The Biology of Hibernation: Can Humans Hibernate?
The days are getting shorter and darker, and, if you’re anything like me, it’s making you want to curl up in a little ball and, well, hibernate. Animals have the right idea—as soon as winter hits they burrow into their caves and settle down to wait it out. So can we do that too? After all, we’re not that biologically different. Could we, at one point, have been able to hibernate as animals do—storing food and sleeping for several months at a time? Have we lost our ability to hibernate? This is what I’m setting out to answer—because I wouldn’t mind, myself, being able to sleep for a few months this winter.
The Science of It All
So what is hibernation? Why have some organisms adapted in order to be able to be dormant for weeks or even months at a time? And what actually happens to an organism’s body when it goes into such a state? “The term hibernation is often loosely used to denote any state of torpor, inactivity, or dormancy that an organism might exhibit. Properly speaking, however, use of the term should be confined solely to warm-blooded homoiotherms; i.e. birds and mammals….[who are] less dependent on many environmental restrictions, particularly those limitations imposed on organisms by ambient temperatures.” . An organism will enter a dormant state to survive environmental extremes—lack of food and water, very cold or very hot temperatures (arctic winters and desert summers), and changes in light. States of inactivity to survive extreme cold are referred to as hibernation, while those to survive extreme heat are referred to as “estivation.” Because humans are able to avoid such extremes, our bodies have not been required, over the years, to adapt to be able to hibernate…but does that mean we have lost the biological mechanisms necessary to achieve true hibernation? Did we ever have them? There are, after all, profound biological shifts that occur when an organism hibernates. There are actually different states of hibernation, and few animals that are able to experience “true” hibernation—hibernation as we generally think of it that is, surviving up to six months without food or water in an inactive state. I will deal here with “true” hibernation, and later touch upon the ways in which different organisms may enter into different states of inactivity for extended periods of time.
True hibernation is not only “characterized by profound reductions in metabolism, oxygen consumption and heart rate,” , but also by the ability for body temperatures to mimic the environment . In this hypometabolic state (a state of decreased metabolism), an organism’s body turns to lipids (fatty acids) rather than carbohydrates for the production of energy. In fact, an organism loses about 40% of its body weight during hibernation, 0.2-0.3 percent a day. The hibernation period itself does not consume much energy, but the waking period consumes a great deal, and this is when the most weight is lost . Organisms also have a decreased heart rate and blood pressure, all of which ensure that the body consumes as little energy as possible to prolong its energy stores. “The hibernator apparently is balanced on a very narrow line between the maintenance of life at a level that makes recovery from hibernation possible and a reduction of metabolism to a level that will lead to death” . It is imperative that organisms have enough fat stores or food stores close by to allow their bodies to basically internally consume themselves during hibernation.
Another characteristic of true hibernation is that of extremely low core body temperatures—often dipping below -2.9 degrees Celsius.  As the body of an organism cools, its metabolic rate decreases, which, in turn, reduces the need for oxygen, which is required to make Adenosine Triphosphate, a molecule used in metabolic processes. Dormant organisms also cease to be sexually active during true hibernation. In short, hibernation is an attempt to conserve energy in as many ways as possible in order to survive environmental extremes.
So who hibernates? Do humans hibernate? As I mentioned before there are only several animals that truly hibernate, but as it turns out, almost every organism enters a state or dormancy in some form or another—even single celled organisms!
Single-celled organisms, or Protozoans, form protective cysts around themselves in order to survive in hostile environments. This is what enables some viruses to spread so easily—the Protozoan that causes Amebic Dystentery, for example, forms a cyst that allows it to survive in water and be passed from one person to another. “Without encystment, which allows the organism to live in a dormant state in an unfavorable environment (e.g. water), amebic dysentery could be much more easily controlled. Protected by the cyst wall, the dormant contents of the cyst can survive for weeks” . Other protozoans “encyst” to protect themselves from lack of nutrients, unfavorable environments, or pollution . Invertebrates also form some type of protective cysts in unfavorable conditions. Snails, for example, cover their shell openings by “secret[ing] a membrane (the epiphragm) of mucus and slime…” , as do slugs, though they also bury themselves in the ground beforehand . Some freshwater sponges form gemmules, (essentially the asexually produced offspring of sponges) which are then covered in a protective coating of organic matter and spicules, skeletal structures that protect the organism from environmental hardships such as lack of oxygen and extreme temperatures. When the environment around the sponge becomes favorable again, these gemmules become adult sponges.
