Grizzlies move into Polar bear territory
July/August 2012 by Rebecca Deatsman
On June 15, 1996, three biologists flew over the newly established Wapusk National Park in north-eastern Manitoba in a small plane. They were conducting surveys of breeding Canada Geese in the area, flying so low over the tundra that they could count individual eggs in the nests. As they headed up the coast of Hudson Bay, Dale Humburg spotted a bear on the ground below them. In disbelief, he asked the pilot to circle back so they could take photographs. There was no mistaking the brown fur, humped back, and dish-shaped face: this was not a polar bear, which they were accustomed to seeing occasionally, but a grizzly bear.
That the park would appeal to grizzlies wasn’t surprising — it’s nearly inaccessible to humans and is full of bear food in the form of berries, fish, caribou, and moose. However, the sighting was significant for one striking reason. It was the first confirmed report of a grizzly bear in the area since regular research had begun there over 30 years earlier. At least eight more grizzly sightings have since been reported in the park, bringing them into territory that has historically been inhabited solely by polar bears. Why are these two species colliding now, and what does it mean for their future? To answer these questions, we first need to understand how they originally diverged.
Most mammal species alive in the world today diverged from their relatives at least a million years ago. For a while, polar bears seemed to be a glaring exception to this rule, with genetic analysis suggesting they split off from their brown bear cousins less than 200 thousand years ago, a mere moment in evolutionary time. In fact, polar bears appeared to lie within the brown bear family tree, rather than being a separate lineage: brown bears from certain Alaskan island chains are more closely related to polar bears than to brown bears from other parts of the world. Or are they?
Polar bears, like all plants and animals, have two distinct sets of DNA. One is carried in the nucleus of each of our cells, 50 per cent of which comes from mom and 50 per cent from dad. However, a second set of DNA exists in our mitochondria, the tiny organs within our cells that provide the energy needed for cells to function. These are inherited exclusively from our mothers, because sperm cells’ mitochondria are destroyed after entering the egg. Until recently, most research on the relationship between brown and polar bears had been carried out using mitochondrial DNA. When a comprehensive study comparing their nuclear DNA was finally done, a different picture emerged.
The study, which appeared in Science in April 2012 and led by Frank Hailer, pushes the actual divergence of polar and brown bears back to at least 600 thousand years ago, still recent but longer ago than was previously believed. It also places polar bears firmly on their own separate branch of the family tree. According to Hailer and his colleagues, the reason that mitochondrial DNA tells a different story is that even after polar bears first branched off into a separate lineage, warm periods between ice ages periodically brought them back into contact with their brown cousins, and when this happened, hybridization was the result. Female brown bears must have mated with male polar bears, and their offspring entered the polar bear population, bringing brown bear mitochondria with them.
Rise of the Grolar Bear?
Under normal conditions, polar bears and grizzly bears encounter each other rarely, if at all. Polar bears spend most of their time out on the sea ice hunting seals, their favourite prey, and even when they return to land to have cubs or avoid the summer break-up of the ice, they stick close to the coastline. Grizzly bears, on the other hand, are faithful land-dwellers, ranging through tundra and forest in search of varied food including caribou, spawning fish, and even berries and other vegetation. However, as climate change takes hold, it appears that grizzlies are expanding northward just as melting ice forces polar bears ashore for longer periods of time.
No one can say for certain whether individual incidents are directly related to global warming, but in the last decade, two polar grizzly hybrids have been confirmed in Canada. The first, shot on Banks Island in the Northwest Territories in 2006, attracted interest when officials noticed that despite having a polar bear’s white fur, it had the shallow face and humped back characteristic of a grizzly. DNA analysis proclaimed that it was the offspring of a female polar bear and a male grizzly. The second case was even more interesting. In 2010, an Inuk hunter shot what he initially believed was a polar bear on Victoria Island.
The bear’s patches of brown fur again caught the attention of biologists, and it turned out to have a polar/grizzly mother and a pure grizzly bear father, the first documented second generation hybrid. Hybrid animals are often sterile — the classic example is the mule, which is the result of a horse/donkey cross and is almost never capable of having young of its own. However, the 2010 find confirmed that not only are the two species of bear interbreeding in the High Arctic, they are producing offspring that are fertile.
An Uncertain Future
Polar bears have weathered periods of warming and hybridization in the past, but the effects of global warming are causing the Arctic climate to change at a faster rate than ever before. Bears aren’t the only animals likely to experience increased hybridization as a result — a 2010 paper in Nature suggested 12 pairs of species that may interbreed as climate change shifts their ranges, and some have already been reported in the wild, such as beluga/ narwhal crosses sighted off Greenland. Is hybridization good or bad? Unfortunately, this question has no simple answer.
As polar bear populations decline and individuals become rarer and more scattered, it will be difficult for them even to find each other. If this causes them to mate with grizzlies instead, the blend of traits hybrids inherit from their parents might be beneficial in the short term, but hybridization is not a solution to the threats posed by climate change. A bear that’s sort of good at catching seals and sort of good at catching caribou might not be good enough at either one to survive very long. Over time, so much of polar bears’ genetic distinctiveness could be lost through mixing with the more abundant grizzly bears that they effectively are hybridized right out of existence — that is, if habitat loss, pollution, and other threats don’t drive them to extinction first.
Because polar bears ‘problems have global causes, conservation efforts have been complicated and slow. The United States has designated them as a threatened species, but with a special stipulation that allows fossil fuel exploration in their habitat to continue. Last November, after years of controversy and debate over polar bear hunting in the North, they were finally declared a “species of special concern “under Canada’s Species At Risk Act. As worldwide awareness of the causes and effects of climate change increases, we can only hope that our efforts to save this symbol of the Arctic wilderness will be successful. For a species that’s been thriving in a harsh environment for 600 thousand years, time may finally be running out.