A Long Nap

When considering winter survival, the American Black Bear is commonly used as an example of hibernation, when in fact their winter rest is simply a long nap. True hibernators, such as bats, are able to drop their body temperature down to near freezing (or ambient temperature) and slow their heart rate for reduced metabolic costs. A bat’s body temperature drops to 32oF and heart rate drops from 210 beats per minute to 8 beats per minute. Shivering is the only mechanism that stands between survival and freezing to death. Another true hibernator, the jumping mouse is able to raise its body temperature every several weeks to prevent the cells from rupturing, and then resume torpor. Chipmunks spend nights or several days in torpor, waking in between to eat food reserves. This mechanism unfortunately uses up most of their fat reserves as they raise their heart rate from 4 beats per minute to 350 beats per minute when “waking up”.

What separates black bears from the mammals presented above is the surface area to volume ratio. A bear’s low surface area to volume ratio makes it challenging to drop body temperature, whereas smaller animals are able to dissipate the heat. Its body temperature drops only 10oF, from 100oF to 90oF, and heart rate drops from 50 beats per minute to 8 beats per minute. The black bear’s sleep is not deep, and they are easily aroused throughout the winter. They tend not to consume food, nor relieve themselves of waste; however, females do give birth to 2-3 young in January every other year, and will produce milk throughout the remainder of the winter using their fat reserves.

To prepare for this season, black bears double their weight in the fall season by gorging themselves on berries, seeds, and nuts. A good mast year tends to bode well for the bears, as it not only provides sufficient fat reserves, but also makes for a shorter nap season. The bears den later in the season if food sources are available. This year marked an excellent mast year, so one can expect the bears to be active into December.

To follow bear den webcams throughout the winter season go to http://www.bear.org/website/ and click on the live cameras link. Several bear dens are monitored each year and it is an incredible resource for learning about bear winter survival!

-H. Cowan

AMC Naturalist Guide