Up north, winter is the season of low temperatures, white landscapes, and survival strategies. For many animals that means migration, why hunker down when there’s warmth a few hundred miles south? But some animals enlist another strategy, something I’d like to on the coldest, darkest days: hibernation.
In warm months, hibernating animals function much like we do; they eat, drink, use energy, and urinate/defecate. But once they go into hibernation, they stop doing all of those things for 5-7 months. Their metabolic rates drop to 25%, or even 2% in some cases, of what they normally are (Tøien et al. 906). And even though scientists have studied hibernating animals for decades, we still don’t know how they pull off such a feat.
Scientists aren’t drawn by curiosity alone to know how bears, bats, lemurs and more can survive a several month slumber. There are potential advances for osteoporosis and muscle atrophy prevention at the root of many hibernations studies, especially studies about bears. Because, even though bears experience “long-term anorexia and limited mobility,” their muscles are still strong and functional and their bones don’t lose density, in fact they may get even stronger (Lohuis et al. 257).
In this video, the hibernating black bear’s heart probably beat 8-12 times while we heard it breath twice in 52 seconds (Tøien et al. 908). In a 2011 Science article, Tøien et al. noted black bears drop both metabolic and heart rates to ~25% of their norm (908). It was hypothesized that cold temperatures induce this biological torpor, but Tøien et al. found that changes in body temperature during hibernation did not correlate with changes in metabolic rates. This means that there must be other internal mechanisms inducing the energy-saving hibernation state. Some researchers think that understanding the biological on/off switch for hibernation could assist in developing new medical treatments and a few are even hopeful that it could assist humans in long distance space travel.
Hibernation as a cure
If humans underwent the extreme conditions of hibernation, severely limiting nutrient intake and movement, even for a shorter time than bears do, their bones would weaken. Even in far more hospitable conditions, many people suffer from osteoporosis, a disease that renders bones brittle and fragile. In situations where people have very limited mobility (e.g. physical disability or wheel-chair bound), disuse of bone may cause osteoporosis. In disuse osteoporosis a primary problem is something called skeletal loading. During skeletal loading bone formation slows but bone resorption of material does not, making the bone weaker and weaker as it tries to reabsorb bone materials that the body isn’t producing ( McGee-Lawrence et al. R1999). It is surprising to researchers who study hibernating bears that they don’t find disuse osteoporosis, leading some to believe that bears may provide a natural model for preventing it in humans.
In 2008, McGee-Lawrence et al. published an article stating that “… bears may have evolved more sophisticated physiological processes to recycle calcium, prevent hypercalcemia, and maintain bone integrity (R1999).” How exactly the bears prevent their bones from weakening despite such little movement is still an enigma, though the synthesis of parathyroid hormone (PTH) may be involved (McGee-Lawrence et al. R2009, Donahue et al. 1630).
Beyond maintaining bone density, researchers would also like to understand how bears prevent muscle atrophy while hibernating. In one study, after 110 days of hibernation, bears had lost 29% of strength (Lohuis et al. 257). This is surprisingly low when compared to human strength reduction during bed confinement, which has been measured at 54% weaker after 90 days (Lohuis et al. 257). In non-hibernating animals, being confined to a small space for months causes severe muscle atrophy, so how do bears conserve strength while hibernating? Lohuis et al. suggest that relying on fat for nourishment, sparing use of vital nutrients like nitrogen, and recycling urea all help bears maintain muscle mass (266). Also, isometric movements and shivering can prevent atrophy through direct muscle use (Lohuis et al. 266).
Hibernation as a quest
On the more bizarre end of hibernation studies is the hope that someday humans can hibernate, enabling travel to distant planets and stars. That idea has been on the table for decades and recently it got a little bit weirder. In 1960, as space travel was coming of age, humans were confronted with the possibility of exploring places far beyond our atmospheric bubble. Subsequently, learning how to hibernate became an attractive idea. Follow the logic of Hock in his 1960 article “The potential application of hibernation to space travel” in Aerospace Medicine:
- Though high speed space travel makes time appear to slow down (i.e. relativity), it does not actually slow biological processes. Therefore, for long, long distance space travel special arrangements will have to be made for astronauts that will be confined and limited in resources.
- Freezing doesn’t work. If a body is held at or below 0º C for 90 minutes it is not possible to recover if ice forms on tissues. If ice doesn’t form, still only 4 hours can be endured (Hock 485).
- So, let’s try to hibernate! “…if such a state may be achieved by man, many of the problems of concern to psychologists and physiologists anticipating the conditions of lengthy space travel will be alleviated or avoided (Hock 486).”
You may be thinking, okay, that was then, this is now. But, a quick search on Web of Knowledge will show that the idea of hibernating for space travel was scientifically approached as recently as 2006. Now it’s called “endogenous hypometabalism,” i.e. lowering metabolism via internal mechanisms. And in the 2006 article, “Human hibernation for space flight: uptopistic vision or realistic possibility?” the discussion takes a bit of a creepy turn as the author follows this logic:
- Fetuses act like an organ of the mother, with much lower metabolic rates than their size warrants until birthed (Singer 139).
- Fetuses adapt to the low level of oxygen in placenta or inside an egg shell. And they’re kept warm so they don’t have to expend energy to keep their body temperature regulated (Singer 140-141). This, he argues, is as close as humans ever get to hibernating.
- So, “..it does not seem fully unreasonable to assume that “hibernation-like” reduction in metabolic rate could be re-inducible in human adults, provided the resulting drop in body temperature would be limited by appropriate “fetal-like” ambient conditions (Singer 139).”
The thought of returning to fetal conditions really doesn’t appeal to me, but I do see the aim of the article: if we could do it once, could we do it again? As a soil scientist, I have absolutely no credibility in attempting to answer that question. What I do know is that, at this moment, many furry creatures are curled up, breathing slowly and barely metabolizing anything. Perhaps we have things to learn from our sleeping friends, or at least can be astonished at their ability to do something so curious and useful.
Donahue S. et al. 2006. Parathyroid hormone may maintain bone formation in hibernating black bears (Ursus americanus) to prevent disuse osteoporosis. The Journal of Experimental Biology. 209: 1630-1638.
Hock R. 1960. The potential application of hibernation to space travel. Aerospace Medicine Association. 31: 485-489.
Lohuis T. D. et al. 2007. Hibernating bears conserve muscle strength and in fatigue resistance. Physiological and Biochemical Zoology. 80 (3): 257-269.
McGee-Lawrence M. E. et al. 2008. Mammalian hibernation as a model of disuse osteoporosis: the effects of physical inactivity on bone metabolism, structure, and strength. American Journal of Physiology – Regulatory, Integrative and Comparative Physiology. 295: R1999-R2014.
Singer D. 2006. Human hibernation for space flight: uptopistic vision or realistic possibility? Journal of the British Interplanetary Society. 59 (3): 139-143.
Tøien Ø. et al. 2011. Hibernation in black bears: independence of metabolic suppression from body temperature. Science. 331: 906-909.