This is Admiralty Island, which is not far from Juneau. It is known by the Tlingit as Xootsnoowú, which translates as “Fortress of the Bears.”
It is home to 1,600 brown bears, which I didn’t see while whale watching. This island has one the highest densities of brown bears anywhere in the world, and it is the highest for North America.
These aren’t just normal brown bears, however. This is where things get really interesting.
About ten years ago, it turned out that many brown bears from Admiralty Island and the neighboring islands of Baranof and Chichagof had mitochondrial DNA that is similar to the polar bear. This caused quite a bit of a sensation, because if these brown bears really were closely related to the polar bears, then we might have found the place where polar bears evolved from brown bears. This was also at the time when there was a growing body of evidence that polar bears evolved very rapidly and relatively recently from brown bears.
A later nuclear DNA study revealed that the similarities between these brown bears and polar bears were the result of ancient hybridization. The genomes of these brown bears is roughly 1 percent polar bear, but 6.5 percent of the X chromosomes come from polar bears.
These islands and Ireland are both places where polar and brown bears hybridized at the end of the last glacial maximum. Polar bears got stranded on islands, which became great brown bear habitat. Male brown bears mated with polar bear sows, and the offspring were fertile. However, they bred back into the brown bear population in such a way that they are almost entirely brown bear in ancestry.
As the arctic is warming, polar bears are finding themselves stranded on land for longer periods during the mating season, and brown bears (mostly grizzlies) are wandering north. Several hybrids have been killed in recent years, including one from this year.
Polar bears could very likely become extinct as a result of climate change, but their genes could still live on in the brown and grizzly bears that manage to hybridize with them during this transition period.
I wish I had been able to see one of those bears on Admiralty Island, but I am just glad I got a photo of the island itself.
Whales were calling. Not bears.
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