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DNA exposes ancient island love affair between polar and brown bears


By David Smith
Picture a polar bear — a regal white giant. Now imagine a group of polar bears isolated on an island and a pack of male brown bears swimming for that island with the hopes of wooing the females.

Although far fetched, recent analyses of polar bear and brown bear DNA suggest that this is precisely what happened thousands of years ago on the islands of the Alexander Archipelago off the southeastern coast of Alaska.

The study, published this spring in the journal PLoS Genetics and headed by researchers from the University of California, Santa Cruz, looked at genes from various polar bears and compared them to those of brown bears, including ones from Alaska’s ABC Islands (Admiralty, Baranof, and Chicagof). The scientists found that polar bears differ very little from each other at the genetic level, but that brown bears are genetically diverse. In other words, if you compare the DNA of one brown bear to that of another, you’re likely to find many differences. Not so for polar bears.

The most intriguing observation, however, came when the researchers compared polar bear and brown bear genes. As expected, they observed many genetic differences, but some – thing surprising jumped out of the genes from the ABC Islands brown bear: long stretches of polar bear-like DNA.

The brown bears that inhabit the ABC Islands look and act just like other types of brown bear, such as the grizzly. They’re large and stout, have a gorgeous dark brown coat, and enjoy fishing for salmon in streams and rivers. Nothing about their appearance indicates that they harbour large segments of polar bear DNA. In fact, polar bears aren’t found anywhere close to the ABC Islands. So then where did the polar bear genes come from?

When the scientists looked closely at the ABC Islands bears’ DNA, they discovered that the parts most similar to polar bears are found on genes inherited only from the mother, such as those on the X chromosome. This suggests that the ABC Islands bears are the product of an ancient rendezvous between female polar bears and male brown bears. But if polar bears are not found on or around the ABC Islands, how could this romantic encounter have taken place?

The authors of the study, after considering data on fossils and climate, think that at the peak of the last ice age, around twenty thousand years ago, polar bears roamed the frozen seas that surrounded the Alexander Archipelago. “As the climate warmed and ice retreated,” they argue, “polar bears may have been stranded on or near the ABC Islands.”

Rising temperatures and an increasingly hospitable habitat could have also attracted mainland brown bears to the area. If a lonely male brown bear stumbled upon a stranded female polar bear, he may not have been able to resist her beautiful blonde fur. Not too bad for a “how I met your mother” story. If true, the offspring of these early encounters would have probably looked like a mix between a brown bear and a polar bear — sometimes called a pizzly bear or grolar bear. But a steady influx of male brown bears onto the island could have gradually eroded the polar bear genes from the population, converting it to one that is essentially brown bear. This theory is supported by recent observations of polar bear/brown bear hybrids in northern Canada, proving that the two types of bear are genetically compatible and do mate in the wild.

Increasing Arctic temperatures and receding sea ice are causing polar bears to spend more time on land and brown bears to spread north – ward into traditionally polar bear territory.

This means that these two magnificent creatures will come into contact with each other more and more often, and that a similar situation to that proposed for the ABC Islands bears may start playing out in diverse regions across the Arctic. The broader consequences of interbreeding between polar bears and brown bears are unknown, but if the ABC Islands story tells us anything, it is that with time the iconic polar bear could slowly melt away.

David Smith is an assistant professor in the biology department at Western University.

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