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Hibernation is a state of minimal activity and metabolic depression. Hibernation is seasonal heterothermy, typically occurring during the winter months. It functions to conserve energy when sufficient food is unavailable, which is accomplished by decreasing metabolic rate, lowering body-temperature, slower breathing and heart-rate. Hibernation may last days, weeks, or months—depending on the species, ambient temperature, time of year, and the individual's body-condition. Before entering hibernation, animals need to store enough energy to last through the duration of their dormant period, possibly as long as an entire winter.

To achieve this energy saving process, ectothermic animals also undergo periods of metabolic suppression and dormancy, which in many invertebrates is referred to as diapause. An endothermic animal decreases its metabolic rate and thereby its body temperature.

Hibernation traditionally was reserved for "deep" hibernators such as rodents, but it has now been redefined to include animals such as bears. It is now applied based on active metabolic suppression rather than any absolute decline in body temperature. Many experts believe that the processes of daily torpor and hibernation form a continuum and utilize similar mechanisms. The equivalent of hibernation during the summer months is aestivation.

Hibernation induction trigger (HIT) is a chemical substance found in the blood of hibernating animals, and functions similar to opiates. Although, there are arguments against it contributing to inducing hibernation.

Mammals
There is a variety of definitions for terms that describe hibernation in mammals, and different mammal clades hibernate differently. The following subsections discuss the terms obligate and facultative hibernation. The other sections point out primates, none of whom were thought to hibernate until recently, bears whose winter torpor had been contested as not being "true hibernation" during the late 20th century since it is dissimilar from hibernation seen in rodents, bats who have a preferred hibernaculum, and hibernation research being conducted on humans.

Obligate hibernation
Obligate hibernators are animals that spontaneously, and annually, enter hibernation regardless of ambient temperature and access to food.

The typical winter season for obligate hibernators is characterized by periods of torpor interrupted by periodic, euthermic arousals, during which body temperatures and heart rates are restored to more typical levels. The cause and purpose of these arousals is still not clear; the question of why hibernators may return periodically to normal body temperatures has plagued researchers for decades, and while there is still no clear-cut explanation, there are multiple hypotheses on the topic.

One favored hypothesis is that hibernators build a "sleep debt" during hibernation, and so must occasionally warm up to sleep. This has been supported by evidence in the Arctic ground squirrel. Hibernating Arctic ground squirrels may exhibit abdominal temperatures as low as −2.9 °C (26.8 °F), maintaining sub-zero abdominal temperatures for more than three weeks at a time, although the temperatures at the head and neck remain at 0 °C (32 °F) or above.

Other theories postulate that brief periods of high body temperature during hibernation allow the animal to restore its available energy sources, or to initiate an immune response.

Obligate hibernators include many species of ground squirrels, other rodents, mouse lemurs, European hedgehogs and other insectivores, monotremes, and marsupials. These species undergo what has been traditionally called "hibernation": a physiological state wherein the body temperature drops to near ambient temperature, and heart and respiration rates slow drastically.

Facultative hibernation
Facultative hibernators enter hibernation only when either cold-stressed, food-deprived, or both, unlike obligate hibernators, who enter hibernation based on seasonal timing cues rather than as a response to stressors from the environment.

A good example of the differences between these two types of hibernation can be seen in prairie dogs:

·       The white-tailed prairie dog is an obligate hibernator.

·       The closely related black-tailed prairie dog is a facultative hibernator.

Primates
While hibernation has long been studied in rodents (namely ground squirrels) no primate or tropical mammal was known to hibernate until the discovery of hibernation in the fat-tailed dwarf lemur of Madagascar, which hibernates in tree holes for seven months of the year. Malagasy winter temperatures sometimes rise to over 30 °C (86 °F), so hibernation is not exclusively an adaptation to low ambient temperatures.

The hibernation of this lemur is strongly dependent on the thermal behaviour of its tree hole: If the hole is poorly insulated, the lemur's body temperature fluctuates widely, passively following the ambient temperature; if well insulated, the body temperature stays fairly constant and the animal undergoes regular spells of arousal. Dausmann found that hypometabolism in hibernating animals is not necessarily coupled with low body temperature.

Bears
Historically there was a question of whether or not bears truly hibernate since they experience only a modest decline in body temperature (3–5 °C) compared with the much larger decreases (often 32 °C or more) seen in other hibernators. Many researchers thought that their deep sleep was not comparable with true, deep hibernation, but research in 2011 refuted this theory in captive black bears and again in 2016 in a study on brown bears.

