Imagine going to sleep for months at a time and then waking up just in time to land on Mars. Hibernation is second nature to bears and other mammals...but there's no reason humans can't do it too.
Exactly how hibernation works and why only certain animals do it remain great mysteries for biologists, but the last few years have seen some huge strides in our understanding of this phenomenon. One fact has become increasingly clear: natural human hibernation isn't just a possibility, it's a reality. We just need to figure out how to trigger hibernation states when and where we want to.
What exactly is hibernation?
Hibernation is just the best known of a bunch of different metabolic processes that allow organisms to enter a dormant state. What sets hibernation apart is the precise changes that go on in the animal's body during its dormancy period. Animals use the late summer and autumn months to build up lots of surplus body fat, which it can then slowly burn off during the winter.
During the hibernation period, animals drop their heart rate to as little as 5% of normal. This in turn causes their body temperature to also substantially decrease. Internal temperatures in the hibernating ground squirrels can drop to just below freezing, at 32 degrees Fahrenheit. Animals spend days or weeks slowly burning off their stored fat, effectively suspending all other biological processes until spring arrives, although animals do occasionally wake up for brief periods during their hibernation cycle.
A crucial concept here is torpor, which is also known as temporary hibernation. It features a lot of the same basic physiological processes as hibernation - reduced metabolism followed by a drop in body temperature and oxygen consumption - but animals switch in and out of torpor on a daily cycle, making it essentially a more extreme version of the sleep cycle. There's some thought that torpor and hibernation exist on a spectrum with each other, but there's still too much that we don't know for us to say for sure.
Which animals hibernate?
As a general rule, mammals are the ones who hibernate, while other types of animals use different ways to survive extreme conditions. Reptiles, for instance, undergo a similar process known as brumation, where they alter their metabolic processes so that they can survive on just water and without any food for as much as eight months of the year. The opposite of these processes is probably aestivation, which is where certain organisms - typically invertebrates - become dormant to avoid hot summer conditions.
That said, there are plenty of exceptions to these basic categories. One of the most important recent discoveries regarding animal hibernation concerns the fat-tailed dwarf lemur, a primate species found on the island of Madagascar. First described in 2004 by researchers at Phillips University, this is the first known primate and the first tropical animal to have a hibernation cycle. Writing that year for National Geographic, James Owen describes the little primate's unusual approach to hibernation:
The fat-tail dwarf lemur gorges on fruit, flowers, and insects during a short rainy season between December and February. As its body weight increases by around 40 percent, the lemur's tail swells massively with stored fat reserves. The animal, which usually weighs five ounces (150 grams), then goes into a torpor, marked by reduced activity and appetite...The "fat-tail" was found to take to holes in tree trunks during prolonged periods of drought, when trees shed their leaves and food becomes scarce...The team revealed that the fat-tail's body temperature varied to an extent previously unknown in mammals. Daily fluctuations ranged almost 20 degrees Celsius (nearly 40 degrees Fahrenheit), with temperatures recorded from as low as 9.3 degrees Celsius (48.7 degrees Fahrenheit) to well over 30 degrees Celsius (86 degrees Fahrenheit).
The fat-tailed dwarf lemur is just one of a long list of hibernating mammals, including rodents, bats, hedgehogs, anteaters, echidnas, and various marsupials. The only known bird to hibernate is the North American Common Poorwill - many other birds go through torpor states, but this is the only one that actually hibernates for extended periods. That's pretty much the complete list of hibernating animals, although I can't help but think I'm forgetting something...
What about bears?
Oh, right, of course. It's a point of contention in the scientific community whether the world's most famous hibernators actually hibernate. While they certainly go to sleep for extended periods during the winter, bears don't actually see much of a drop in their body temperature - a typical decrease is from 99 degrees Fahrenheit to 88, which is nothing compared to other hibernating mammals. Their metabolic processes also remain relatively close to normal. As such, experts are split over whether this technically even counts as hibernation...or is in fact a particularly sophisticated advanced form of the phenomenon that gets the same basic results as the more extreme version utilized by smaller animals.
For their part, the good folks at the North American Bear Center offer an awesomely straightforward overview of why that debate is silly:
When people defined hibernation simply in terms of temperature reduction, bears were not considered hibernators. However, when biologists discovered the many metabolic changes that let black and grizzly bears hibernate up to 7 ½ months without eating, drinking, urinating, or defecating, they realized that body temperature was only a small part of hibernation. They redefined mammalian hibernation as a specialized, seasonal reduction in metabolism concurrent with scarce food and cold weather...People have called black and grizzly bear hibernation torpor, winter sleep, dormancy, and carnivorean lethargy. The leading physiologists now simply call it hibernation.
