By Ed Struzik, edmontonjournal.com
I am standing on the edge of a frozen lake 80 kilometres inland from the west coast of Hudson Bay. It is –33 C. The skies are clear and the bright sun is hovering over the horizon, lighting the spindly forest around us with that alpenglow that makes everything look surreal. Twenty metres away is the polar bear den we have been looking for, nestled into the side of a south-facing hill.
Polar Bear with newly born pups emerges from winter sleep
Helicopter pilot Justin Seniuk keeps the engine running as Manitoba biologists Darryl Hedman and Vicki Trim and I crunch through the waist-deep snow, crossing days-old tracks of playful cubs zigzagging around us. Having worked on polar bears since 1986, Hedman could make light of what we are about to do; he is fairly certain the female and her cubs have recently left. The tracks that led us to the site indicate this family is on the long march to Hudson Bay.
Hedman isn't taking any chances, though. He has his gun loaded and the safety off. When he lets out a shout, it is loud enough to startle Trim and me as we look over his shoulder, searching, as he is, for any movement inside. There is none we can see. Still, I can't stop thinking of the story that Ian Stirling, a University of Alberta scientist, told me several years ago when he was doing exactly what we are doing now. In his case, he didn't notice there was a polar bear standing in a second chamber of the den until his eyes adjusted to the dim light inside.
Mother Bear leading her cubs to the sea
"Unreal," says Hedman as he does his best to squeeze into the den a few minutes later. "Imagine spending a winter in a space like this where you can barely move and not have any food to eat or water to drink."
Since British explorer Samuel Hearne first described the denning behaviour of polar bears in this part of the world back in 1795, nothing much has changed. "The females that are pregnant seek shelter at the skirts of the woods," he wrote, "and dig themselves dens in the deepest drifts of snow they can find. There they remain in a state of inactivity, and without food, from the latter end of December or January till the latter end of March: at which time they leave their dens and bend their course toward the sea with their cubs."
Biologist Darryl Hedman examining abandoned den
Now that climate change is rapidly warming this part of the world, this centuries-old tradition may be in peril. Forest fires, which were once rare in this part of the world because it has been too cold and wet, have already destroyed dozens of dens in the past decade. Milder winters are also resulting in snow dens collapsing and killing bears and their cubs in some cases.
Building new ones, according to research by scientist Evan Richardson while he was a graduate student at the University of Alberta, could further tax the polar bear's already strained energy supplies.Not all dens are created in the same way. Some, like this one that I crawl into, has been dug into a peat bank. Like many dens, it has a single chamber just big enough to allow a female to nurse one or two and, in very rare cases, three cubs. Others have as many as three or four chambers to accommodate more adventurous cubs or a two-year-old that hasn't been chased off by its mother.
site of Polar Bear den
Most bears den in snow caves on land, but some animals in Alaska den on the sea ice.
The common denominator in these cases is the quality of the snow.
"Like Goldilocks' bed, snow has to be just right, not too soft and not too hard," says Andrew Derocher, a University of Alberta scientist who devotes a chapter to the art of polar bear denning in his book Polar Bears, A Complete Guide to Their Biology and Behavior. Fresh snow, Derocher points out, lacks structure and strength. Year-old snow may be too compact to allow enough oxygen in.
Once I get over the fear that there is a bear waiting for me, I am struck by how warm, snug and remarkably clean the den is inside.
Considering how much snow and peat moss there is insulating these chambers, it is not surprising that the temperature inside is just below the freezing mark when it –30C outside or colder.
In his pioneering study on polar bear denning in the 1960s, Dick Harington, the first scientist to study polar bears in Canada, dropped a thermometer into two active dens and found that temperatures inside were 7.8 C and 21 C warmer than they were outside.
Dens are clean because the female can go several months without defecating or urinating.Bears also move their dens throughout the winter when snowstorms pile things up to the point where oxygen is scarce.Harington's study on polar bear dens in the Canadian Arctic between 1961 and 1964 is remarkable not only because he did it by dogsled over three years, but because he somehow managed to find 113 dens. He attributes his success to the Inuit, who helped him identify the sites.
No one knows exactly how many dens there are in the Churchill region; Hedman is still discovering new ones on the spring surveys he started doing several years ago. But scientists such Stirling, Derocher and Nick Lunn, who have studied this population of 1,000 or so animals for decades, believe 90 per cent of the pregnant females den in a relatively small area 40 to 80 kilometres south of Churchill, from the Nelson River east to the Ontario border.