Insects experience what is called “Diapuse,” which is just another word for reduced metabolic activity. Because insects cannot develop without the hormone ecdysone, and ecdysone in turn cannot be produced in extreme temperatures or lack of light, they must essentially “hibernate” and wait for favorable conditions to return so that they may resume the metamorphic process. If they are fully grown adults, sexual reproduction ceases during these periods. During such periods, insects will find some sort of shelter and emerge when conditions are favorable.
Vertebrates are divided into two categories: warm-blooded (also called homoiotherms, or those whose body temperature is more or less stable), and cold-blooded (also called poikilothermous, or those whose body temperature corresponds with that of the environment). Both types experience some form of hibernation, though warm-blooded animals are the “true” hibernators. Cold-blooded vertebrates, such as fish, amphibians, and reptiles, generally “hibernate” by sheltering themselves from the elements. While they are incapable of lowering their body temperature to that of the environment, they do require less food and water. Most fish do not experience hibernation in the way other species do—after all, lack of water provides a larger problem for them than for most organisms. One type of fish, however, can survive without water for a time—the Lungfish. African lungfish (Protopterus), for example, during periods of drought, bury themselves in the mud and “use a lunglike air bladder” to breathe, rather than their gills. Much like mammals, they burn fat for energy (rather than oxidizing carbohydrates) “and in order to conserve water, they excrete urea rather than ammonia. This is because ammonia as an excretory product is highly toxic; animals that excrete ammonia require large quantities of water to dilute it below toxic levels. Urea…requires little or no water for its excretion” . So if even fish can hibernate, it would seem as though we, too, should be able to, in a way. But can we?
And now to the big guns—the mammals. While we generally think of bears as true hibernators, their dormancy periods resemble sleeping rather than hibernating. Their core temperatures do not approach those of their habitat, and they are also able to give birth to young, whereas, in true hibernating mammals, gonad activity ceases completely. “Perpetuation of the species requires that the animal be warm and active during the mating and pregnancy periods” . There are three types of mammals who exhibit “true” hibernation—the Insectivora (such as hedgehogs, shrews, and moles), the Chiroptera (bats), and the Rodentia (marmots, hamsters, dormice, hazel mice, and ground squirrels) . They prepare themselves by finding a suitable shelter, then storing food or gaining weight. “Generally, as the season advances and as the hibernator becomes progressively more prepared for hibernation, there is an increase of fat deposition and a general readjustment of body temperature, metabolism, and heart rate to lowered levels of activity” . Rather than oxidizing carbohydrates for energy, an organism turns to its lipids (fats), and burns them instead. This is due to the fact that lipids, when combusted, contain up to twice as much energy than carbohydrates . Arctic ground squirrels exhibit the most marked reductions in metabolism, temperature, and inactivity of any organism, and are used as the “measuring stick” for true hibernation. They find a protected space and curl up in the fetal position, then allowing their core temperature (which is initially about the same as that of a bear) to drop below freezing (0 degrees Celsius). They barely breathe, their internal organs (digestive tract and endocrine glands) effectively shutdown, and their bones and teeth deteriorate. If the animal is picked up or “uncurled” it will not immediately awaken, though “such handling will trigger wakening mechanisms,” causing it to eventually rouse from its dormant state . It can take up to two days for an animal to fully emerge from hibernation and its temperature to return to normal. They are, essentially, near death for months at a time, emerging every three weeks or so to renew energy stores and move around a bit.