The big takeaway here is that what constitutes hibernation really is a matter of definition, and that's important as we consider whether humans can hibernate. If you stick to a strict definition built around the massive body temperature reduction seen in small animals, then hibernation is definitely beyond the abilities of humans...but you'd have to exclude bears as well, and that seems a bit ridiculous. But more on that in a moment. First, let's look back at the origins of hibernation.
How did hibernation evolve?
The origins of hibernation are probably tied up in the origins of warm-bloodedness, or endothermy - of course, no one's really sure how that evolved either. Zoologist Fritz Geiser of Australia's University of New England tackled the subject back in 1998, he found that the inability to change core temperature and hibernate - known as homeothermy - is actually a more recent innovation than being able to enter torpor. That's backed up by the fact that mammals with a more ancient evolutionary history tend to be able to hibernate, while more recent species can't.
Exactly why some species evolved their current proclivity towards hibernation and other species didn't is still a matter of debate. One possible explanation is that it's determined by body size, as most mammals and birds that either hibernate or enter torpor on a regular basis are small animals with varied diets, and so the argument goes that these animals needed to take extreme measures to survive wintry conditions and sharply reduced food supplies.
There's of course also a genetic component to hibernation, but this also remains difficult to understand. The genes of hibernating animals are expressed differently when they're dormant as opposed to when they're awake. For instance, a gene called PDK-4 becomes more active during squirrel hibernation, controlling their cardiac function and allowing them to sleep for extended periods. Most of these "hibernation genes" are highly conserved in species that don't hibernate - humans also possess the PDK-4 gene, for instance. Taken together, the evolutionary profiles of hibernation suggests that we humans may well still carry the genetic mechanisms needed to hibernate.
Can humans hibernate?
The answer, remarkable as it might seem, is an unequivocal "yes." In fact, until relatively recently the idea of humans sleeping through most of the winter wasn't even seen as uncommon. There are stories of peasants effectively hibernating as late as the 19th century in both frigid Siberia and the comparatively temperate French countryside. From what we can tell, this wasn't strictly hibernation - the peasants' core temperatures didn't drop, and they still woke up once a day or so to eat a small biscuit before going back to bed. Still, they changed their lifestyles to use just a fraction of their normal energy requirements, which is essentially what hibernation is.
And there are even more dramatic individual examples. Consider the case of the then 35-year-old Japanese man Mitsuka Uchikoshi, who in 2006 spent over three weeks unconscious on the freezing Rokko Mountain before making a full recovery. Writing for Discover in 2007, Alex Stone recounts the remarkable story:
He had no detectable pulse or respiration, and his body temperature was 71 degrees Fahrenheit, 27 hatch marks shy of normal. While returning alone from a party on the mountain, Uchikoshi had stumbled and hit his head; he spent the next 24 days sprawled unconscious in the frigid air, without food or water. But when he arrived at Kobe City General Hospital, something remarkable occurred: He woke up. To the astonishment of the doctors who treated him for severe hypothermia and blood loss, Uchikoshi made a full recovery without a trace of brain damage.
Assuming the story isn't some elaborate hoax - and there's been no suggestion so far that it is - this is the most dramatic evidence yet that humans are capable of hibernating in exactly the same way that smaller mammals can. Of course, we lack the natural ability to go into hibernation, and it although Uchikoshi recovered from his ordeal, it's hardly the same thing as bears who wake up exactly the same as when they started sleeping, as he had to be treated for multiple organ failure and severe blood loss. Still, if the mechanism for human hibernation exists at all, then there's the chance that we can make it work for us.
But how can humans hibernate?
As a species, we lack the very basic knowledge of how to hibernate that is second nature to other animals, but it seems that humans both pursue behaviors that mimic actual hibernation and, under the most extreme of circumstances, can actually enter a hibernation state. Exactly why Uchikoshi and other examples of human hibernation were able to enter this state is not even remotely understood. As suggested earlier, the genetic mechanisms of hibernation may be very ancient, and it's entirely possible that we still carry the basic ability to hibernate in our DNA. But what activates this otherwise forgotten ability?
We reported on one possibility last year that was advanced by Dr. Mark Roth, a researcher at Seattle's Fred Hutchinson Cancer Research Center. He compared these human hibernation cases to the worms and nematodes in his lab. These organisms perish when frozen, but can be revived without any ill effects if they are deprived of oxygen before freezing. Roth explained:
"There are many examples in the scientific literature of humans who appear frozen to death. They have no heartbeat and are clinically dead. But they can be reanimated. We wondered if what was happening with the organisms in my laboratory was also happening in people like the toddler and the Japanese mountain climber. Before they got cold did they somehow manage to decrease their oxygen consumption? Is that what protected them? Our work in nematodes and yeast suggests that this may be the case, and it may bring us a step closer to understanding what happens to people who appear to freeze to death but can be reanimated."
