Polar bears are physiologically unable to compensate for extended periods of food deprivation due to the ongoing loss of sea ice, despite demonstrating an unusual ability to minimize heat loss while swimming in Arctic waters, according to new research published Thursday.
“Our results suggest that polar bears can’t extend their fast for months (as bears do during winter hibernation) because they can’t reduce their metabolic rate to hibernation levels in summer,” one of the authors of the study, University of Wyoming professor Merav Ben-David, told redOrbit in an email. “Therefore, increasing ice loss and especially increasing the length of the summer melt period will likely negatively affect polar bear survival and thus abundance.”
Professor Ben-David and her colleagues explained their first-of-its-kind project revealed that polar bears are unable to prolong their reliance on stored energy, thus confirming the creatures are vulnerable to hunting opportunities lost due to melting sea ice. The results contradict previous research suggesting that the creatures could at least partially compensate for longer summer food deprivation by entering into a state of lowered activity and reduced metabolic rate known as a “walking hibernation.”
The study authors found that the summer activity levels and body temperature of bears on shore and on ice were typical of fasting, non-hibernating mammals, with no signs of this so-called state of “walking hibernation.” The results suggest that polar bears are not likely to avoid the physical declines associated with continued ice loss and longer periods of ice melting, and their very survival could be at stake.
Highlighting the threats of sea ice loss
Professor Ben-David told redOrbit that it is “important to keep in mind that different areas of the Arctic show different sea ice dynamics so to project what will happen to polar bears globally will require some sophisticated modeling using our results (as well as other parameters).”
Her colleague, Dr. Steven Amstrup, chief scientist at Polar Bears International, added, “The bottom line is that we have no evidence anywhere that polar bears can persist without adequate access to sea ice. When sea ice is unavailable, or not available over productive waters preferred by the seals on which polar bears prey, polar bears are largely food deprived.”
“As Merav points out we do not expect the pattern of ice loss or its effects to be identical in all parts of the polar bear’s range, and observations thus far show variation in response depending on things like basic ecosystem productivity,” he noted. “That variation is transitory, however. Regardless of where they live, polar bears need ice as a foraging substrate. If we allow continued warming, we ultimately will be out of polar bear habitat throughout their current range.”
The researchers captured more than two-dozen polar bears and implanted each with temperature loggers. They then tracked their movements both on shore and on the ice in the Arctic Ocean’s Beaufort Sea, which is located north of Alaska and Canada, during 2008-2010.
Polar bears have no special way to cope with ice loss
Dr. Amstrup explained that the data showed that, even though many claimed that polar bears are able to employ the “walking hibernation” trick to prolong their ability to go without eating food, this was not the case. The study revealed that the creatures lack this special metabolic trick, and even though they’re able to hibernate in winter, when they’re food deprived during the summer they’re just like any other food-deprived mammal, whether on the ice or on land.
“Many people would have you believe that polar bears will somehow escape the negative impacts of sea ice disappearance,” he told redOrbit. “This evidence that they have no special ‘metabolic escape’ further confirms that retreating sea ice can only negatively affect polar bears, [and] it supports a statement I made some time ago: ‘Unless we mitigate the rise in greenhouse concentrations, we polar bear researchers will simply become polar bear historians.’”
A positive note
One positive that came out of the research, however, is that polar bears can employ an unusual physiological response to avoid unsustainable heat loss while swimming in cold Arctic waters. They are able to maintain their interior body temperature, allowing them to survive longer and more frequent swimming sessions, by temporarily cooling down the outermost tissues of their core to form an insulating shell. This phenomenon is known as regional heterothermy.
This regional heterothermy could represent an adaption to long-distance swims, although they note that the limits of this ability are currently unknown. Previous research by the same team found that one bear was able to survive a nine-day, 400-mile journey from shore to ice. The study shed new light on the potential mechanism used to pull off this feat, and could provide new insights into the overall metabolism and activity of bears.
(Image credit: Thinkstock)
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