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Polar Bear Metabolism Is Less Resilient to Summer Food Shortages Than Expected

When polar bears' feeding opportunities decrease during the summer ice melt, the animals can reduce their energy expenditure a little, but not enough to make up for the food shortages, a study in the 17 July issue of Science shows.

The summer months have always been challenging for polar bears, when receding ice makes it more difficult for the bears to ambush seals that surface for air. As their hunting territories dwindle, many bears relocate to the shore, where food is less plentiful but there may be some opportunities for the bears to forage.

For the first time, researchers have measured the energy expenditure of polar bears in the wild during the summer months. Before this, some scientists suggested that polar bears forced ashore compensate by entering a low-energy state called "walking hibernation," and that this strategy could help the animals survive the food shortages associated with reduced ice.

"If [walking hibernation] were true, it may have helped bears cope with lost hunting opportunities in the summer, as they would have had lower energy requirements," explained lead author John Whiteman of the University of Wyoming. "However, our data indicate that this is not the case."

As a warming climate continues to accelerate the summer ice melts, it is important to understand how polar bears are — or are not — adapting to even more extreme food shortages.

To measure polar bears' energy expenditure during the summer months, Whiteman and his colleagues used satellite collars to track the movement of individual bears, both on shore and on ice. Flying via helicopter into remote areas to reach the bears, the team surgically implanted temperature loggers in the bears' abdomens.

By monitoring the bears' core body temperature and movement over the course of the summer in 2009, the team realized that the iconic white bears of the Arctic are probably feeling hunger pangs more than previously thought.

The results suggest that polar bears, both on ice and shore, cannot slow down their metabolism enough to make it through the summer without burning through much of their stored fat. Bears in both habitats reduced their body temperatures and activity levels below those of bears actively hunting and feeding, but not to levels as low as those observed during energy-saving hibernation.

As the climate continues to warm, Whiteman said there is a particular urgency to understand the functioning of organisms and ecosystems that are threatened by climate change, which can inform conservation efforts. His team will continue to evaluate the polar bear's ability to adapt to a warming climate.

"Now that we have a better understanding of polar bear energy expenditures during summer because of this study, we are analyzing additional data we collected which relates to their energy storage," Whiteman said. "A clear understanding of energy use and energy storage will help improve models of how bears will respond to future changes in the sea ice."

[Credit for associated teaser image: Shawn Harper]