The term hibernation comes from the Latin word hibernare, which means to pass the winter. The term is commonly used for a type of deep winter dormancy in some animals (i.e., mammals, birds, reptiles, and amphibians). A similar type of dormancy that occurs in summer is referred to as estivation. Some insects and snails also exhibit a similar state of dormancy in winter that is often called winter estivation or diapause.
Hibernation (winter dormancy) involves a periodic (seasonal) drastic reduction of the animal’s metabolic rate and body temperature for an extended period of time. Generally, in hibernation, the body temperature drops almost to that of the surroundings (the bear being a well-known exception). Basic body processes, like breathing and the rate at which the heart beats, are drastically reduced so that the hibernator appears almost to be in a comatose state. The hibernating animal then lives through the winter on a reserve of body fat and/or externally stored food until it awakens in the spring.
Hibernation is therefore considered to be a fundamental adaptation to the environment since it is essentially an adjustment in the animal to allow its survival during those periods of the year when environmental conditions are so severe that exposure to the elements will be fatal. To survive in these harsh conditions, some animals can temporarily migrate from the hostile environment (e.g., birds). Some animals can remain active but adapt by
- growing thicker fur (e.g., weasels and snowshoe rabbits),
- storing extra food for the coming winter months (e.g., beavers, some squirrels and mice),
- changing their diet to that which is available during the winter (e.g., the red fox normally eats fruit and insects but eats small rodents in the winter),
- using shelters to protect them from the cold.
For others (e.g., reptiles, ground squirrels, and certain bears), migration is impossible and other adaptations are either impossible and/or ineffective as protection. These animals then adapt by means of hibernation.
Hibernation is seen mainly in small animals such as marmots, ground squirrels, chipmunks, dormice, and northern bats whose food supply is very limited or nonexistent in winter. Coldblooded hibernators include such amphibians as frogs and toads and such reptiles as lizards and snakes that pass the winter with body temperatures that are the same as the ambient temperature (i.e., near freezing). More recently, a tropical primate, the fat-tailed dwarf lemur (Cheirogaleus medius), endemic to Madagascar, has also been found to hibernate.
Outside of winter, natural hibernators lacking food and facing other adverse conditions cannot choose to hibernate to survive. Indeed, unless an animal is genetically disposed to hibernation or other states of dormancy, it cannot become dormant at will to escape inhospitable conditions.
Some refer to hibernation as “time migration” since hibernation allows the animals to skip over the entire period of the inhospitable winter season and truly “live” only in the periods of plentiful food and higher temperatures that occur outside of winter. Indeed, to the hibernating animal, time passes unnoticed since it is insensible and unaware during winter and becomes active or “alive” only at the end of hibernation.
Interestingly, according to an American legend, the hibernating groundhog (woodchuck) can be used to predict a period of time, namely, the length of the winter season. It is said that the groundhog creeps out of its hole on February 2 of each year (Groundhog Day) and that if the day happens to be sunny and the groundhog sees its shadow, there will be 6 more weeks of winter. On seeing its shadow, the groundhog supposedly returns to hibernation again. If, on the other hand, the groundhog does not see its shadow, it stays above ground, cutting short its hibernation. This is supposed to indicate the imminent arrival of spring, or the end of winter. Of related interest is a 1993 comedic movie by the same name, Groundhog Day. It describes a television reporter (a weatherman) who, while waiting to report on the arrival of the hibernating groundhog, becomes “stuck in time” on Groundhog Day. In the movie, time stands still since the events of that day are repeated without end, making it impossible to move on to the next day!
Hibernation is very different from sleep since it involves an extreme reduction in metabolism. For example, a black bear’s heart normally beats about 60 times per minute but this is reduced to as little as 5 times per minute during hibernation (about a 90% reduction). The blood pressure of an active mouse varies between about 80 and 120 millimeters of mercury (mm Hg). By contrast, blood pressure in a hibernating mouse varies between about 30 and 50 mm Hg. A hibernating marmot may reduce its breathing from 16 to 2 breaths a minute and its heartbeats from 88 to 15 per minute. Hibernating turtles reduce their metabolic rate by as much as 95%.
