For all the publicity over discoveries of “pizzly” bear hybrids, new research suggests polar bears’ genetic purity isn’t threatened by crossbreeding with grizzlies. But that doesn’t mean polar bears aren’t in trouble.
On april 16, 2006, Jim Martell, a 65-year-old hunter from Idaho, was on a hunting trip on the southern tip of Banks Island in Canada’s Northwest Territories when he shot and killed what would later turn out to be a half-polar bear, half-grizzly bear hybrid. The bear, featuring white fur pocked with brown patches, long claws, a humped back and a concave face, was the first evidence of a wild hybrid.
Ten years later, in May 2016, Inuit hunters in Nunavut thought they had shot and killed yet another hybrid – their “pizzly” or “grolar” bear made headlines around the world – but a pizzly bear it was not. Rather, DNA analysis revealed they had simply caught a very blonde grizzly bear.
In a changing Arctic, grizzly bears are king. Though polar bear and grizzly bear habitat has long overlapped, the latter are moving farther north as temperatures warm, while the former are fast disappearing in parts of their historic range as the sea ice they use as hunting grounds disappears.
In recent years, some have theorized that we may soon see widespread hybridization as two bear subspecies roam the Arctic tundra, but researchers are skeptical such a takeover could ever occur. In a recently released study, scientists found that all four hybrids found over the past decade were the offspring of the same female polar bear and different grizzly bear fathers. The hybridization so far could, after all, merely be the result of one female’s unusual penchant for humpbacked grizzlies. But that doesn’t mean things won’t change for the Arctic’s apex predator.
In evolutionary terms, polar bears branched off from grizzly bears relatively recently. Genetic models show polar bears likely emerged as a separate subspecies between 70,000 and 1.5 million years ago. Unlike mules and other hybrids, pizzly bears are capable of reproducing in the wild. “They are really not different ‘species,’ in the biological sense of the word,” said Marsha Branigan, a wildlife manager with the Northwest Territories government and coauthor of the study. In fact, during periods of extreme cold, polar bears and grizzly bears likely bred with one another as polar bears moved south when the sea ice became too thick to hunt prey.
Though what initially appeared to be a sudden spate of hybridization in the western Canadian Arctic ultimately originated with the “unusual mating” between three non-hybrid parents, that’s not to say the breakdown of species barriers can’t simply start with the strange mating preferences of certain individuals. In the conservation world, there’s concern that the polar bear population could eventually be subsumed by the gene flow from male grizzly bears, but so far the resulting four first-generation hybrids have in every documented instance bred with grizzly bears, not polar bears. This means that polar bears are largely continuing to have genetically pure young. “Until we see evidence of successful matings between hybrids and polar bears, this threat is not substantiated,” the researchers concluded.
Among experts, hybridization – and possible grizzly gene swamping – isn’t seen as a primary threat. Andrew Derocher, the head of the University of Alberta’s Polar Bear Lab, said that polar bears are likely to disappear at a rate that far outpaces hybridization, though there have been isolated incidents of polar bears being subsumed into grizzlies throughout history. “There were polar bears on Alaska’s ABC Islands at one time, but once the climate warmed the polar bears left, and the individuals that remained saw their genes flooded by brown bear genes coming from the mainland,” said Derocher.
Since the early 1980s, Canada’s Western Hudson Bay polar bear population has declined by 22 percent, while the Southern Beaufort Sea population dropped by roughly 40 percent between 2001 and 2010. Polar bears depend on sea ice to travel, hunt and raise their young, but climate change has caused sea ice to melt earlier in the spring and form later in the fall, disrupting the bears’ habitat.
Globally, polar bear populations could decline by as much as one-third by 2025 due to disappearing ice, and by 50 percent by 2050. At the same time, male grizzly bears are moving northward. Thanks to reduced hunting, a warming climate and better management, “grizzlies are expanding their range in all directions right now, after going through a couple hundred years of range retractions,” said Derocher. Recently, he recalled seeing a grizzly bear out on the edge of the sea ice during fieldwork east of Churchill, Manitoba. “The was the first time I’ve seen a grizzly bear in that part of the world.” Branigan, too, said there have been incidents of grizzly bears seen hunting seals off the sea ice near the mainland.
In any species that disperses, males are almost always the vanguard of the range expansion, and that’s essentially what scientists are seeing happening with grizzly bears and polar bears in the Arctic. When the males emerge from hibernation, ready to mate, if there are no female grizzlies nearby, eventually they’ll cross tracks with the scent of a female polar bear, in breeding condition.
Ultimately, “it’s a bit of a race to see whether or not polar bear range will contract faster than grizzly bear range expands,” said Derocher. “I think polar bears are going to be the big loser here, and I think grizzly bears will take over the High Arctic.”