Lead researcher Anthony Pagano noted that the polar bears were monitored during the period that polar bears catch most of their prey, and are expected to put on most of the body mass needed to sustain them throughout the year.
IUCN experts estimate that the population could decrease by 30 to 50% if the loss of sea ice continues. The remote location has made it hard for scientists to study the bears' movement and activities until now.
The iconic animals require 1.6 times more energy than was estimated in the 1990s, researchers at the US Geological Survey, the Alaska Science Center in Anchorage, and at the University of California at Santa Cruz report in the journal "Science".
They collared nine adult female polar bears on the sea ice of the Beaufort Sea in Alaska with a Global Positioning System video camera and observed the bears for discreet time periods over three consecutive years. They used collars equipped with point-of-view cameras and location-tracking tech to follow the bears' movements and feeding behavior.
During the test, researchers saw about half of hunting bears lost 10 percent of their body weight in just over a week.
The data showed the bears were active about 35 percent of the time and resting for the remainder, yet they burned through 12,325 calories a day, much of it from their body reserves.
The research by Pagano and his colleagues, as well as previous work by other scientists, suggests that as sea ice becomes increasingly fragmented and short-lived year by year, polar bears are likely to experience more and more stress - and to die in ever greater numbers - leading to continuing declines in their populations. That may be partly because the carnivores typically spend more energy than animals with herbivorous or mixed diets, pointed out John P. Whiteman of the University of New Mexico in Albuquerque, who was not involved in the study. But scientists have struggled to quantify how much energy free-ranging polar bears need and how many seals they need to catch to avoid this negative energy imbalance. But because the ice is shrinking, the bears are having a harder time catching seal pups even during prime hunting time, Pagano said. Getting stuck on land simply wouldn't keep these apex predators alive.
Analysis of the bears' metabolism revealed they required at least one seal every 10 days to satisfy their dietary needs, but numerous bears were unable to capture enough.
Polar bears are now listed as vulnerable on the International Union for Conservation of Nature's Red List of Threatened Species.
"However", the authors wrote, "this rate of consumption would be necessary simply to energetically break even".
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.
As more sea ice melts in a warming Arctic, polar bears are forced to travel longer distances to find prey, as well as move farther north with the retreating ice in the summer. Thanks to the metabolic rates documented in the new findings, he added, "these costs can now be modeled more precisely". Now, scientists can see the effects: Polar bears are swimming themselves to the brink of starvation. The new study shows that's not the case. These animals lost weight during the period that they were observed.
Bears can lose weight fast but also gain it back quickly if they can catch seals.
Polar bears are listed as threatened under the US Endangered Species Act and as vulnerable by the International Union for the Conservation of Nature - a designation just short of endangered on both lists.
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