The ABC’s of Hybridization of Brown Bears and Polar Bears

By Bjorn Dihle

First published in Alaska Magazine, December 2019.


The rainforest brown bear of Southeast Alaska’s ABC Islands (Admiralty, Baranof, Chichagof, Yakobi and Kruzof Islands) might be more interesting and mysterious than Sasquatch. Those are fighting words for people invested in big hairy critters, particularly the “Squatch-Truthers,” a growing community devoted to exposing the government conspiracy to keep the existence of cryptids hidden from the public. Before you Squatch-Truthers fling this magazine against the wall, light it on fire and condemn it to perdition as blasphemy, let me explain.

ABC Island bears look and behave like brown bears, but have DNA more similar to polar bears. For years, biologists have theorized as to how this was possible. In 2013, a paper co-authored by Beth Shapiro, Professor of Ecology and Evolutionary Biology at the University of Santa Cruz California, attempted to explain the phenomenon. By examining the genetics of a number of bears, Shapiro and her team found a significant degree of similarity in the X chromosomal DNA between ABC Island bears and polar bears. Being that the X chromosome is passed down from a mother to her young, the team concluded that ABC Island bears are the product of generations of female polar bears, and their hybrid offspring, mating with male brown bears.

This theory is based on the fact that during the Pleistocene epoch, Southeast Alaska was a very different place than it is today. Besides there being no cruise ships or reality film crews, there were glaciers covering just about all the land, excluding an ice-free refugia down around Prince of Wales Island. The ocean was covered in ice much of the year, and polar bears used that ice as a platform to hunt seals. At a certain point, when temperatures suddenly rose around 12,000 years ago, ice began to disappear from ocean and land. The theory goes that a population of female polar bears became marooned on the ABC Islands. Male brown bears swam across ocean or traveled across ice – behavior that male brown bears still have today, particularly in the spring during mating season – and successfully courted the female polar bears. There’s a strong possibility the two species were interbreeding long before that. Not everyone agrees with this theory, but there’s no disputing that ABC bears’ DNA indicates significant hybridization with polar bears.

Stranger things have happened in the lonesome wilderness. Close your eyes and imagine the soft evening light as a dominant yet graceful male brown bear, his muscles rippling and fur blowing in the wind, meets a lithe polar bear whose only flaw is that she’s sometimes clumsy. Serenaded by the music of returning migratory birds and suffering the pent-up passion of spring, there’s only two ways this can go. One, the bigger bear will try to eat the smaller. Two, they’ll make love with reckless abandonment. It basically sums up the dating scene in Alaska.

A few years back, I almost got to play a part in attempting to answer the question of the origin of the ABC Island brown bears. I was contacted by someone involved with a film production who asked if I was interested in being in a documentary delving into the heart of the mystery. They were looking for a bear expert, and seeing that I was hairy, large and lived in Alaska, they thought I would be a good candidate. The job, as I understood it, would be trying to look tough and uttering cool and cryptic one-liners as I followed a biologist gathering bear hair and bagging scat samples on Admiralty Island. At first I was excited. I even came up with cool titles for the scientific papers I planned on writing and imagined how respected I would become by bear biologists around the world. After a few more phone calls, my enthusiasm waned when I learned the film was actually about something very different. The producer told me he had it on good evidence that there was a beast – some sort of unknown species of giant bear or, perhaps, Sasquatch – on the island and that film would be about searching for the cryptid. I reluctantly declined.

In our rapidly warming world, crossbreeding between brown and polar bears is on the rise. Numerous viable offspring of the two species – sometimes called the “pizzly” or “grolar” – have been sighted and shot by hunters. The future of polar bears remains to be seen, but there’s a decent probability a similar process of hybridization that long ago happened between the two species on the ABC Islands will occur in parts of the Arctic. Some biologists might remain skeptical, but it doesn’t take a molecular ecologist to know you can’t stop the heat.



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