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By Root 1290 0

beaver ponds had dried up and the water seal to a beaver’s den was gone, I also entered one of their dwellings. I carried a flashlight, to have a look around. The den I examined contained two platforms. One was slightly above the other, and in both the floors were liberally strewn with small debarked twigs, the remnants of take-home meals. (There were no remnants of fecal matter.) The chambers were roomy enough for me to be able to turn around, but it must have been a tight squeeze for a mated pair of beavers, together weighing about one hundred pounds. Add their two to five yearling offspring, and an equal number of young from the current year, and it must have been a tight, cozy fit. The space seemed awfully small for a crowd of perhaps a dozen beavers to spend half a year there in total darkness. Beavers do not hibernate and must therefore be quite tolerant of one another, as they would almost literally be rubbing against each other all winter. By spring, however, the yearlings get the wanderlust—or the cold shoulder as the parents evict them. After the ice melts off their ponds, I find these yearlings as common roadkill on highways that cross brooks or streams.

Even larger animals may build shelters. Usually before snow starts to cover the ground, a bear (or a family of a sow and her previous yearling cubs) chooses a den site for hibernation. Grizzlies dig tunnels into a hillside and excavate a cavity at the end. Black bears excavate depressions under a pile of brush, or the roots of a fallen tree, or use a rock cave or a hollow tree, and sometimes make or use no cave at all. The typical den cavity, if there is one, is about five feet wide and two or three feet high and upholstered with leaves, grass, and other debris raked in from the surroundings. Cedar bark and conifer twigs may also be bitten and torn from nearby trees and carried in. Yearling cubs help their mother build the den that they share with their mother. A bear’s den retards convective cooling by the wind, but temperatures inside the den are not much different from air temperature outside; bears rely mainly on their fur for insulation, which doubles its insulative capacity in winter. Once settled in, they don’t feed for seven months, living off their body fat until late spring. In January, sows give birth to two to three naked cubs, which don’t hibernate. They snuggle up to their mother and suckle for the three additional months that she hibernates all the while losing about twice as much body weight as males. Both sexes, however, stand a 99 percent chance of surviving the winter. The den stays clean because the sow does not urinate or defecate all winter. The cubs do, but despite a greatly depressed appetite the mother eats their feces.

The biologist “Bearman” Lynn Rogers, studying black bears in northwestern Minnesota in the 1970s, routinely visited them in their dens. “Most of those I visited in dens were wakeful enough that they lifted their heads and looked at me,” he wrote. “Although, in general, they seemed less sensitive to danger than they had been in summer, some were moderately aggressive…. Some did not wake even after gentle prodding and jostling.” And: “In one case, on March 27, 1970, I accidentally fell on a six-year-old female in her den. She didn’t wake up for at least eight minutes, even though her cub bawled loudly and I began prodding her. On January 8, 1972, I tried to hear the heartbeat of a soundly sleeping five-year-old female by pressing my ear against her chest. I could hear nothing. After about two minutes, though, I suddenly heard a strong, rapid heartbeat. The bear was waking up. Within a few seconds she lifted her head as I tried to squeeze backward through the den entrance. Outside, I could still hear the heartbeat, which I timed (after checking to make sure it wasn’t my own) at approximately 175 beats per minute.” A bear’s heart rate during the day is normally 50 to 90 beats per minute, although the heart rate of a sleeping bear in winter may decline to as low as 8 beats per minute. I wonder who was the more surprised by