Hibernating bears are able to recycle their proteins and urine, allowing them to stop urinating for months and to avoid muscle atrophy. They stay hydrated with the metabolic fat that is produced in sufficient quantities that satisfy the water needs of the bear. They also do not eat or drink while hibernating and live off of their fat storage.

Female black bears go into hibernation during the cold winter months in order to give birth to their offspring. The pregnant mothers significantly increase their body mass prior to hibernation, and this increase is further reflected in the weight of the offspring. The fat accumulation enables them to provide a sufficiently warm and nurturing environment for their newborns. During hibernation, they subsequently lose 15–27% of their pre-hibernation weight by using their stored fats for energy.

Despite long-term inactivity and lack of food intake, hibernating bears are believed to maintain their bone mass and do not suffer from osteoporosis. They also increase the availability of certain essential amino acids in the muscle, as well as regulate the transcription of a suite of genes that limit muscle wasting. In a study done by G. Edgar Folk, Jill M. Hunt and Mary A. Folk, they compared EKG of typical hibernators to three different bear species with respect to season, activity and dormancy, and found that the reduced relaxation (QT) interval of small hibernators was the same for the three bear species. They also found the QT interval changed for both typical hibernators and the bears from summer to winter. This 1977 study was one of the first indications used to show that bears are hibernators.

In a 2016 study done by wildlife veterinarian and associate professor at Inland Norway University of Applied Sciences, Alina L. Evans researched 14 brown bears over three winters. Their movement, heart rate (HR), heart rate variability (HRV), body temperature (Tb), physical activity, ambient temperature (TA), and snow depth were measured to identify the drivers of the start and end of hibernation for bears. This study built the first chronology of both ecological and physiological events from before the start to the end of hibernation in the field. This research found that bears would enter their den when snow arrived, and the TA dropped to 0 °C but even up to several weeks before they entered their den, Activity, HR, and Tb started to drop slowly. Their HRV dropped dramatically once in their den which indirectly suggests metabolic suppression being related to the bears' hibernation. Toward the end of hibernation and two months before arousal, the bears' Tb rise, unrelated to HRV but rather driven by the TA. The HRV only increases around three weeks before arousal and the bears only leave their den once outside temperatures are at their lower critical temperature. These findings suggest that bears are thermoconforming and bear hibernation is driven by environmental cues, but arousal is driven by physiological cues.

Bats
Bats exhibit heterothermy, they have a complex physiology that is dependent on climate which requires them to find suitable hibernaculum to hibernate over winter. In 2008, a study by Yrjo Siivonen and Terhi Wermundsen was published where they researched 5 bat species in Finland for 2 winter seasons from 2002-2003 and 2005-2006. They measured hibernaculum locations, microclimate conditions, formation of clusters, bat temperature and their relative humidity. Their research usually found all 5 species hibernating within the same hibernaculum, however cluster sizes were relatively small. The findings indicated each species of bat except for one, were typically found at natural stone hibernacula rather than man-made hibernacula. These findings suggest natural stone is a preferred hibernaculum that the bats actively seek out, because it is less prone to cooling due to its irregular surfaces and gaps. The variability in the natural rock structure is more effective at retaining temperature and humidity when compared to the man-made structures. Thus contributing to the bat’s hibernation survival in Finland during the winter season.

Humans
Researchers have studied how to induce hibernation in humans. The ability to hibernate would be useful for a number of reasons, such as saving the lives of seriously ill or injured people by temporarily putting them in a state of hibernation until treatment can be given. For space travel, human hibernation is also under consideration, such as for missions to Mars.

The question is also being studied by archeologists.

Birds
Ancient people believed that swallows hibernated, and ornithologist Gilbert White documented anecdotal evidence in his 1789 book The Natural History of Selborne that indicated the belief was still current in his time. It is now understood that the vast majority of bird species typically do not hibernate, instead utilizing torpor. One known exception is the common poorwill (Phalaenoptilus nuttallii), for which hibernation was first documented by Edmund Jaeger.

