Polar bears are excellent swimmers, their massive front paws perfectly designed for propelling through the water between Arctic ice floes. But swimming comes at an energetic cost, and swimming any kind of distance is something bears in the wild generally avoid if they can. Unfortunately, a new study has found, as climate change causes, avoiding long swims is becoming increasingly hard for some polar bears to do.
Researchers from the University of Alberta and Environment and Climate Change Canada studied telemetry data from collars with GPS transmitters, affixed to 58 adult females and 18 subadults from the Beaufort Sea and 59 adult females from Hudson Bay, for evidence of long-distance swimming during seasonal migrations in 2007–2012. (Fully mature adult males are unsuitable for such studies, as their necks are too large for the collars; researchers continue to develop alternative means of satellite tracking for these bears.) They published their findings in the journal Ecography.
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During the study period, the tracked bears undertook a total of 115 swims greater than 31 miles (50 kilometers) in length, 100 of them in the Beaufort Sea, where ice conditions were more varied; there, subadults swam as frequently as lone adult females, but more frequently than adult females with offspring, which tend to swim less to avoid submersion of youngsters in cold waters.
During 2012, when summer Arctic sea ice extent hit a record low, 69 percent of tracked adult females in the Beaufort Sea swam 31 miles or more at least once. The single longest swim recorded by any bear was by a subadult female that swam more than 249 miles (400 kilometers) over nine days.
According to lead author Nicholas Pilfold, now a postdoctoral fellow at San Diego Zoo Global, "the pattern of long-distance swimming by polar bears in the Beaufort Sea shows the fingerprint of climate change. Swims are occurring more often, in association with sea ice melting faster and moving farther from shore in the summer."
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In an interview with Radio Canada International, co-author Andrew Derocher of the University of Alberta noted that, "When I started studying polar bears in 1984, sea ice in the Beaufort Sea was visible from shore year round. In recent years, the ice has retreated several hundred kilometers offshore by September and it's a much more challenging habitat for the bears to live in." He added that, while polar bears as a species are eminently suited to swimming, not all bears are equally able to swim long distances.
"The youngest, oldest, and skinniest bears are much more vulnerable to drowning," he said. "With more open water, we can expect increased mortality associated with more long distance swimming."
"Recent studies indicate that swimming may be energetically costly to polar bears," echoed Pilfold. "Given the continued trend of sea ice loss, we recognize that an increased frequency in the need to engage in this behavior may have serious implications for populations of polar bears living around the Arctic Basin."