For now, the oxygen deprivation hypothesis is probably the best explanation we've got as to how people can enter stasis for so long with no permanent damage (though admittedly plenty of problems in the short-term). We're still only working from a few cases here, but the fact that anyone can survive in what is essentially stasis for over three weeks suggests humans are capable of some pretty extraordinary feats, and something unexpected must serve as the trigger for this otherwise unknown ability. Oxygen deprivation is as good an explanation as any I can see, though obviously that's far from conclusive.
Could hibernation be the best way to achieve suspended animation?
There's a lot of reasons why we might want to be able to hibernate. The most obvious, and the one seen most often in science fiction, is to be able to endure long journeys in space to other planets, even other star systems. Medically, being able to put humans in stasis for extended periods and halt any oncoming diseases could save countless lives. Basically, hibernation means a naturally occurring alternative to external methods of suspended animation, like the much mooted but still mostly hypothetical field of cryonics.
We're a long way from knowing exactly how humans could hibernate in the first place, let alone how to make them do it artificially, but we're making serious progress elsewhere in the animal kingdom. Earlier this year, researchers at the University of Alaska Fairbanks discovered how to make Arctic ground squirrels start hibernating and wake up at will. They seized upon the fact that the molecule adenosine slows down neural activity in the brain, as researcher Kelly Drew explains:
We devised an experiment in which non-hibernating arctic ground squirrels were given a substance that stimulated adenosine receptors in their brains. We expected the substance to induce hibernation. We also gave a substance similar to caffeine to arouse hibernating ground squirrels."
The researchers generally had no problem controlling the squirrels' hibernation state, though only about a third of squirrels could be returned to torpor after being woken up at the beginning of the hibernation season. There appears to be a seasonal component to when animals are most able to hibernate, but the exact nature of that relationship is not yet well understood.
What do bears have to teach us about hibernation?
It's probably not surprising that bears are the animals researchers are most interested in when it comes to human hibernation - after all, they're the hibernators most similar to us, at least in terms of body size (though those dwarf fat-tailed lemurs are the most closely related hibernators, since they're also primates). Back in February, we covered a study by researchers, also at the University of Alaska Fairbanks, who looked to black bears to see what lessons could be learned about human hibernation. Professor Brian Barnes, the director of the Institute of Arctic Biology, explained all the amazing things hibernating bears can do:
"When black bears emerge from hibernation, it has been shown that they have not suffered the losses in muscle and bone mass that would be expected to occur in humans. If we could discover the genetic and molecular basis for this protection, and for the mechanisms that underlie the reduction in metabolic demand, we could derive new therapies and medicines to prevent osteoporosis, disuse atrophy of muscle, or even place injured people in a type of suspended or reduced animation until they can be delivered to advanced medical care - extending the golden hour to a golden day or a golden week."
The idea that humans could go to sleep for weeks at a time and awake without any health problems or loss in bone or muscle mass is an amazing, seemingly impossible thought - but if bears can do it, there's very little reason to think that we couldn't do it to. Recent years have provided us with both cutting-edge new research and dramatic real-life examples of the hidden human ability to hibernate. With any luck, the next few years will see us start to work out and unlock the exact mechanisms that control our dormant ability for...well, dormancy.
"What will suspended animation really be like? A new study offers hints."
Suspended Animation is no longer a pipe dream
The Curious Case of Human Hibernation
"Hibernation Technique Might Work on Humans" by Robert Roy Britt
"Evolution of Daily Torpor and Hibernation in Birds and Mammals: Importance of Body Size" by Fritz Geiser
"The Evolution of Endothermy and Its Diversity in Mammals and Birds" by Gordon C. Grigg, Lyn A. Beard, and Michael L. Augee
"Season Primes the Brain in an Arctic Hibernator to Facilitate Entrance into Torpor Mediated by Adenosine A1 Receptors" by T. R. Jinka, O. Toien, and K. L. Drew.
"Suspended Animation Extends Survival Limits of Caenorhabditis elegans and Saccharomyces cerevisiae at Low Temperature" by Kin Chan, Jesse P. Goldmark, and Mark Roth
Top image from Alien.
Fat-tailed dwarf lemur by Petra Lahann on Wikimedia
Hibernating bat by Magne Flåten on Wikimedia
Hibernating bear via the North American Bear Center
Sleeping peasants by Gerard Dubois
Arctic ground squirrel by Ianaré Sévi on Wikimedia