Polar bears' survival tactics even worse than previously thought

Polar bears’ survival tactics are worse than previously thought, according to a new study published this week in Science.

The team’s findings show that polar bears don’t enter a hibernation-like and energy-saving state during summer that would help them cope with our warming climate. We spoke with author Steven Amstrup from Polar Bears International in Bozeman, USA, about how their results compare with past research, and what needs to be done to protect the bears from their ever-melting ice habitat.

ResearchGate: First off, what’s the future looking like for polar bears?

Steven Amstrup: This study makes it clear that polar bears have no special ability to survive very long periods with no food. They will only persist in places where sea ice is around for long enough that they can gain enough weight to survive when the sea ice is gone. If we continue the build-up of CO2 and other greenhouse gases in the atmosphere, there will ultimately not be enough ice, anywhere, to support polar bears.

RG: You uncovered novel results about the limits of polar bears’ summer fast. Can you discuss?

Steven Amstrup

SA: The long held hypothesis that polar bears can enter a hibernation-like state when food deprived in summer, appears not to be true. Polar bears are just like other mammals in that they can save some energy by small declines in body temperature associated with reduced activity, but they cannot dramatically reduce body temperatures, or recycle products of nitrogen metabolism—critical components of winter hibernation. This means that polar bears have no special metabolic trick to help them avoid negative effects of the loss of the sea ice habitat they need to catch their seal prey.

Ever-longer ice absence caused by global warming, therefore, can only negatively affect bears and ultimately result in their disappearance. They will not disappear everywhere all at once, but all available evidence indicates they will not persist where sea ice is not around long enough for them to gain sufficient weight to survive the times it is absent. If warming continues, that ultimately will be all of the current polar bear range.

RG: How did you find this out?

SA: Bears showed gradual declines summer time core body temperatures down to about 1 degree C below normal active temperatures.  Such gradual declines in core temperatures, along with reduced activity levels are typical patterns for other food deprived mammals.  But  they are in contrast with abrupt and much lower declines in body temperature (3+ degrees), and even greater reductions in activity, characteristic of bears entering hibernation.  We saw no evidence that summer-active bears were somehow flipping a metabolic switch and entering a hibernation like state.

RG: Previous studies have suggested polar bears are inefficient walkers. How do your results compare?

SA: Previous studies, in which bears were walking on treadmills, suggested polar bears generated too much body heat while walking and may be subject to overheating - especially in summer. This finding always seemed puzzling to me because my early work showed that polar bears were by far the most mobile of all four-legged creatures. This study shows that the earlier studies were flawed in that they forced the bears to walk too rapidly on treadmills. My early work and this study confirmed polar bears seldom walk as fast as they were forced to do on the treadmill, and indeed they seldom overheat while moving about in their normal habitat.

RG: Summer ice-loss may increase the need and frequency for long distance-swims. How do polar bears mitigate energy levels and body temperature during swimming, and what are their limits?

SA: This study revealed that polar bears have a great ability to control the temperature of their skin and other peripheral tissues, and even the outer layer of their core during swimming. This can assure stable temperatures around critical organs. It’s apparently an adaptation to the periodic need to make long swims. This is a very interesting result as this sort of heterothermy is not well documented in other species.

We long have known that polar bears will swim among ice floes and across leads (cracks and other openings in the sea ice) to get to new hunting territory. But, the 22% weight loss of one bear we observed to make a very long swim makes it clear there are limits to this ability as well. And if food is unavailable because of sea ice loss, where can a bear swim to find a source of food?

RG: What’s next for polar bear researchers?

SA: Polar bear researchers continue to try to understand the limits of fasting, and the details of how polar bears in different areas may be affected by ongoing sea ice loss. Work like that reported here, however, is unlikely to ever be repeated because of the expense and logistical challenges. Regardless of whether people can muster the ability to repeat some of this work, the outcomes are already clear. If we do not stop the rise in global temperatures, polar bears will ultimately disappear. There are many interesting questions new research still could answer about polar bears. But we already know the answer to “what do we need to do to save them from extinction?”  All the new research in the world will not save polar bears if we do not stop the rise in atmospheric GHG levels.

Feature image courtesy of Matthew Studebaker