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Polar-grizzly bear interbreeding still rare, but expected to increase as habitats overlap: new report

A male polar bear eats a piece of whale meat as it walks along the shore of Hudson Bay near Churchill, Manitoba, Aug. 23, 2010. (Sean Kilpatrick/The Canadian Press via AP) A male polar bear eats a piece of whale meat as it walks along the shore of Hudson Bay near Churchill, Manitoba, Aug. 23, 2010. (Sean Kilpatrick/The Canadian Press via AP)
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Interbreeding between polar bears and grizzlies is not threatening the Arctic's polar bear population – at least not yet. 

The risk of interbreeding, also called hybridization, and the decrease in genetic diversity within a species it could bring, increases as the planet grows warmer. 

That this could happen to polar bears became less of a theory and more of a concern when a polar-grizzly hybrid bear was discovered in 2006 in the Northwest Territories

In total, scientists knew of eight hybrid polar-grizzly bears connected to this family group in the Canadian Western Arctic, but not how frequently polar bears and grizzlies were interbreeding as warmer temperatures forced grizzlies northward, nor how much genetic variation existed within the polar bears there. 

"Which is an important question because we think genetic variation is really the fuel for evolution," explained molecular ecologist and MacEwan University assistant professor Joshua Miller in a recent interview. 

"Polar bears adapted to colder climates and foraging on ice, whereas grizzly bears are adapted to living in lower latitudes," he pointed out.

"If you have a hybrid that's kind of a mix between those two, it might not be well suited to either environment and just less likely to survive. And the concern being that if a bunch of polar bears and grizzly bears are mating – rather than polar bears mating with other polar bears – you could have a bunch of these unfit hybrids instead of having more fit polar bears on their own."

Analyzing 8,000 genetic markers

To answer these questions, Miller and a team of researchers from Environment and Climate Change Canada, Polar Bears International, University of Manitoba, the Northwest Territories government, and San Diego Zoo Wildlife Alliance compared the DNA of 371 polar bears, 440 grizzly bears, and the eight known hybrids. 

The DNA had been collected by various agencies between 1975 and 2015 mainly in Canada, but also the U.S. and Greenland. 

Before comparing the DNA, however, they needed to improve and greatly expand the capabilities of the chip that had been used to study the genetic profile of polar bears. 

The chip Miller and his partners made – called Ursus maritimus V2 SNP chip, borrowing the polar bear's latin name like its predecessor – looks like a microscope slide or computer chip with room to examine four samples of DNA at once. It cannot be reused.  

Molecular ecologist and MacEwan University assistant professor Joshua Miller holds up the Ursus maritimus V2 SNP chip, which was used to make and compare the genetic profiles of some 800 polar and grizzly bears from the Arctic, during a Zoom interview with CTV News Edmonton about the research on June 14, 2024. (CTV News Edmonton)

The chip references more than 8,000 genetic markers, chosen by the researchers, to not only measure genetic diversity between polar bears but also to determine whether a DNA sample is a polar bear, grizzly bear, or hybrid. 

"Before, you were looking at maybe 10 or 20 (markers)," Miller said. 

Their findings

Out of the 819 bears profiled, Miller and his colleagues found no other hybrids nor any evidence of ancestral interbreeding in the polar bear gene pool, publishing Thursday in the Conservation Genetics Resources journal

"With this, we can say at the moment, or from the samples that we looked at, (hybridization) doesn't seem to be a big concern. That doesn't mean it couldn't be in the future, but up until 2015, which is when our last sample was from, it doesn't seem to be widespread," Miller told CTV News Edmonton. 

The researchers also concluded that it didn't seem polar bears were mating with grizzlies as a survival mechanism. 

"That hybrids are less fit, less adapted to either niche of their parents, is one line of thinking. A kind of counter line of thinking is that, hey, maybe you can get the best of both worlds: It could be that a grizzly bear-polar bear hybrid is just a superbear and would be more likely to survive than either parent," Miller said.

"But that doesn't seem to be the case based on the fact that we don't see any of these hybrids, so it's not occurring very often. And if you look at the physiology of the few hybrids that are out there, as far as I know, they don't seem to be better adapted. They seem to be less adapted."

He believes the findings highlight the necessity of continuing monitoring for hybridization and to expand the genetic diversity research beyond the Arctic in case it could help inform polar bear conservation efforts, now possible thanks to the chip. 

Canada is estimated to be home to about 16,000 of the world's 20,000 to 24,000 polar bears. 

The animal is classified as a species of special concern under the Canadian Species at Risk Act. 

Hybrid fun facts

Offspring of polar bears and grizzly bears are named according to their paternal lineage: "grolar bear" for those born to a grizzly father and polar bear mother, and "pizzly bear" for those born to a polar bear father and grizzly mother. 

Their fur is often lightly coloured, a combination of both parents.