My column in the November/December issue of BHM was the first in a two-part series examining the science of winter denning in bears. During this period of torpor, bears refrain from eating, drinking, defecating, urinating, and moving outside of the confines of their carefully selected dens. Under other circumstances, this would leave an animal starving, dehydrated, and at risk of organ failure, but not so for denning bears. They not only tolerate their self-imposed seclusion, they thrive. The question is…how? 

If you were to examine a denning bear, one of the first things you might notice is, despite their dormant state, they still radiate warmth. The normal body temperature of an American black bear (Ursus americanus) is 98°F-100°F (37°C-38°C). During denning, this drops and cycles between 86°F-97°F (30°C-36°C), but that is still a far cry from smaller mammals in true hibernation whose body temperatures closely mimic their environment, in some cases near freezing. Bears are believed to maintain these higher body temperatures due to their size and composition as well as the curled position they assume in their dens and periodic muscle movements (shivering). Anecdotally, researchers have observed that sows preparing to have cubs may maintain higher body temperatures until after they give birth. 

Another thing you would notice examining a denning bear is a very, very slow heartbeat. When active, a black bear’s heart typically beats 45-60 or more times every minute, but for a denning bear, that rate slows to an average of 14 beats per minute. Their breathing can also slow to less than two breaths each minute. Normally, this combination would be a serious problem because less oxygen is distributed to the body. Even though oxygen is required for cells to carry out their normal functions, many of those functions slow down significantly during torpor, which lowers oxygen demand. That is the part you can’t see when you look at a denning bear—the changes in its metabolism. 

The word “metabolism” gets thrown around a lot (mostly related to diets and such), but its technical meaning is sometimes lost in the shuffle. Metabolism is the overarching term for all chemical processes in a body necessary to sustain life. It includes packaging energy into usable forms, using that energy, building things up, breaking things down, and getting rid of any resulting waste. Bears in torpor can reduce their metabolism by 20-50% compared to active seasons. This explains, at least in part, how denning bears can go months without eating. Just like oxygen demand, slower metabolic processes mean less energy is required for cells to survive, but slower metabolism doesn’t mean a complete stoppage. 

To prepare for their extended winter fast, bears spend each autumn in a state of “hyperphagia” or extreme food consumption. During this time, they can more than double their caloric intake and pack on fat stores that will fuel them through the winter. Being omnivores, bears use a mix of carbohydrates, proteins, and fats to acquire energy throughout the year; as a result, their near exclusive reliance on fat during the winter requires them to retool their metabolic machinery. By using fats instead of proteins, bears can lose up to a quarter of their total body weight during denning but lose only 10-15 % of their lean muscle mass. 

Another advantage of using fat instead of protein during denning is reduced production of nitrogen-containing waste. These byproducts of protein metabolism are usually excreted in urine and can damage tissues if they accumulate. Although denning bears don’t drink water and don’t appear to urinate, their kidneys continue to produce small amounts of urine. Bears are unique because they have adapted to reabsorb water and waste products from their bladders and recycle them back into their bodies. The recycled water is supplemented with water molecules released during fat metabolism (“metabolic water”) to keep the animal hydrated. The nitrogen wastes are turned into amino acids, the building blocks of new protein.  

Long periods of confinement or inactivity in mammals usually result in decreased muscle mass and bone strength. If this happened during denning, it would leave bears weak and vulnerable come spring. Bears have developed several adaptations to avoid this. By slowing the breakdown of proteins and recycling nitrogen wastes to build new proteins, bears save energy and prevent a net loss of lean muscle mass. Contractions in the form of shivering may also help to maintain muscle mass during denning. Bears appear to protect their bone strength with similar mechanisms. At a cellular level, bones are continuously broken down and rebuilt in equal measure. The processes are driven by chemical signals, but rebuilding is also heavily influenced by the forces and pressures that come with use. Long periods without these forces can cause breakdown to outpace rebuilding, leaving bones brittle and at risk of fracture. Research suggests that bears avoid this by equally suppressing both processes; neither is allowed to outpace the other, which saves energy and protects bone integrity. 

It would be easy to mistake denning bears for metabolic superheroes, but no good thing lasts forever. It turns out that during the summer, bears are just as susceptible to dehydration and starvation as other species. The metabolic changes that allow them to survive the winter don’t persist year around. Those changes start several weeks before denning, and their metabolism remains slow for days to weeks after they emerge. One older reference I found described this latter period as “walking hibernation.” The need for weeks of adaptation to implement and reverse these drastic metabolic changes speaks to the magnitude of the phenomenon.  

Knowing the nitty gritty details of how bears survive winter torpor is really interesting if you’re a bear nerd like me, but the information doesn’t necessarily lend itself to managing bear populations. Moreover, what I’ve covered here is only the tip of the iceberg for what is known about biochemical pathways and the like. So why have researchers invested so much time and attention in these topics? The answer is biomimicry, the examination of nature and its processes to find solutions and pathways for addressing human health issues. Research on denning bears may hold clues for treating kidney failure, muscle wasting, vascular disease, osteoporosis, and many other conditions. After all, sometimes nature does know best. 

References: 

McGee-Lawrence M, Buckendahl P, Carpenter C, Henriksen K, Vaughan M, Donahue S. 2015. Suppressed bone remodeling in black bears conserves energy and bone mass during hibernation. The Journal of Experimental Biology 218:2067-2074. 

Nelson RA, Folk Jr GE, Pfeiffer EW, Craighead JJ, Jonkel CJ, Steiger DL. 1980. Behavior, biochemistry, and hibernation in black, grizzly, and polar bears. Vol. 5 A Selection of Papers from the Fifth International Conference on Bear Research and Management, Madison, Wisconsin, USA. Pp. 284-290. [abstract] 

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

Tøien Ø, Blake J, Edgar DM, Grahn DA, Heller HC, Barnes BM. 2011. Hibernation in Black Bears: Independence of Metabolic Suppression from Body Temperature. Science 331: 906. DOI: 10.1126/science.1199435.