An innovative use of radio collars has allowed researchers to gauge the long-distance swimming skills of polar bears in the Arctic Ocean waters north of Alaska. The research, by United States Geological Survey biologists, shows that the predator has a truly formidable ability to routinely cover extraordinary distances in the water. The bear clearly earns its designation under federal law as a marine mammal.

The researchers tracked 52 females from 2004 to 2009 (I was told that the necks of male bears are too thick to accommodate radio collars), then compared the recorded tracks of the bears with maps of sea ice through the same period. The biologists documented 50 swims with an average length of 96 miles. The paper, “Long-distance swimming by polar bears (Ursus maritimus) of the southern Beaufort Sea during years of extensive open water,” is published in the Canadian Journal of Zoology.

The study was too limited to clarify whether the warming Arctic climate and related summer expansion of open water in the Arctic Ocean is necessitating more long swims — or whether that is reducing the bear’s survival rate or reproductive success. The paper notes that cubs accompanied mothers on a number of the marathon swims. In a phone interview this morning, the lead author, Anthony Pagano, noted that the bear population in the study region, the southern Beaufort Sea, appears to be stable at about 1,500 animals. I asked him to consider these findings in relation to the much-discussed reports of drowned polar bears a few years ago. He said that mortality appeared linked to a powerful storm, but said “generally speaking, polar bears seem capable of swimming amazing distances.” Pagano said there is concern that the stresses from continuing ice retreats could threaten the bears’ prospects in the long run. Still, this is quite a different picture of the issue than that painted by some climate campaigners in years past.

An agency news release has more details on the work. Here’s an excerpt:

Scientists have no way of knowing if long-distance swims are a new feature of polar bear life. “We did not have the GPS technology on collars to document this type of swimming behavior in polar bears in prior decades,” explains Karen Oakley, of the USGS Alaska Science Center . “However, summer sea ice conditions in the southern Beaufort Sea have changed considerably over the last 20 to 30 years, such that there is much more open water during summer and fall. Historically, there had not been enough open water for polar bears in this region to swim the long distances we observed in these recent summers of extreme sea ice retreat.”

While it is encouraging that polar bears can swim so far, it is also a potential risk for the bears, the researchers noted. The energy and physical costs of such long-distance swimming are unknown, but scientists did note polar bears moved, on average, 2.3 times more than when the same individuals were on sea ice. The movement data also suggest the bears were not pausing to rest or feed during long-distance swims. Twelve of the twenty documented swimming bears were adult females that had yearlings or cubs-of-the-year at the time they were outfitted with the GPS collar.

“We were able to recapture or observe 10 of these females within a year of collaring, and 6 of these females still had their cubs,” said Anthony Pagano, a USGS scientist and lead author of the study. “These observations suggest that some cubs are also capable of swimming long distances. For the other four females with cubs, we don’t know if they lost their cubs before, during, or at some point after their long swims.” (Source)

Here’s the rest of the release: Polar Bears, Long-Distance Swimming