In other words, polar bears were burning more energy from hunting than they were gaining energy from food. That could explain the 40% decline in polar bear population in the southern Beaufort Sea between 2001 and 2010 as noted by an earlier study.
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.
The GPS told them the distances the bears wandered, and the video camera recorded if the bears were successful in killing and eating prey. More than half of the bears were decreasing in body mass during their main hunting period, the study found.
"Two-thirds of the world's polar bears could die out by 2050", cautions WWF, demanding an immediate decline in greenhouse gas emissions so the bears can stand a chance of survival. And every additional mile that a polar bear has to traverse under its own power is that much more food the bear has to eat. For now, the study demonstrates that bears' energy demands are likely higher than expected-meaning they require more food to survive than previously thought-and that failure to catch seals can result in rapid, significant weight loss.
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.
Polar bears are considered endangered in the US and are listed as "vulnerable" by the IUCN, because their sea ice habitat is under threat from climate change.
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. To minimize their energy consumption the bears still-hunt, waiting for hours by seals' cone-shaped breathing holes in the sea ice. One female bear Griffen studied swam 426 miles over nine days.
"We've been documenting declines in polar bear survival rates, body condition, and population numbers over the past decade", Pagano said.
The fact that climate change might hit polar bears hard is not a surprise.
This polar bear wears a high-tech collar for tracking its behavior, location and hunting success. During that time the bear crossed into Canada, having walked almost 270 miles (430 kilometers) since her collar had been applied.
In total, the bears would have to eat one adult ringed seal or 19 newborn seal pups every 10 to 12 days to avoid starvation, the researchers say.
"By simultaneously measuring field metabolic rates, daily activity patterns, body condition, and foraging success of polar bears moving on the spring sea ice, we found that high metabolic rates (1.6 times greater than previously assumed) coupled with low intake of fat-rich marine mammal prey resulted in an energy deficit for more than half of the bears examined". A few of the bears travelled more than 155 miles (250 kilometers) in about 10 days off the northern coast of Alaska in the Beaufort Sea, Pagano said.
Previous studies had tried to estimate polar bear metabolic rates and energy expenditures based on some assumptions about their behavior and physiology.
"I hope we will have an awakening, but we haven't really done much to save polar bears over the past decade", said Amstrup.