Canada, having walked almost 270 miles (430 kilometers) since her collar had been applied. In the Beaufort Sea, sea ice starts to retreat away from the continental shelf in July, and most of the bears move north on the ice as it retreats. Our measures of metabolism were considerably higher than previous estimates of polar bear metabolic rates.

As the ice dwindles, "we are essentially pulling the rug out from underneath the polar bears", Durner said.

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