Twenty years ago, that meant nearly 200 bears in a good year. But now that the bears' energy reserves are being taxed by a shorter ice season that limits their ability to put on fat, the numbers are much lower.
The dens of female polar bears of Churchill aren't quite as clustered as they are in Kongs Karls Land in Svalbard, the northernmost part of Norway, where as many as 25 dens have been counted in a small area. But they are close enough together to warrant the attention given by Richardson's study, which was done under the supervision of Stirling and the collaboration of Bob Kochtubadja of Environment Canada.
The historical forest fire data for northern Manitoba are scant. But if climate models are correct, says Mike Flannigan, University of Alberta fire and vegetation specialist, one could expect to see more forest fires in the denning region of western Hudson Bay. All it would take, he says, is fuel, which there is plenty of; a source of ignition, which in this case is most likely to come from lightning, and warming temperatures, which is pretty much a given for most parts of Northern Canada.
Why Churchill's polar bears choose sites so far inland when bears in Svalbard hunker down in clusters as close as 10 metres from the edge of shore remains a mystery.The fear of hungry males tracking them down could be a factor. So may be sea ice patterns and snow conditions. Hunting by humans might also have played into the bear's decision-making. Up until 1957, when the Hudson's Bay post at York Factory closed, First Nations people living on the coast harvested them for fur and trade.
Big as polar bears can be, pregnant females pay a price for denning so far inland. When they come off the ice in late June or early July, most pregnant females have so much fat stored in their bodies that it sometimes looks as if they can hardly walk. By the time they settle into their dens, however, most haven't eaten for four months and it will be another four before they get their first seal. Some of them are little more than a bag of bones by the time that happens.
The good news is that some dens in this part of the world are passed from one generation to the next so all that most polar bears need to do is clean up whatever debris may have accumulated from one year to the next.
How a young female bear instinctively knows where to go after being out of the den for four or five years is one of the enduring mysteries of polar bear science. They are just roly-poly cubs when they emerge, and at least four years will pass before they are ready to reproduce. Yet like salmon that return to the same spawning stream after a year living in the ocean, these bears instinctively know where to go to find the place they were born.
In a study of tree rings that have been damaged by the digging done by pregnant females, Stirling has found evidence to suggest this hand-me-down practice has been going on for more than 100 years in some cases, and probably longer.Given the distances these bears have to travel, one would expect they would follow a relatively straight line. Yet as we followed the tracks of another family group along the Nelson River the next day, it was clear this is not the case. At one point, the bears made a full circle before heading back in the direction they were supposed to be going.
It seemed absurd they would plow through shoulder-deep snow in an open meadow rather than follow the solid path of a frozen creek heading north. At the rate this family group was progressing, Hedman figured it would take them three days or more to get to the coast.
The long walk is not the only challenge these family groups face in late winter, as we saw first-hand when we picked up a set of wolf tracks in pursuit of another family group. There is no shortage of wolves in northern Manitoba, and based on the experiences that Hedman and his colleagues have had over the years, the animals aren't shy about trying to take down a young adult.
While this one wolf wouldn't have a hope of taking down a female protecting her cubs, a pack of five or more animals would have a much better chance.Cubs, Hedman points out, just don't have the energy to walk or run for an unlimited time. All the wolves need to do is distract the mother long enough for one or more of them to get at one of the defenceless cubs. "We don't know how often this happens, but I've seen tracks which suggest that it may not be all that uncommon. It's something that we are going to be watching for in future surveys."
Surveys such as this one are complicated by bitterly cold weather and wild windstorms that seem to come out of nowhere in western Hudson Bay. Base camp for us was a one-room cabin on the coast that was heated by a single wood stove. We don't dare to walk to the outhouse at night for fear we will run into one of the bears that we know are around us from the fresh tracks we see.
Justin Seniuk, however, has no choice but to venture into the dark when the generator he is using to heat the helicopter's engine keeps breaking down.
Back at home in Edmonton, I sort through the photos I have taken on the trip. I see one that looks like nothing more than a stand of trees close to another den site we had investigated.I figure I must have taken this photo by accident. But just as I am about to delete it, I see it shows tracks in the forest. I follow the tracks and discover they lead to the back end of a polar bear, which is mostly hidden in the trees."Can't be," I say to myself. I study it more. There is no doubt about it. It is a polar bear and it was obviously hiding in the trees when we were crawling into that den.
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