When Bears Go To Sleep
by Sterling Miller
Most wildlife biologists experience great satisfaction in their careers. I’ve been particularly lucky in this regard because I was able to study species that are highly interesting. I spent 20 years researching grizzly (also called brown) and black bears in a truly remarkable setting—Alaska. SuzAnne asked me to provide you with some information on bear hibernation, as she thought it might be of interest to you as you begin your own preparations for winter. Part of my research involved studying where bears den during the winter, how much time they spend in their dens, and what their dens look like. You can find details of these studies in the references provided below.
Winter is a stressful time for most wildlife species, because sources of food are covered up by snow or are otherwise unavailable. Some species respond by migrating, including most ospreys, neo-tropic bird species, geese, and ducks, but also hoofed animals like caribou and antelope. Less-mobile species sometimes adapt by changing their color to better match snowy winter backgrounds, including snowshoe hares, weasels, and the grouse-like ptarmigan. Others, including North American bear species, ground squirrels, marmots, and bats, hibernate, which is a period of winter sleep characterized by lowered body temperature, reduced metabolism, slower breathing, and cessation of some body functions like urination and defecation.
Among hibernators, bears are considered to be “light” hibernators, as body temperature and metabolic rates drop relatively little during hibernation compared to “deep” hibernators like bats, marmots, and ground squirrels. Bears in their winter dens are easily awakened from their relatively light sleep by noises from outside. When I located the dens of hibernating (radio-collared) black bears during winter, I could tell when it heard me and woke up because the radio collar would switch from inactive (infrequent beeps) to more frequent beeps indicating it heard me, woke up, and started to move around. The same would sometimes happen when I flew over the dens of hibernating grizzly bears and they were disturbed by the aircraft’s noise.
In my Alaskan study area, grizzly bears usually excavated their dens on southern exposures of mountainsides above timberline. Elsewhere, they are sometimes known to also use natural rock crevices. I have never seen a dug grizzly bear den used for more than a single year in Alaska as the dens collapse during summer. Dens in rock crevices may well be used for multiple years.
Black bears, in contrast, den in a wide variety of different den types, including cracks/caves in solid rock cliffs, under jumbles of boulders, holes dug under bushes or trees that provide roof support, and within naturally hollow trees (cottonwood trees in Alaska). Many black bears dens appear to be reused for many years, perhaps centuries, while others are used only once.
Polar bears usually hibernate in dens dug in the snow but sometimes in dens dug into the tundra. Only pregnant female polar bears den. Males and females with older cubs are able to catch their primary source of food (seals) all winter long, so there is no winter scarcity of food requiring them to den.
Newborn cubs are born in winter dens. Since the cubs are very small when born (under a pound in weight), the den provides a warm, secure, and safe place to grow until spring when they are large enough to venture out into the open closely guarded by their vigilant and very protective mothers. While in the dens, the cubs are nourished by their mother’s rich milk and grow until they are large enough to survive outside the den 4-5 months after birth. The den can be thought of as an extension of the womb of a mother bear, as it is a protective environment for the young. The necessity of the den’s protective environment is illustrated by contrasting denning behavior of polar bears with that of black and grizzly bears.
Grizzly cubs typically re-den with their mothers as yearlings and 2-year-olds, and sometimes longer. Black bears re-den with their yearling cubs and sometimes with their 2-year-old cubs. I once entered a very complicated den occupied by a mother black bear and four 2-year-old cubs. In that den it was hard to keep straight which bears I’d already injected with tranquilizer (in a syringe stuck on the end of a stick) and which ones I hadn’t. There is no need to tranquilize the small cubs which are captured by hand and measured outside of the den. Once when I was lying on top of a tranquilized female in her den, one of her cubs squealed when I grabbed it. This noise was enough to overcome the mother’s drug and caused her to respond to her cub’s squeal by lifting her head and putting her nose directly on mine. This gave me a scare for a brief moment that she’d been inadequately tranquilized.
Grizzly bears in my Alaska study area averaged 201 days in winter dens, while black bears spent 218 days in dens. Bears living in more southern latitudes—where the growing season is longer—spend less time in winter dens. Typically, pregnant females are the first to enter dens in the fall and the last to exit (with their newborn cubs) the following spring. Adult males are the last to enter dens and the first to exit, so these spend the shortest period in winter dens.
Black bear researchers enter black bear dens during winter to replace collars on adult bears. At this time it is minimally dangerous to the bear compared to other ways of capturing bears (in traps, snares, or darting from a helicopter). It is also less expensive and a lot more exciting and fun! We follow the radio-collar signal and then dig down to find the entrance and enter the den with a jab stick to tranquilize the adult. Biologists usually don’t enter occupied grizzly or polar bear dens because these species are more likely to be aggressive.
Prior to entering dens, bears are intent on eating as many calories as possible. This is called a period of hyperphagia. At this time bears must get as fat as possible, as the stored fat reserves will be metabolized during hibernation when bears will lose 25 percent or more of their body mass. Females that give birth must also metabolize body fat to produce the milk needed by their cubs. Pregnant females unable to get fat enough to provide for their newborn cubs during hibernation and the period of lactation following emergence will typically reabsorb their fetuses and try again the following year. It is nature’s way of not wasting energy and effort in a doomed effort to give birth and raise cubs when next year might be more successful.
In contrast, as we enter our winters most of us must strive to avoid the human equivalent of hyperphagia brought on by too many Christmas/New Years cookies, fruitcake and eggnog.
Some publications on denning of bears in Alaska by myself and others can be obtained from the website of the International Association for Bear Research and Management (www.bearbiology.com) by clicking on the link to the journal Ursus and searching for the volume and publication in the publication’s (back issues) listed below:
Schwartz, C.C., S.D. Miller, and A. Franzmann. 1987. Denning ecology of three black bear populations in Alaska. Int. Conf. Bear Res. and Manage. 7:281‑291. (Schwartz, Miller, and Franzmann).
Miller, S.D. 1990. Denning ecology of brown bears in southcentral Alaska and comparisons with a sympatric black bear population. Int. Conf. Bear Res. and Manage. 8:279‑287.
Waller, B.W., J.L. Belant, B.W. Young, B.D. Leopold, and S.L. Simek. Denning chronology and den characteristics of American black bears in Mississippi. Ursus 23(1):6-11.
Ciarniello, L.M., M.S. Boyce, D.C. Heard, and D.R. Seip. Denning behavior and den site selection of grizzly bears along the Parsnip River, British Columbia, Canada. Ursus 16(1):47-58.