‘Walking hibernation’ won’t save polar bears

In 2008 and 2009, researchers searched by helicopter or icebreaker for polar bears in the Beaufort Sea and on nearby shores. After tranquilizing the bears with dart guns, the team attached collars with tracking devices, temperature sensors, and accelerometers to the animals or glued transmitters to their fur. The researchers also implanted temperature loggers in some of the bears’ abdomens or rumps.

The scientists had a pressing question in mind: How well could polar bears adapt to the accelerating loss of Arctic sea ice? Earlier work had suggested that the animals’ situation might not be so bad. Perhaps bears could enter a state called “walking hibernation,” in which they reduce their metabolic rates and activity levels when food is scarce. That adaptation could keep the animals from losing too much weight, even if sea-ice loss reduced their ability to hunt seals.

Unfortunately, the authors of the new study found little evidence to support that idea. They tracked the activity of 26 bears and the core or peripheral temperatures of 17 bears. The animals did become less active from August to October, when less food is available. But even during that period, the bears were still active 12 to 22 percent of the time, whereas those in winter hibernation are active only 1 to 2 percent of the time. Their core body temperatures during the summer also suggested the animals were not in a hibernating state.

The ice bears’ core body temperatures dropped from an average of 37.3 degrees Celsius in May to 36.6 degrees in September, perhaps because the animals were fasting. But fasting “offers limited to no energy savings,” the authors write. “Thus, our data indicate that bears cannot use a hibernation-like metabolism to meaningfully prolong their summer period of fasting and reliance on energy stores.”

The team also found that some polar bears’ core temperatures dropped quickly — as much as 5 degrees Celsius per hour — while they were swimming. This physiological change may keep the bears from losing heat during long swims, the authors speculate. So polar bears do have some strategies to help them cope with tough conditions — but going into “walking hibernation” doesn’t seem to be one of them. Roberta Kwok | 16 July 2015

Source: Whiteman, J.P. et al. 2015. Summer declines in activity and body temperature offer polar bears limited energy savings. Science doi: 10.1126/science.aaa8623.

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