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Hibernation is a state of minimal activity and Metabolism reduction entered by some animal species. Hibernation is a seasonal heterothermy characterized by low body-temperature, slow breathing and heart-rate, and low metabolic rate. It is most commonly used to pass through winter months – called overwintering.
Although traditionally reserved for "deep" hibernators such as rodents, the term has been redefined to include animals such as and 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 use similar mechanisms. The equivalent during the summer months is aestivation.
Hibernation functions to conserve energy when sufficient food is not available. To achieve this energy saving, an endothermic animal decreases its metabolic rate and thereby its body temperature. 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. Larger species become Hyperphagia, eating a large amount of food and storing the energy in their bodies in the form of fat deposits. In many small species, food caching replaces eating and becoming fat.
Some species of mammals hibernate while gestation young, which are born either while the mother hibernates or shortly afterwards. For example, female black bears go into hibernation during the 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.
Ectothermic animals also undergo periods of metabolic suppression and dormancy, which in many invertebrates is referred to as Diapause. Some researchers and members of the public use the term brumation to describe winter dormancy of reptiles, but the more general term hibernation is believed adequate to refer to any winter dormancy.Ultsch, Gordon R. 1989. Ecology and Physiology of Hibernation and Overwintering Among Freshwater Fishes, Turtles, and Snakes. Biological Reviews 64(4), pp. 435-515. doi:10.1111/j.1469-185X.1989.tb00683.x Many insects, such as the wasp Polistes exclamans and the beetle Bolitotherus, exhibit periods of dormancy which have often been referred to as hibernation, despite their ectothermy.
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 are 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. 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.
Hibernating Arctic ground squirrels may exhibit abdominal temperatures as low as , maintaining sub-zero abdominal temperatures for more than three weeks at a time, although the temperatures at the head and neck remain at or above.
A chipmunk, for example, is a facultative hibernator. Even though it sleeps for a long period of time, it is not a true obligate hibernator. This is because during the long period of sleep, its temperatures do not decrease to the low levels of hibernation. It only truly hibernates if food is scarce. (2025). 9780761471417, Marshall Cavendish. ISBN 9780761471417
A good example of the differences between these two types of hibernation can be seen in . The white-tailed prairie dog is an obligate hibernator, while the closely related black-tailed prairie dog is a facultative hibernator.
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.
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 water that is produced in sufficient quantities to satisfy the water needs of the bear. They also do not eat or drink while hibernating, but live off their stored fat. 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 in the muscle, as well as regulate the transcription of a suite of genes that limit muscle wasting. A study by G. Edgar Folk, Jill M. Hunt and Mary A. Folk 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 evidences used to show that bears are hibernators.
In a 2016 study, 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, heart rate variability, body temperature, physical activity, ambient temperature, 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 ambient temperature dropped to . However, physical activity, heart rate, and body temperature started to drop slowly even several weeks before this. Once in their dens, the bears' heart rate variability dropped dramatically, indirectly suggesting metabolic suppression is related to their hibernation. Two months before the end of hibernation, the bears' body temperature starts to rise, unrelated to heart rate variability but rather driven by the ambient temperature. The heart rate variability 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.
Some animals can literally survive winter by freezing. For example, some fish, , and can naturally freeze and then "wake" up in the spring. These species have evolved freeze tolerance mechanism such as antifreeze proteins.
Recent advances in recombinant protein technology make it possible for scientists to manufacture hibernation induction trigger (HIT) proteins in the laboratory without the need for animal euthanasia. Bioengineering of proteins can aid in the protection of vulnerable populations of bears and other mammals that produce valuable proteins. Protein sequencing of HIT proteins, such as α 1-glycoprotein-like 88 kDa hibernation-related protein HRP, contributes to this research pool. A study in 2014 uses recombinant technology to construct, express, purify, and isolate animal proteins (HP-20, HP-25, and HP-27) outside of the animal to study key hibernation proteins (HP).
Anthropologists are also studying whether hibernation was possible in early hominid species.
In both cases, hibernation likely evolved simultaneously with endothermy, with the earliest suggested instance of hibernation being in Thrinaxodon, an ancestor of mammals that lived roughly 252 million years ago. The evolution of endothermy allowed animals to have greater levels of activity and better incubation of embryos, among other benefits for animals in the Permian and Triassic periods. In order to conserve energy, the ancestors of birds and mammals would likely have experienced an early form of torpor or hibernation when they were not using their thermoregulatory abilities during the transition from ectothermy to endothermy. This is opposed to the previously dominant hypothesis that hibernation evolved after endothermy in response to the emergence of colder habitats. Body size also had an effect on the evolution of hibernation, as endotherms which grow large enough tend to lose their ability to be selectively heterothermic, with bears being one of very few exceptions. After torpor and hibernation diverged from a common proto-hibernating ancestor of birds and mammals, the ability to hibernate or go through torpor would have been lost in most larger mammals and birds. Hibernation would be less favored in larger animals because as animals increase in size, the surface area to volume ratio decreases, and it takes less energy to keep a high internal body temperature, and thus hibernation becomes unnecessary.
There is evidence that hibernation evolved separately in marsupials and placental mammals, though it is not settled. That evidence stems from development, where as soon as young marsupials from hibernating species are able to regulate their own heat, they have the capability to hibernate. In contrast, placental mammals that hibernate first develop homeothermy, only developing the ability to hibernate at a later point. This difference in development is evidence, though inconclusive, that they evolved by slightly different mechanisms and thus at different times.
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