The crossbreeds found in Alaska and Canada are not genetic anomalies. Scientists have found the mix in the islands off Southeast Alaska, where bears resemble grizzlies but contain polar bear DNA. That indicates decades of sporadic interbreeding, said Steven Amstrup, chief scientist at Polar Bears International.
The polar-grizzly cocktail is also far from the only recent animal hybrid. The coywolf - a coyote-dog-wolf amalgamation - and a lynx-bobcat mix have been popping up along the northern Atlantic coast. The more scientists analyze species' genomes, the more they realize that animals we label as "pure breeds" actually share DNA - and that includes us.
Many humans carry traces of DNA from Neanderthals, which means we're all hybrids. It also means there's no such thing as genetic purity. The concept is a romantic construct, an anthropomorphised take on nature. And what may be most surprising about this, researchers say, is the role interbreeding plays in the futures of endangered species - or, as the case may be with polar bears, accelerating their end.
Amstrup has studied bears in the Arctic since the 1970s and was instrumental in helping list the polar bear as a threatened species in 2008. He, like other experts, characterises this "new" bear relationship as more beneficial to grizzlies than polar bears. That's because there are more grizzlies than polar bears and because grizzly territory is expanding while polar bear territory is contracting. What that adds up to is a good chance grizzlies could essentially dilute the polar bear population until it doesn't exist at all, they say.
Polar bears are getting the short end of the stick in this relationship, not "gaining any genetic diversity," said Geoff York, who led research on polar bears at the World Wildlife Fund for almost a decade before joining Amstrup at PBI.
Andrew Derocher, a professor of biological studies at the University of Alberta, has spent three decades studying bears throughout the Arctic. He, too, has a sobering view about where the hybridisation is heading.
"I hate to say it, but from a genetic perspective, it's quite likely grizzly bears will eat polar bears up, genetically," he told me. And he says the changes are already at play.
All hybrids that have been analyzed had grizzly fathers, because grizzly males roam to establish territory and come in contact with receptive female polar bears. Female grizzlies tend not to stray far from their home ranges, and male polar bears don't usually creep into grizzly habitats.
Polar bears need the ice - that's where the seals and walruses they eat live. They don't hibernate, and they don't travel south of the tundra. Grizzlies, historically, rarely ventured north of the treeline. Permafrost is too cold for their liking, and they sink into the snow easily. (Polar bears have padded paws that act as snowshoes). Hunting is more challenging in the north, where prey is scarce. They're not really swimmers.
But shifts are afoot.
"What we're starting to see in the Canadian Arctic is three-fourth grizzlies," Derocher said, referring to the offspring of 50-50 hybrids that then mated with grizzlies. "How do they act? Probably more like grizzly bears, living on land. As climate change continues, terrestrial habitat is going to increase, and the likelihood is the habitat for grizzlies, a terrestrial bear, is going to get better. That means a longer warming period and greater food potential."
Derocher said it will not be long before we start seeing female grizzlies bump into male polar bears, further straining the polar bear's genetic variation. "I suspect at the same time that that's occurring, we'll start to see polar bears on their way out."
When will that be? Impossible to say, but some experts think that as the Arctic continues warming, it may be only a few decades, perhaps a century. There are about 20,000 to 25,000 polar bears in the Circumpolar Arctic, and "an order of magnitude higher for grizzlies in that area" and other brown bears, Derocher said. "It shouldn't be a big surprise that grizzlies are moving north - everything is."
Right now, polar bears are also threatened by polychlorinated biphenyls, or PCBs, and other toxic pollution - primarily from eating seals and other animals affected by these carcinogens - that has been linked to brain damage, even causing some bears' baculums, or penis bones, to break off.
And those outcomes could affect polar-grizzly hybrids as badly as pure breeds. No matter what bear ends up as the Arctic's future apex predator, scientists say, if the issues up north aren't solved, it won't matter what bears are there.
Hybrids are "a normal part of the evolutionary process," Derocher said. But if the ice disappears, "we won't have grizzlies or polar bears in this area. If you roll the clock ahead another number of decades or a century, quite clearly it's going to be no bears eventually."
