“Bears rose to the top of the food chain solely so they can dedicate half their lives to sleep. It’s the world’s most intimidating power nap.” - James Breakwell 

I can’t talk about bear biology without addressing the subject of denning. Indeed, few things are more quintessentially bruin, but denning is a lot more than just a power nap for bears. If anything, it is a marvel of the natural world. So as the weather gets colder and another denning season approaches, this feels like the perfect time to take a closer look at this fascinating and complex topic. 

For starters, what is denning? Denning is the common term for a bear’s annual period of winter dormancy. During denning, bears do not eat, drink, urinate or defecate and remain relatively immobile for an average of five to seven months. Although it sounds a lot like hibernation, denning is better described as a form of torpor. In true hibernation, mammals (usually small ones) reduce their metabolic rates by 90% or more and their body temperatures drop dramatically. Due to the extent of these changes, animals waking up from true hibernation do so slowly, requiring time to readjust. In contrast, torpor is a milder state of dormancy. Bears in torpor lower their metabolic rates by 20-50% and, while their heart rate and oxygen use are significantly reduced, decreases in body temperature are relatively small. From this state, bears are able to wake and respond to changes in their environment almost immediately. Admittedly, the difference between hibernation and torpor is not set in stone. Some sources describe denning as hibernation or use the terms interchangeably, but the general consensus seems to lean toward torpor as a better description for this process. Other terms for animal dormancy you may come across include “estivation” (dormancy during hot, dry periods) and “brumation” (dormancy in cold-blooded animals), but these have specific meanings that don’t apply to denning bears. 

In bears, denning serves two distinct purposes. The first is survival during periods of low food availability (specifically winter) and the second is reproduction. As discussed in previous columns, it is a unique feature of bears that they give birth while in their winter dens, and this is reflected in the denning strategies of the three North American bear species. In American black bears (Ursus americanus) and brown bears (Ursus arctos), males and females both use dens to survive harsh winters, but their patterns of use often vary by sex, age and reproductive status. In the warmest parts of their range, male black bears and females without cubs may forgo denning or den for very short periods because food is readily available year-round. Similarly, polar bears (Ursus maritimus) are well adapted to frigid conditions and able to hunt seals throughout the winter. Therefore, denning in this species is solely for the purpose of reproduction and only occurs in females preparing to have cubs. 

For many people the idea of a “bear den” conjures images of caves or abandoned mines, and in some cases, they may be right. But because bears occupy a wide variety of ecosystems and habitats, the kinds of dens they actually use vary tremendously. Black bears can occupy almost any space they set their minds to – including caves, rocky crevices, hollow trees, excavated root wads, ground nests, thickets and even manmade structures. Earlier this year, I saw two black bears denning in a culvert so small they seemed to be defying the laws of physics. Brown bears can also be somewhat versatile in their den selection. They often dig earthen dens into hillsides or under trees using their powerful claws, but they have been documented using rock cavities and ground nests as well. While they do not necessarily use the same dugouts each year, brown bears are known for returning to the same general area year after year to make their dens, a characteristic known as site fidelity. Black bears can also demonstrate site fidelity depending on individual animal behaviors. Uniquely, female polar bears often dig their dens into snow drifts (making them notoriously hard to study) but they can also use earthen dens when available. Regardless of the species or environment, the most important characteristic for any bear den is its ability to protect its occupant from the threats that are likely to occur in that area, be they extreme cold, heavy accumulations of snow or seasonal flooding. 

Beyond the why and the where of denning behavior, it is really the HOW that makes this process extraordinary. How can such a large animal survive months at a time without any of the activities that normally sustain life? And not only do bears survive denning, but they emerge essentially no worse for the wear. For example, bears accumulate large fat stores to sustain themselves through the winter. During denning, they breakdown these stores to produce energy and, in the process, release high levels of lipid to circulate in their blood stream. Unlike people, bears do not experience damaging plaque formations or blockages in their blood vessels as a result of this circulating fat. Bears also experience very low heart rates during torpor. Extreme decreases in heart rate and circulation in a human would put them at risk of heart failure, blot clots, kidney and other tissue damage, but bears experience none of the effects. Finally, long periods of inactivity in humans can result in significant loss of muscle mass, osteoporosis and pressure wounds. In contrast, bears demonstrate a phenomenal ability to maintain their bone density and lean muscle mass despite months of disuse. They emerge from torpor strong and healthy. In some cases, research has shed light on the mechanisms behind this biological wizardry, but in other cases, we still have a lot to learn. In the second half of this series on bear denning, I’ll take a deeper dive into these mechanisms and what they can tell us about bears and our own bodies…stay tuned! 

References: 

Breakwell, J. 2019. How to save your child from ostrich attacks, accidental time travel, and anything else that might happen on an average Tuesday. BenBella Books, Dallas, Texas, USA. 

Sorum MS, Joly K, Wells AG, Cameron MD, Hilderbrand GV, Gustine DD. 2019. Den-site characteristics and selection by brown bears (Ursus arctos) in the central Brooks Range of Alaska. Ecosphere 10(8):e02822. 

Stenvinkel P, Jani AH, Johnson RJ. 2013. Hibernating bears (Ursidae): metabolic magicians of the definite interest for the nephrologist. Kidney International 83: 207-212.