Terms like adaptive hypothermia, or the lowering of body temperature to adapt to changes in the external environment; torpor (state of dormancy and inactivity); and suspended animation (temporary suspension of vital functions) are often associated with hibernation.
To hibernate, most animals go into shelters such as burrows, caves, and dens as a buffer from the lethally low ambient temperatures. The temperature of these shelters is normally slightly above the ambient temperature and does not fluctuate as much. This shelter is called a hibernaculum. There is no standard type of hibernaculum. For example, most rodents hibernate in underground burrows. Bog turtles (Clemmys muhlenbergii) use hibernacula that include abandoned animal burrows, mud cavities, and the base of tree stumps. Tortoises use underground sites such as burrows excavated in soil or rocky caves. Bats choose a hibernaculum that is not only cold but humid. For example, in winter, the Indiana bat (Myotis soDalíis) hibernates in limestone caves and also in some manmade shelters, like underground mines. Adders (Vipera berus) in cold areas hibernate in clusters in underground areas made by other animals, for example, in abandoned mammal or tortoise burrows. Animals that are not in the wild will even use crawl spaces and basements in buildings.
Most hibernacula must remain at temperatures slightly above freezing (not too cold or too warm), must have a relatively high humidity, must be safe from predators and other intruders, and in some cases must have space for the animal to store food. Indeed, hibernacula are usually dark, protected, and secluded to keep the hibernating animal safe. A suitable hibernaculum is key to the survival of the animal when it is so vulnerable during hibernation.
Length of Hibernation
The period of hibernation may vary, lasting several days or weeks depending on the animal’s species, the ambient temperature, length of winter, and the animal’s latitudinal location. Woodchucks hibernate in underground burrows from September or October until March (about 5 to 6 months). The fat-tailed dwarf lemur hibernates for around 7 months of each year in tropical winter temperatures that are relatively high.
Typically, whatever the exact length of hibernation, it is generally characterized by periods of sporadic arousals. The intervals between these arousals depend on the animal’s size, body temperature, and other internal factors. For example, in one study, hibernating hedgehogs were observed to have 12-18 arousals. The average duration of these arousals was 34 to 44 hours. Some arousals are initiated by external stimuli like noise and are called alarm arousals. Other arousals do not appear to be initiated by any external trigger and seem to be under the control of signals internal to the animal (endogenous signals). Final arousal from hibernation always occurs at the end of winter, and it is believed that this may be controlled by a combination of exogenous (environmental) and endogenous signals that work to prevent the animal from reentering hibernation.
What Initiates/Triggers Hibernation
Entry to hibernation may be triggered by environmental factors such as temperature, day length, and shortage of food. Indeed, animals such as hamsters and chipmunks are sometimes called facultative hibernators because they hibernate in response to environmental conditions.
For obligate hibernators (e.g., ground squirrels, marmots, and white-tailed prairie dogs), on the other hand, the cycle of storing food, hibernating, and arousing seems to be controlled mainly by a signal or cue that originates internally from the animal itself (an endogenous signal). For example, when the golden-mantled ground squirrel (Spermophilus lateralis) is kept in the laboratory under constant environmental conditions (i.e., in the absence of environmental triggers), hibernation occurs just as if it were in its natural habitat. This indicates that environmental triggers (exogenous cues) are not essential for hibernation in this animal and so the golden- mantled ground squirrel can be considered to be an obligate hibernator.
On the other hand, some birds, like the common whippoorwill, enter a hibernation-like state once their food is removed (environmental trigger). Similarly, food availability has been shown to play a major role in the start of hibernation in the Japanese dormouse (Glirulus japonicus). At the same time, another environmental trigger, low ambient temperature, does not appear to have significant influence on the hibernation of the Japanese dormouse once there is an adequate supply of food. Hibernation in the desert tortoise (Gophents agassizii) also appears to be only weakly influenced by exogenous environmental cues.
Thus, for some animals some type of endogenous signal or cue is the main determinant of entry to and exit from hibernation. It is believed that this internal signal is regulated by a biological rhythm based on a 24-hour cycle (called a circadian rhythm). The onset and end of hibernation in most animals appear to be triggered by both endogenous signals and exogenous environmental cues.
