Scientists have long wondered whether polar bears are able to enter a physiological state resembling hibernation in response to food shortages, an adaptation some researchers have speculated could protect the species even as their hunting grounds melt away. Today, we have an answer—though it’s not the one we’ve been hoping for.
Polar bears, you’ve no doubt heard, are in trouble. The simplest explanation why relates to habitat loss. Climate change is causing ice in the Arctic sea to disappear even faster than predicted. And polar bears need ice. They use it to prowl for seals, their primary prey, especially between April and July, when the fin-footed mammals haul their nutrient-rich bodies onto slabs of solid seawater to molt and rear their pups. As temperatures warm, seals retreat with the melting ice toward the central Arctic Basin, where seawater remains frozen throughout the summer. Some bears follow the seals. Others take to the shore to forage, though prospects there are typically poor.
Researchers have long recognized a clear connection between a loss of sea ice and an increase in nutritional stress for polar bears: As the global climate has warmed, sea ice area has decreased across all months of the year, and the loss of summer ice, in particular, has required bears to endure longer and longer periods with limited access to food. But what’s less clear is how much stress this actually places on the bears. Perhaps they have a way of adjusting to these pressures, a physiological adaptation that would allow them to weather extending periods of food deprivation.
This line of thinking isn’t as wishful as it sounds. Scientists have hypothesized that polar bears can compensate for a scarcity of food by entering a physiological state called “walking hibernation.” In 1983, researchers led by bear metabolism and hibernation expert Ralph A. Nelson described the state as one “in which the biochemistry of hibernation is integrated with physical activity, but food and water intake are minimal.” For decades now, researchers have pointed to the walking hibernation hypothesis and speculated that polar bears might dramatically reduce their body temperature and metabolism to conserve energy in the face of a dwindling food supply. What’s been missing all this time is a comprehensive investigation of the physiological state of polar bears in summer, one that looks at whether their metabolism is really as resilient as some some scientists have hoped.
Now, the results of such a study have been published, and the findings are inauspicious.
In this week’s issue of Science, researchers led by University of Wyoming’s John Whiteman report that, while polar bears do reduce their activity and body temperatures during summer, these reductions are not indicative of a significantly depressed metabolism. In fact, Whiteman says they look a lot like the declines you’d expect to see in most hungry, non-hibernating mammals. This was found to be the case not only of polar bears on shore, but also those adrift in the Arctic sea. “[The] loss of hunting opportunities in summer probably affects polar bears the same way that loss of food would affect most mammals in a normal metabolic state,” he tells io9.
Collecting detailed physiological measurements from a polar bear is not easy. To record the data they needed, Whiteman and his colleagues had to search for bears on coasts, islands, and offshore sea ice at latitudes above 66° 33’ North, the lower boundary of the Arctic Circle. They tracked their quarry via helicopter and sedated them with a cocktail of tiletamine hydrochloride and zolazepam hydrochloride, delivered via dart gun. In this way, Whiteman and his colleagues managed to capture dozens of polar bears.
Scenes from polar bear research captured between 2008 and 2010. Credit: Greg Marshall and Birgit Buhleier.
While the bears were immobilized, the researchers weighed them, recorded their temperatures and respiratory rates, and determined their ages by inspecting their teeth. Finally, and most critically, the bears were outfitted with a range of monitoring equipment. Location transmitters. Activity loggers. Some bears had temperature loggers implanted in their abdomens or their rumps, to measure core and periphery temperatures, respectively. Once they’d been outfitted with their biometric sensors, the bears were released. A year and a half later, they were recaptured, instrumentation retrieved and their data logged. Finally, the bears were re-released. “Many colleagues—even some on our research team—doubted whether the study was possible, until we actually did it,” said University of Wyoming co-author Merav Ben-David, in a statement. “This project was logistically so intense that it may never be replicated.”
The effort appears to have been worthwhile. University of Alberta scientist Andrew Derocher, who has studied polar bears for more than thirty years and was unaffiliated with the investigation, tells io9 the researcher’s findings are significant. We knew from other studies “that the bears could change their metabolic activity,” he says, “but the degree was unknown from an energetics perspective.” Neither did we know what bears on sea ice far offshore were experiencing metabolically, as those bears had never been sampled.
“This study provides further insights into the long-term challenges facing polar bears throughout the circumpolar Arctic over the coming decades,” he adds. That said, Derocher says it doesn’t substantially alter our predictions on the fate of the species over the coming years, which he says remain “dire.”
Whiteman and his colleagues agree. “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 researchers report. Polar bears, they conclude, “are unlikely to avoid deleterious declines in body condition, and ultimately survival, that are expected with continued ice loss and lengthening of the ice melt period.”
Moving forward, Whiteman thinks that the concept of walking hibernation in polar bears is in need of revision. When Nelson and his colleagues characterized the physiological state in 1983, they based their description on observations that had been made not only of polar bears, but black and grizzly bears, as well. “A new model of the annual physiological cycle of bears would indeed likely group black bears and brown bears as more similar to each other than to polar bears,” he says, “and would include more nuance for transitions between [physiological states] and for understanding the biochemical mechanisms that lead to the hallmark characteristics of each [state].”
Whiteman’s team’s findings are published in the latest issue of Science.