More Alaskan Polar Bears Giving Birth on Land

Polar bears in northern Alaska that once gave birth on sea ice are increasingly building their dens on land, most likely because sea ice is forming later in the fall and melting earlier in the spring, according to a new study by the U.S. Geological Survey.

The 20-year study found that only 37 percent of northern Alaska polar bears were denning on sea ice between 1998 and 2004, compared to 62 percent between 1985 and 1994. The researchers located dens throughout those time periods using satellite telemetry.

Unlike polar bears throughout the rest of the Arctic, which make their birthing dens primarily on land, polar bears in northern Alaska have typically denned on sea ice. Pregnant polar bears usually build their snow dens in fall or early winter, give birth in mid-winter, and remain in their dens until the spring, when it's warm enough for their cubs to emerge.

After determining that more Alaska polar bears were denning on land, the U.S.G.S. researchers examined three possible reasons for the shift: changes in polar bear hunting patterns, changes in the availability of human-hunted whale carcasses on land, or changes in the formation and distribution of sea ice. They concluded that changes in sea ice were largely responsible for the alteration of polar bear denning practices.

"In recent years, Arctic pack ice has formed progressively later, melted earlier and lost much of its older and thicker multi-year component," said researcher Anthony Fischbach. "Together, these changes have resulted in pack ice that is a less stable platform on which to give birth and raise new cubs. Previous research had already shown that unstable ice can result in failures of on-ice denning attempts. Less ice that is suitable for denning apparently has led to an increased frequency of pregnant polar bears in this region choosing to den on land."

The threat to polar bears could become especially severe if sea ice declines to the point where the falltime ice is too far from shore for pregnant females to swim to land.