A male Grizzly Bear crosses the arctic coastal plain of the Arctic National Wildlife Refuge.
I recently returned from leading my first wilderness trip of the season. I flew with two clients into the Jago River in the Arctic National Wildlife Refuge. There we donned our packs and hiked about 15 miles over a pass into the Aichilik River valley where we inflated packrafts and paddled our way out of the mountains, through the threatened coastal plain, all the way to the coast. It was pretty awesome, as most trips to Alaska’s Brooks Range are.
One evening, about 11pm, at our first camp on the coastal plain, we were sitting outside in the sun, starting to think about bed, and watching a small herd of caribou 1/4 mile to our north. I took a moment to glass the sprawling tundra, and spotted a lone Grizzly ambling down a slope on the far side of the river. Through my binoculars I could tell it was likely an adult male (males have more defined humps over their shoulders and a broader head). The big guy dropped down onto the river bar a few hundred meters from our camp, wandered across the gravel, swam across the clear water of the Aichilik and climbed up onto our side of the river.
Though I’m certain there are pragmatic reasons for this behavior like personal safety, avoiding conflict, and defending resources, I prefer to think of them as “polite”.
Brooks Range bears tend to have a large personal space, and generally don’t like to get close to people, or other bears. Though I’m certain there are pragmatic reasons for this behavior like personal safety, avoiding conflict, and defending resources, I prefer to think of them as “polite”.
This guy was no different and swung in a big arc around our camp before heading off across the tundra to the west. We watched him for 40 minutes, as he stopped occasionally to sniff, dig up a root, or on one occasion, raid the nest of a pair of American Golden Plovers.
Jacob, one of my clients, asked the occasional question as we watched, but when he got to denning and hibernation of bears, my answers stalled out. I knew that bears are not really “true hibernators” at least not when compared to species like Arctic Ground Squirrels but knew little more about the subject, and I knew almost nothing about where they chose to spend their winters. When I got home, I looked into it.
Do bears really “hibernate”? That’s a tricky question and one that seems to be a matter of semantics. Technically speaking, hibernation is a period of inactivity and metabolic depression. Meaning that body temperature, respiration, and heart rate all decline while the animal appears to “sleep” (though this is unlikely to be the best comparison). Traditionally, the term has been applied strictly to species that enter a “deep” period of hibernation. The Arctic Ground Squirrel, for example, has a body temperature that drops near to, or even below, the freezing point, and their heart rate and respiration are hardly detectable. So our bear on the north slope of Alaska, when compared to the much smaller ground squirrel, is a weak hibernator.
That said, bears do enter a period of extended dormancy: their heart rates can drop to less than half their summer rate, respiration declines (overall oxygen use can decline by 50%), and their body temperature decreases. So, sure, why not go ahead and call it hibernation? And that is, in fact, the same conclusion reached by a group of researchers who studied winter behavior of bears. (I love it when my own opinions are supported by people who actually study such things.)
So, what about denning? Dens, it turns out, vary quite a bit based on locale, so I focused on bears of the far north of Alaska. At the Third International Conference on Bears, in the 1970s a rather lengthy tome of papers was created. Mixed into the hundreds of pages of behavior, diet, and physiology was a chapter entitled: “Denning Ecology of Grizzly Bears in Northeastern Alaska”. In other words, the Arctic National Wildlife Refuge. I had my answer with just a few clicks of my computer’s mouse.
The three authors, Harry Reynolds, James Curatolo, and Roland Quimby used a mix of aerial surveys and tracking of radio-collared bears to find 52 dens in the eastern Brooks Range and the nearby arctic coastal plain from 1972-1974. They used a mix of aerial surveys and tracking of radio-tagged animals to find the dens. Of these, 39 were excavated burrows, while the remaining 13 were in natural caves. 90% were on south-facing slopes, 8% on north-facing, and 2% on east-facing. (West-facing slopes were avoided entirely). This choice of a southerly aspect, I found interesting, but not surprising. After all, that warm spring sun, the bear’s wake-up call, will be strongest on the south slopes of the mountains. The authors add that on north-facing slopes the permafrost is too shallow (30-60cm below the surface) to allow easy excavation. Slopes facing south, however, can melt to a depth of more than 2m.
Come autumn, our bear, after gorging on fall berries, and stuffing packing on as much fat as he can, will select a spot on a steep (20-35 degree) south-facing slope, at around 1000m above sea level. Though he might find a natural cave beneath a rock overhang, it’s more likely he’ll dig out his own den.
I can picture our bear roaming around on a late-autumn hillside, summer’s fat jiggling around his middle. The sun is fading quickly by that time of year, and there is a darn good chance that the tundra on his mountainside is already covered in termination dust, that first layer of snow indicating the arrival of winter. With long-clawed paws he’ll peel back the layer of vegetation and start pulling out great wads of rocky soil, which tumble down the hillside below.
He’ll remain in that den for months, until winter begins to blow out and the sun returns.
When it’s deep enough, (as much as 5m), he’ll hollow out a sleeping chamber and drag in a pile of moss, grasses, and other vegetation to serve as an insulating bed. There he’ll curl up and drift off, as winter’s snow builds up around the opening. Occasionally, during his long sleep, he may stand and turn like a dog, choosing a comfortable position before collapsing again in a great blonde heap. He’ll remain in that den for months, until winter begins to blow out and the sun returns. By April he’ll be getting restless, hungry perhaps, or maybe bored, and will venture outside. There is likely to be snow on the ground, but it won’t last long. Spring, he knows, has arrived.
By June, the bear will be back on the coastal plain, pursuing the herds of caribou. And perhaps I’ll be back out there to watch him pass politely by our Aichilik River camp.