Can Humans Hibernate?
Now what about humans? While we don’t have the need to hibernate for protection against the elements as animals do, did we once have the biological mechanisms to regulate our metabolic activity and temperature for long periods of time? Do we still have those mechanisms—and just not use them? “What was before a blind reflexive response developed by certain organisms for survival in natural habitats reemerges in humans as the product of a rational, everyday waking consciousness…Now instead of being merely reactive to environmental variables, such as temperature change or lack of food, human beings must be trained to reenter this conservative and restorative state” . The closest most humans come to hibernation these days seems be through meditation, sleep, and starvation. All three states are characterized by many of the same things as hibernation—decreased metabolic activity, decreased oxygen consumption, muscle relaxation, and decreased hormone production. While body temperatures drop in all four states, meditation comes closest to hibernation—when the Yogi Satyamurti meditated in an underground pit for eight days, his heartbeat decreased to such a level that it barely registered, and his body temperature fell to 34.8 degrees Celsius, matching his surroundings . A fundamental difference, however, between hibernation and meditation is that hibernating animals are not conscious during dormancy, while humans have demonstrated alpha-theta brain waves during meditation, which are those that are also most closely related to being awake. “In wakeful alertness, one’s state of consciousness is characterized as empty of any particular content but nevertheless active and alert above the threshold of awareness” . Animals are also, unlike humans, able to control the rate at which their bodies use lipids rather than carbohydrates for energy .
It would appear that while humans have some of the tools for hibernation, we have not evolved them to be used for long periods of time because we have not been required to. Perhaps if we were at the mercy of the elements, we would have evolved to be able to hibernate, to be able to lower our body temperatures and severely reduce the amount of energy we use and where such energy comes from. It seems, however, unlikely that we will ever have the need to hibernate in the future, considering our wealth of indoor heating systems and warehouses full of winter coats. For now we’ll have to settle for bundling up, hunkering down, and waiting out the winter fully conscious, and, sadly, awake.
Sources & Further Information
1. Advances in Molecular Biology of Hibernation in Mammals, Matthew T. Andrews http://www.ncbi.nlm.nih.gov/sites/entrez?Db=pubmed&Cmd=ShowDetailView&TermToSearch=17450592&ordinalpos=2&itool=EntrezSystem2.PEntrez.Pubmed.Pubmed_ResultsPanel.Pubmed_RVDocSum
2. “Dormancy.” Encyclopedia Brittanica. 2007. Encyclopedia Britannica Online. 10 2007. http://www.britannica.com/eb/article-48521.
3. Meditation as a Voluntary Hypometabolic State of Biological Estivation, by John Ding-E Young and Eugene Taylor http://physiologyonline.physiology.org/cgi/content/full/13/3/149
4. Focus On: “Coordinate Expression of the PDK4 gene: a means of regulating fuel selection in a hibernating mammal.” http://physiolgenomics.physiology.org/cgi/reprint/8/1/3.pdf
5. Mammalian Hibernation, University of Calgary http://www.ucalgary.ca/~kmuldrew/cryo_course/cryo_chap12_1.html
6. Overview of Lipid Function,http://www.elmhurst.edu/~chm/vchembook/620fattyacid.html
7. A Simple Molecular Mathematical Model of Mammalian Hibernation, Marshall Hampton & Matthew T. Andrews, http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6WMD-4N85BF0-5&_user=400777&_coverDate=07%2F21%2F2007&_alid=646055634&_rdoc=23&_fmt=full&_orig=search&_cdi=6932&_sort=d&_docanchor=&view=c&_ct=1301&_acct=C000018819&_version=1&_urlVersion=0&_userid=400777&md5=c69ef9479b5db9bc7164db18b28c6165
While I found this article too late to incorporate it into my paper, it is still fascinating. It deals with the idea of “suspended animation” or “inducing hibernation” in humans by attempting to reduce metabolism in various ways. A great read.