Hibernation is a great plan to deal with adversity – just sleep right through the bad moments! I’ve tried this a few times in my life, when things seem particularly awful (or cold), and have only managed to lounge about in bed complaining about the weather for about a weekend. I wish I could go on snoozing for longer, waking up to a more beautiful world some months later, like mice can.
When mice hibernate, they undergo widespread physiological changes that allow them to use less than 15% of the energy they normally would. These are the true masters of hibernation, but not every other hibernator is so lucky. Other mammals, such as bears, withdraw from society but exist in an extremely fragile state of rest. Their metabolic rates and body temperatures show only the slightest of reductions, and unfortunately, all sorts of things wake them up, which must be particularly annoying (a feeling we can all relate to I’m sure).
These disturbances to hibernation often have important downstream effects – affecting reproductive success and overall energy conditions for the animal.
In North America, black bears (Ursus americanus) and grizzly bears (Ursus arctos) hibernate for 4-7 months a year. Researchers from the Foothills Research Institute Grizzly Bear Program in Alberta, Canada, have investigated the causes of grizzly bear hibernation disturbances in the context of different den locations. The results of their 11 year study was recently reported in the Journal of Mammalogy.
They focused on 79 dens selected by 15 male and 35 female grizzly bears that were at least 4 years old. Over the course of two years, all of these bears were darted and radio collars affixed to them. What they then discovered fits very well into the general framework for wildlife survival – avoiding humans makes everything better, including hibernation.
The avoidance of areas with roads in them was particularly startling. Not a single bear denned closer than 150 m from a road or truck trail. Dens were 30% less likely to be placed in areas with road densities of 0.6km/sq-km, and 70% less likely if densities hit 1.2km/sq-km. Once road density reached 2km/sq-km, grizzly bears completely avoided putting dens in the area at all.
Sex differences in den selection were surprisingly negligible. Females didn’t seem to select dens in habitats that were different from males – and having cubs did not alter how they selected their dens either. They also did not appear to be actively avoiding areas in which males were denning, despite some fears researchers have had that males might injure or kill cubs, if encountered.
In general, the authors concluded, “grizzly bears selected dens that were in steep slopes at high elevations, dry conifer stands, areas farther from roads, and areas with low amounts of high-ranked autumn food and high proportions of high-ranked spring food.”
These results, one hopes, will inform conservation planning for the grizzly bear in Canada, which were declared a threatened species due to low population size, reproductive rates and immigration from adjacent populations. In addition, Alberta’s grizzlies are subject to a fair amount of human-induced mortality.
We can only hope that some day they will be able to really sleep through the winters undisturbed by us.