Polar bears are on the move! Researchers studying genetic variation in polar bears (Ursus maritimus) over the last two decades of their habitat decline reveal that recent generations are heading towards the Canadian Archipelago—converging in an Arctic region that’s most likely to retain sea ice into the future. The findings were published in PLOS ONE this week.
To see if polar bear genetic diversity and structure have changed during this recent, sad period of change, a large team led by Elizabeth Peacock from the U.S. Geological Survey analyzed DNA from 2,748 polar bear samples collected as long ago as 1973. “By examining the genetic makeup of polar bears, we can estimate levels and directions of gene flow, which represents the past story of mating and movement, and population expansion and contraction,” Peacock explains in a news release. “Gene flow occurs over generations, and would not be detectable by using data from satellite-collars which can only be deployed on a few polar bears for short periods of time.” Also, males have necks that are wider than their heads, making them impossible to collar.
Polar bears from nearly 20 recognized subpopulations, they found, group into four genetically-similar clusters: Eastern Polar Basin, Western Polar Basin, Canadian Archipelago, and Southern Canada. And these clusters span five countries: U.S., Canada, Russia, Greenland, and Norway. In the figure below (imagine looking down on the Arctic from above), the arrow widths represent directional gene flow:
In the last one to three generations, the predominant directional gene flow has been from Southern Canada and the Eastern Polar Basin towards the Canadian Archipelago—where the sea ice is more resilient to summer melt thanks to its geography, circulation patterns, and northern latitude temps. Based on climate warming projections, the area will be an important refuge in the near future. “The polar bear’s recent directional gene flow northward is something new,” Peacock says. “In our analyses that focused on more historic gene flow, we did not detect movement in this direction.”
Additionally, the team found that polar bear populations expanded while brown bear populations contracted during periods when there’s more ice. In times with less ice, it’s the opposite. The researchers were also able to confirm earlier work suggesting how modern polar bears come from hybridization events with brown bears. But they didn’t find new evidence of hybridization; that means the hybrids found in Canada’s Northern Beaufort Sea region are a new and localized phenomenon. Another fun fact they uncovered: Female polar bears tend to show more loyalty to their birth place than males.
Images: shutterstock.com (top), Peacock E, Sonsthagen SA, Obbard ME, Boltunov A, Regehr EV, et al., 2015 (middle)