The endogenous cue that triggers hibernation is believed to be based on an internal or biological clock. The biological clock is believed to control not only the hibernation of animals but also many repetitive physiological functions in humans, as well as the migration of birds and the flowering of plants. It is also believed that the periodic arousals characteristic of some hibernators are regulated in part by the biological clock. Also, final arousal from hibernation in spring may be triggered partly by the biological clock.
Although it is endogenous, it is believed that the biological clock responds to several external environmental cues, in particular day length or more accurately the photoperiod (the ratio of daylight to darkness). The photoperiod is an exogenous environmental cue that is “sensed” by the animal from certain sensory organs, like the eyes and also from light-sensitive receptors in its brain. The location of the biological clock in animals, its properties, and the mechanisms by which it operates are only now beginning to be discovered. Indeed, the study of biological clocks and their associated rhythms has led to the scientific discipline called chronobiology.
Thus, the timing of the entry into and exit from hibernation may be due primarily to an endogenous circannual (about a year) or circadian (about a day) rhythm with the timing being reset annually by environmental factors such as day length.
Effects of Global Warming/Climate Change
There has been some concern that higher global temperatures may have led to shortened winters that could have a negative effect on animals that hibernate. For example, some research done in Italy suggests that climate change is indeed bringing animals out of hibernation early. This is believed to put their feeding and breeding habits out of synchronization with the environment, causing unusual weight loss and stress in the animal. For instance, one study observed the effect of annual temperatures in southern England (between 1983 and 2005) on the body condition of female common toads (Bufo bufo). This study suggested that “partial” hibernation (early emergence from hibernation) occurred in the female toad as a result of the mild winters experienced during those years. It is thought that this contributed to a decline in the bodily condition of the toad, which negatively affected its survival.
Another study of the effect of temperature change on several traits of over 1,000 temperate- zone animal species worldwide revealed that springtime events (e.g., blooming of flowers, laying of eggs, and the end of hibernation) now occur about 5.1 days earlier per decade on average. Some studies on marmots in Colorado also show that they are ending their hibernations about 3 weeks earlier than they used to in the late 1970s. It has been reported that dormice now hibernate 5-1/2 weeks less on average than they did 20 years ago. Reports indicate that some bears in Spain (slightly different genetically from bears in other parts of the world) have stopped hibernating altogether, remaining active throughout recent winters. Research and debate continues on climate change and its effects on animals’ biology and behavior, including hibernation.
Artificially Induced Hibernation
There has always been interest among medical researchers in the chemical induction of hibernation. The potential benefit of putting trauma patients into hibernation and giving doctors more time to repair severely damaged tissue is just one advantage of artificially induced hibernation in humans. Science fiction writers have also explored the possibility of preserving human life in a reversible state of so-called suspended animation. Supposedly, one could exist in such a state for several years without aging, without the need for food and the elimination of waste, and reawaken unaffected by a journey through space of many years to another planet. Inducing artificial hibernation in humans (even in fiction) is therefore potentially a way of “buying time.”
Hibernation cannot be induced in nonnatural hibernators by manipulating the environment, so researchers have searched for a possible bloodborne chemical that could induce hibernation. It has been discovered that a serum extracted from hibernating animals such as the woodchuck, when injected into active animals, can induce a hibernation-like state. This serum contains a substance called the Hibernation Induction Trigger (HIT). Experiments with hydrogen sulfide have also shown potential for inducing a hibernation-like state in nonhibernating animals.
Other Types of Hibernation
The term hibernation is now used widely outside of the life sciences to refer to any period or state of temporary dormancy or inactivity. For example, it is used in computer technology to refer to the state of dormancy that can be achieved by powering down a computer while retaining data about running programs and the status of the input/output devices. When the computer resumes from the state of hibernation (on powering up), it reads the saved state data and restores the system to its previous state.
Companies have been said to go into hibernation as a survival strategy to avoid closure (analogous to animal hibernation, the business operations might be reduced drastically to maintenance levels until, for example, brighter market conditions emerge). Projects that are temporarily dormant have sometimes been said to be placed “on ice” and “in hibernation.” The term hibernation is also sometimes used for any temporary loss in function as seen, for example, in organs in the human body.
See also Clocks, Biological; Cryonics; Ecology; Global
Warming; Heartbeat; Seasons, Change of; Sleep;
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