Every summer, more sea ice melts, leaving polar bears with less territory for hunting. New genetic analysis reveals that recent generations of polar bears are migrating north to the Canadian archipelagos, a region where sea ice more reliably survives the warm summer months.

Top image: Young female polar bear on an ice floe in northeaster Svalbard in summer 2013. Credit: Michael Bamford

Polar bear near Churchill in northern Manitoba. Image credit: Alex Berger

Polar bears love sea ice. Spending time on the ice is crucial to their life cycle, most critically hunting in the summer to build up winter reserves. While technically they can eat terrestrial critters, seals are the main stable of a polar bear's diet. Sea ice extent has been shrinking in recent decades between extended summer melt and the loss of old, stable ice. The result has been a surge in polar bear mortality and a drop in polar bear populations as the bears struggle to find suitable hunting grounds.

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The United States Geologic Survey has been leading research into how polar bear populations are responding to sea ice loss. Along with producing adorable documentaries from a polar bear point of view, their most recent research has been looking at genetic analysis to understand the bulk behaviour of the expansion, contraction, and migration of polar bear populations. Lead by the USGS and the Government of Nunavut, the study also included scientists from Greenland, Norway, and Russia, covering all five countries with polar bear populations.

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Polar bear enjoying the results of a successful hunt. Image credit: Juan-Vidal Díaz

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Genetic analysis confirms that recent generations of polar bears are on the move, migrating in response to declining habitat. Of the 19 subpopulations of polar bears in four genetically-similar clusters, the past one to three generations have been moving north, expanding the Canadian Archipelago cluster, mostly from the Eastern Polar Basin and Southern Canada genetic clusters. The sea ice in the archipelagos is more resistant to summer melt due to cooler northern latitudes, circulation patterns, and complex geographic factors.

USGS polar bear researcher Elizabeth Peacock explains how this research supplements existing satellite tracking data for the bears:

"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. 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."

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Female polar bear on the hunt near Velkomstpynten, Svalbard. Image credit: Allan Hopkins

Satellite tracking of polar bears is also biased by a hilarious constraint: male polar bears are so burly that their necks are wider than their heads, making it impossible to fit them with tracking collars. Therefore, only female bears are tracked by satellite, while until recently, the males were only tracked when researchers spot one, tranquilize it, and read its identifying lip tattoo. This is slowly changing with better technology as some of the males are being fitted with an ear-tag.

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From genetic analysis, it looks like we'll find some marked differences in polar bear roaming between the sexes as realtime tracking improves: female bears seem more likely to stay in their birth region while the male bears are more likely to roam into a new territory to sow their seed.

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A pair of polar bears contemplates how to get at tasty, squishy humans within their armoured vehicle. Image credit: Pennsylvania State University

The study also confirmed what we already know: sometimes, polar bears and brown bears get it on. From the over 2,800 samples analyzed in the current study, the researchers found no evidence of recent hybridization but plenty of evidence that it's happened in the past. This means that the recent hybrids spotted in the Northern Beaufort Sea represent a new, localized phenomena and not a regional trend. At least, not yet: historically, the population of brown bears surges in warm climates with little ice while polar bear populations shrink, and the inverse happens in cooler climates when ice extent grows.

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Polar bear on an ice floe in northeaster Svalbard in summer 2013. Image credit: Michael Bamford

Read the full study: Implications of the Circumpolar Genetic Structure of Polar Bears for Their Conservation in a Rapidly Warming Arctic

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International Polar Bear Day is my favourite day of the year. Need more of these beautiful, violent creatures? Enjoy this polar bear spying through the ice, this one wearing a go-pro, mauling a spy-camera, or attempting to eat a photographer, a cub taking its first bath or seeing snow for the first time, or learn about annual polar bear gatherings.

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