Natural History Moment

Its spring time again: time for the snow to melt, plants to bud and bloom and hibernating animals to wake from their seasonal slumber. Many small creatures in the Rocky Mountains hibernate in the winter months, such as the yellow-bellied marmot (Marmota flaviventris), ground squirrels (Spermophilus sp.) and chipmunks (Tamias sp.). The most famous hibernator however, black bears (Ursus americanus), do not actually undergo true hibernation. Black bears indeed sleep away most of the cold seasons yet they occasionally wake and venture from their dens. This pseudo-hibernation is referred to as torpor.

Preparation for the winter months begins by late summer. Black bears are omnivorous eating both plants and animals, but are primarily vegetarian with most of their diet consisting of emerging plants in early summer and fruits and berries as the summer progresses. Insects, small rodents and carrion (meat scavenged from carcasses) add needed protein to this diet. As summer turns to fall, eating intensifies up to ten fold in order to put on a layer of fat which will sustain the bear though the winter.

In the Colorado Rockies, black bears tend to enter their winter dens around November as snow falls and food runs out. Dens can range from a cave or crevice in a boulder field to a simple depression amongst conifers or even within underbrush. No matter the setting, all dens must provide some shelter from the elements, camouflage from predators and must also be well drained as not to collect water.

Once asleep, a black bear’s body temperature drops by around 8 degrees Celsius (approximately 15 °F) from a normal body temperature of 38 °C (101°F). While this drop is substantial, it remains small in comparison to a chipmunk which undergoes true hibernation and whose body temperature can drop by as much as 26 °C (47 °F). Respiratory, circulatory and digestive rates also decrease. This decrease in body function allows the bear to greatly reduce its energy consumption in order to survive for such a long period of time on only its fat reserves.

One major exception to their prolonged sleep (and thus the classification of torpor) is that in late January to early February pregnant sows give birth to their young. First time mothers usually only have one cub while twins are standard after a sow’s initial birth. At birth, the cubs are extremely small weighing only 170-280g (6-10oz) and only around 20cm (8in) long. A mother must be awake enough to provide basic care to her young, but both the nursing mother and her cubs will remain in the den for another few months until they emerge between late March and early May. Cubs will remain with their mother for another 18 months. During this time cubs will grow to over 45-70kg (100-150lbs) before heading into the world on their own.

Providing milk for her cubs is a notable drain on the mother’s energy stores and by the time a bear emerges (nursing mother or not) it will have lost up to 20% of its fall body mass. Despite the near starvation state, bears start eating slowly by grazing on new shoots and grasses as their digestive tract gears back up after many months of non-use.

As for ourselves, now that temperatures are warming and snow is melting, its time for us to begin thinking about sharing our environment with bears again. This means not only hanging food while camping, but also securing our garbage at our homes and businesses. As you’re hiking start looking for signs of bears such as tracks, scat, opened decaying logs or fur left on scratching trees and maybe you’ll be lucky enough to spot a mother with her new cubs!

Past Natural History Moments

Color change is not just for Aspens, October 2006

Snow In Springtime, February 2006

Aspens in Autumn, October 2005