It's been an ongoing worry for years, and last week the United States Fish and Wildlife Service drove it home again with a new conservation management plan, which identifies climate change and sea ice loss as the primary threat to polar bears.
Despite all the doom and gloom, some research conducted in the early 1980s has helped conservationists maintain a glimmer of hope about the polar bear's ability to survive long periods of time without food. This research found evidence in polar bear blood samples to suggest that the bears might go into a kind of "walking hibernation" when food is scarce, staying awake but significantly lowering their metabolism to use less energy. This would be a useful adaptation during the summer, when sea ice is at its lowest extent and hunting is most difficult.
It's been a tempting theory for more than 30 years - but once again, we're looking at bad news for the polar bear. A new study, published yesterday in Science, debunks the "walking hibernation" idea with data collected from more than two dozen captured polar bears in the Arctic's Beaufort Sea, which the researchers spotted and tranquillised from helicopters.
The researchers, led by biologist John Whiteman at the University of Wyoming, outfitted bears with devices that collect and transit data remotely to collect data on the bears' movement and activity and their body temperature. Their sample included both "ice bears" and "shore bears" - that is, both bears who choose to chase the ice as it retreats north in the summer, looking for seals, and bears who choose to spend their summer on shore.
The researchers expected that if bears did indeed exhibit walking hibernation, their activity and temperature would drop down to the kinds of levels usually observed in other bears during true hibernation - that is, very low levels.
"If there was hibernation metabolism ... you would see all of them have a very steep, abrupt decline in body temperature to about 35C and then remain like that the whole period," says senior author Merav Ben-David, a professor of wildlife ecology at the University of Wyoming. "But we don't see that."
Instead, the results showed that both ice bears and shore bears experience much more moderate declines in body temperatures during the summer, when food is scarce - just the kinds of declines you would expect to see in any mammal that wasn't getting enough food.
"If you went into an extended fast, your body temperature would decline, too," Ben-David says. "It's a normal mammalian response to fasting and losing metabolically active tissue - losing weight."
Unfortunately, this means polar bears have no special protections against starvation as previously thought. They simply exhibit a typical fasting response to food deprivation, and - given a long enough time without food - will starve.
There are an estimated 20,000-25,000 polar bears, grouped into 19 sub-populations. While scientists don't have adequate data for all the populations, we know at least some of them are declining, and the polar bear is currently protected under the Endangered Species Act.
Scientists worry that their populations will suffer worse declines in the future as sea ice continues to melt.
Having more information about the bears' activity can help scientists develop more accurate models about how they will withstand food deprivation and what will happen to their populations, says Whiteman, the lead author. When producing these models, scientists now know to plug in a normal metabolic rate for the bears rather than assuming they may go into walking hibernation.
It's worth noting that while polar bears didn't show any signs of walking hibernation, the study did produce evidence for another kind of special adaptation that may help bears survive under stressful conditions.
The data suggests that when bears swim in the Arctic water for extended periods of time - longer than a few minutes, according to Ben-David - they can cool down the outer parts of their body cores so less body heat is lost to the frigid water.
"This is an exciting new finding and something we didn't know about the bears," Whiteman says. And it may be increasingly useful in the future when retreating sea ice forces bears to swim for longer periods of time to get between the ice and the shore.
But it's not to say they can swim forever. One of the bears being tracked swam for nine days and amazingly survived the journey, but lost nearly a quarter of her body mass by the time she came out of the water.
When it's all said and done, the results of the study confirm that polar bears aren't superheroes - they're susceptible to the same kinds of stresses, including food stress, that other animals are.
But while the paper gives us some deeper insights into the bears' physiological responses to tough times, Whiteman cautions that there's still a lot left to learn.
"We're not looking at any quantitative predictions for what will happen to the bears in the future in terms of climate change," he says. "There still are some fundamental aspects of polar bear biology that we have yet to understand."