Dormancy and freezing in Ectotherms
There are misconceptions about ectothermic organisms. One reason is that certain organisms may or may not participate in the hibernation process. Brumation is utilized by ectotherms such as reptiles. Brumation can be triggered by lack of resources such as food, drop of temperature or the change in photoperiod. In preparation for brumation many animals will not eat as following the entrance into brumation there is a significant drop in the animal's metabolism, and it would not be able to digest the food. During this time, consciousness can still be maintained meaning it is not true hibernation. It was once thought that basking sharks settled to the floor of the North Sea and became dormant, but research by David Sims in 2003 dispelled this hypothesis, showing that the sharks traveled long distances throughout the seasons, tracking the areas with the highest quantity of plankton. Epaulette sharks have been documented to be able to survive for three hours without oxygen and at temperatures of up to 26 °C (79 °F) as a means to survive in their shoreline habitat, where water and oxygen levels vary with the tide. Other animals able to survive long periods with no or very little oxygen include goldfish, red-eared sliders, wood frogs, and bar-headed geese. The ability to survive hypoxic or anoxic conditions is not closely related to endotherm hibernation. Some animals can literally survive winter by freezing. For example, some fish, amphibians, and reptiles can naturally freeze and then "wake" up in the spring. These species have evolved freeze tolerance mechanisms such as antifreeze proteins. Hibernating terrestrial frogs like the wood frog (Rana sylvatica), use Cryoprotectants and Osmoprotectants to tolerate sub-zero temperatures in the winter to maintain minimal metabolic function. The prominent cryoprotectant contributing to overwintering survival is glucose. Glycogen in the liver is converted into glucose, which is stored inside every cell overwinter to resist ice crystal formation and binds water molecules to the inside of the cell to avoid dehydration. Another important cryoprotectant and osmoprotectant is urea, which is stored in tissues before freezing occurs. This buildup of urea before winter aids in resisting ice crystal formation and reduces thawing injury when the temperature fluctuates.

Insects
Diapause is a period of lowered activity or dormancy that is most common in insects. In this state, the organism will lower their metabolism, and their activity will decrease or halter. Diapause also plays a role in the development as it may occur during specific stages in the animal's life cycle. During this time, no development of the individual will occur. The purpose of diapause is a survival mechanism as individuals enter this state just before the onset of unfavorable conditions. This idea allows the individual to better line up energy requiring processes, such as development with a time where resources are plentiful.

Induction
The induction of diapause is trigger by changes within the environment. The conditions for this to occur are specific and have a genetic basis. The animal must also be within a sensitive period within its development. Induction usually occurs before the decline of conditions in order to better prepare the animal for what is to come. Stimulus that contribution to induction include photoperiods and pheromones, these stimuli are referred to as token stimuli.

Preparation
Preparation occurs after the induction and before the initiation in order to prepare for the eventual entrance into diapause. In preparation the animal may undertake different activities or exhibit behavioral changes. Animals in the preparation stage will commonly attempt to increase the amount of energy reserves available and may also seek out suitable environmental conditions for diapause to occur.

Initiation
When direct development of the animal stops the initiation phase begins. The defining feature of the initiation phase is the decrease in metabolic rate. In some species the entrance into this phase may be able to be visibly observe through morphological changes. Although there is a decrease in metabolic rate some species will remain somewhat active and mobile during this phase.

Maintenance
Animals in this phase are sensitive to token stimuli such as photoperiods, which keep them in this stage characterize by a slow constant metabolism. There is still no development in this phase, however over time physiological changes will occur and the animal will become sensitive to stimuli that will initiate termination of diapause.

Termination
The ending of diapause can be abrupt, and can be caused by a change in environmental conditions such as chilling or contact with water. However, in certain cases there is no stimulus required for the termination of diapause to occur. Once termination has occurred the organism may continue on developing unless conditions are not favorable in which it will enter a state in which the induvial is prepare to continue to develop once conditions are acceptable. This state is call Quiescence.

Many insects, such as the wasp Polistes exclamans, exhibit periods of dormancy which have often been referred to as hibernation, despite their ectothermy.

Hibernation induction trigger
Hibernation induction trigger (HIT) is somewhat of a misnomer. Although, research in the 1990’s hinted at the ability to induce torpor in animals by injection of blood taken from a hibernating animal, further research has been unable to reproduce this phenomenon. Despite the inability to induce torpor, there are substances in the blood of hibernators that can lend protection to organs for possible transplant. Researchers were able to prolong the life of an isolated pig's heart with an HIT. This may have potentially important implications for organ transplant, as it could allow organs to survive for up to 18 hours outside the human body. This would be a great improvement from the current 6 hours.

The supposed HIT is a mixture derived from blood serum, including at least one opioid-like substance. DADLE is an opioid that in some experiments has been shown to have similar functional properties.