Bear Update: A bear’s winter sleep
By Michael Allen
Black Bear Researcher
One last objective for the Whistler Bear Project: to find where black bears will spend the next 4-6 months sleeping, escaping winter’s weather and shortage of food. This is accomplished by tracking bears to their potential denning areas during the first snowfalls in October and November. Dens are also found coincidentally during other fieldwork throughout the year.
Hibernation begins in mid-late August, during the stage called hyperphagia — an increase in the bear’s daily food intake. Bears will spend 15-20 hours a day feeding in August and September, compared to 8-15 hours a day feeding in July, to prepare for winter sleep. Hyperphagia lasts through October, until bouts of sleepiness creep up on the bear.
Satisfied with their weight, black bears begin moving to their denning areas in mid-October. Bears usually travel to their denning area up to two weeks in advance and sleep outside the den on a daybed until inclement weather forces them inside. Denning is dependent on the age/sex of the bear, the available food supply and the onset of the first snowfalls. Mothers with cubs traditionally enter their dens first, followed by mothers with yearling cubs (1 year olds), immature single bears and finally adults.
While in the den, black bears do not eat, drink, urinate or defecate. Bears sustain themselves with their fat reserves, built up from their voracious feeding during hyperphagia. Rates of heart and metabolism decrease significantly. I took pulse measurements from a 2-year-old black bear in the B.C. Interior during a previous bear study in 1990. The bear’s pulse while sleeping in early August was between 28 and 48. The same sleeping bear’s pulse in late December was between 6 and 12 beats per minute.
Hibernation in black bears differ from that of true or "deep" hibernators (bats, insectivores and rodents) in that bears hibernate close to their normal body temperature (31-35 degrees Celsius) and their sleep is relatively continuous for 4-6 months. Deep hibernators, such as squirrels, hibernate at near 0 C and rise frequently through the winter to feed. However, black bears become alert and defensive in their dens when disturbed. Bears often abandon their dens if disturbed while entering and, to a lesser extent, as winter progresses.
From 1993 through 1996 44 black bear dens have been located in the Whistler ecosystem. Forty-three per cent (19) were active with denning bears. The remaining 25 were "used" dens, unoccupied. Bears den in old-growth "hollowed out" trees, in excavated chambers in the ground, in depressions underneath boulders, stumps or tree wells, and in the bottom of slash piles on cutblock landings. Nine of the 19 active den sites were inside trees, five were excavated in the ground, three were underneath stumps, one was under a boulder and one was inside a tree well. Black bears don’t typically re-use their dens but instead re-use the habitat in the vicinity constructing another den.
In a population where black bears inhabit human-developed areas, such as Whistler, bears have adapted to entering dens later because of a supplemental, prolonged food supply. The landfill provides edible, fattening human foods for bears. Bears have learned to tolerate early winter inclement weather to continue their feeding at the dump. In 1995, a large male remained near the dump until Dec. 29. Snow depth was 30-40 cm and temperatures plunged to -30 C. Although the bear continued to consume food, the majority of his day was spent sleeping.
This year, all bear families studied left for their denning areas before Nov. 1 The first to leave were the families on Blackcomb and Whistler, around the middle of October during the first heavy snowfalls. The last family to leave was Suse and her three, now very plump, cubs in late October, having supplemented their diet all year by feeding at the dump. It is very possible for families to emerge and feed again, depending on weather and food available (late berries or accessible garbage). Due to the poor berry crop in 1996 it is probable that some bears will remain active through November. For example, adult males who are accustomed to 100-200 pound weight gains during the fall are still active throughout the valley, ski areas and around the landfill during November.
Nearing the end of my fieldwork for this year, I check in on one of my largest study bears in the Whistler Interpretive Forest. I sit back in the snow exhausted. It has been an exceptional year for collecting data on Whistler’s bears, but also a long physical one. I watch the 600-pound male sleeping 5 metres away, wondering how in the hell he has survived all these years living so close to people. What it really comes down to is bears have adapted to living close to us far easier than we have ever tried to co-exist with them. For Whistler’s unhunted bear population the future depends solely on how we perceive and justify their existence.
I forced myself up out of the wet, heavy snow and took one last glance at the massive sleepy omnivore. In about 130 days he will be emerging from his den to begin again what I had studied all year — and I will be waiting.