Hibernation

Hibernation is a state of inactivity (deep sleep) and metabolic depression in animals, characterized by lower body temperature, slower heart beat and breathing, and lower metabolic rate. Hibernation helps to conserve energy, during winter, when there is scarecity of food. So, it is also known as energy saving mode or standby mode of life. During hibernation, animals drastically lower their metabolism so as to tap energy reserves stored as body fat at a slower rate. Hibernation may last several days or weeks depending on species, ambient temperature, and time of the year. The typical winter season for a hibernator is characterized by periods of hibernation interrupted by sporadic euthermic arousals wherein body temperature is restored to typical values.

European Hedgehog

Hibernators

Animals that hibernate include bats, ground squirrels and other rodents, mouse lemurs, the European Hedgehog and other insectivores, monotremes and marsupials. Even some rattlesnakes, such as the Western Diamondback, are known to hibernate in caves every winter. Historically, Pliny the Elder believed that swallows hibernated, and ornithologist Gilbert White pointed to anecdotal evidence in The Natural History of Selborne that indicated as much. However, birds typically do not hibernate, instead utilizing torpor. An exceptional bird known as the Poorwill does hibernate. Walter Arnold of The Research Institute for the Study of Wild Animals and Ecology at the University of Vienna and Gerhard Heldmaier, a physiologist at Germany's University of Marburg have generalized that winter sleep is the general reaction by mammals and other animals in stressful situation regardless of season. Some species of mammals hibernate while gestating young, which are born shortly after the mother stops hibernating.

There are animals that hibernate, or become dormant, daily. Many tiny warm-blooded animals have huge energy requirements. It is difficult for some of them to consume enough food to maintain themselves. To conserve energy, animals like hummingbirds and little brown bats become dormant for part of each day. Many experts believe that the processes of daily torpor and hibernation form a continuum.^{[citation needed]} However, the two differ in the degree of deepness of the sleep and the reduction of body temperature. Some reptile species are said to brumate, or undergo brumation, but the connection to this phenomenon with hibernation is not clear. In many instances, we find that hibernation has been extensively used for warm blooded birds and mammals, where as brumation been preserved for cold blooded animals like reptiles, etc.

For a couple of generations during the 20th century it was thought that basking sharks settled to the floor of the North Sea and hibernated; however, research by Dr David Sims in 2003 dispelled this hypothesis, showing that the sharks actively traveled huge distances throughout the seasons, tracking the areas with the highest quantity of plankton. The epaulette sharks have been documented to be able to survive for long periods of time without oxygen, even being left high and dry, and at temperatures of up to 26 °C. Other animals able to survive long periods without oxygen include the goldfish, the red-eared slider turtle, the wood frog, and the bar-headed goose. They might have used hibernation or other forms of dormancy for survival.

Until recently no primate, and no tropical mammal, was known to hibernate. However, animal physiologist Kathrin Dausmann of Philipps University of Marburg, Germany, and coworkers presented evidence in the 24 June 2004 edition of Nature that the Fat-tailed Dwarf Lemur of Madagascar hibernates in tree holes for seven months of the year. This is interesting because 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 behavior 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 to a low body temperature.

In October 2006, a Japanese man, Mitsutaka Uchikoshi, was believed to have been in a "denning"-like state for three weeks. He had fallen asleep on a snowy mountain and claimed he had only woken up after being discovered 23 days later; doctors who treated him believed his temperature had fallen to 22 °C (71 °F) during that period.

Hibernation - a survival strategy

Winter is characterized by cold weather and shortage of resources especially the food. Most animals either migrate to favorable environment before the dawn of harsh conditions or develop suitable adaptation to cope with the conditions. Some halt their development and remain dormant to escape such situations, while other hibernate. Hibernation can be considered as the last but economic choice to escape the extreme environmental conditions. Ecologists sometimes prefer to term hibernation as time migration. Thus, hibernation allows the animals to skip over the cold, stressful seasons and only expend itself fully in those months of abundant food and moderate climatic conditions. The major disadvantage of hibernation is that the animal is quite defenseless during hibernation, if it is not using a very secure, protected hibernating den (the hibernaculum).

Signal and preparation for winter sleep

Hibernating animals have something in their blood called HIT, or Hibernation Inducement Trigger. Recent research suggests that it is some kind of opiate, chemically related to morphine. As the days get shorter, the temperature changes, and food becomes scarce, HIT triggers hibernation. However, animals may display obligatory (predictive) hibernative responses as day length decreases or enter their hibernative state only after being exposed to adversely environmental conditions (consequential hibernation). Before entering hibernation most species eat a large amount of food in the fall while it is plentiful and store energy in fat deposits in order to survive the winter. Hibernators have two kinds of fat: regular white fat and a special brown fat. The brown fat forms patches near the animal's brain, heart and lungs. It sends a quick burst of energy to warm these organs first when it is time to wake up. Some store food in their burrows or dens, to eat when they awake for short periods. Most hibernating animals build themselves a nest (hibernaculum).

Physiological state during hibernation

As mentioned above, shutting their metabolisms to minimal levels, hibernators lower their body temperature almost to that of the environment, slow down their heart beat and breathing, and tap energy reserves at a slower rate. For example, a hibernating woodchuck's heart rate slows from 80 to 4 beats per minute, and its temperature drops from 37°C (98°F) to as low as 4°C (38°F). If its temperature falls too low, it will awaken slightly and shiver to warm up a bit. The hibernating arctic ground squirrels may have core body temperatures as low as -2°C. Even in this "supercooling", the body's fluids are held in a "meta-stable state" and are not allowed to freeze. It is said that small animals can reduce their energy requirements by up to 98 percent by engaging in such behavior. Genetic sequences are no longer decoded in their cell nuclei, proteins are no longer synthesized and entire reaction chains are essentially frozen. The immune system also practically ceases to operate. There is no longer any visible brain activity on the EEG. One animal that some famously consider a hibernator is the bear. However, during a bear's winter sleep state, the degree of temperature decrease is much less than what is observed in smaller mammals. Some prefer to use the term "denning" in place of hibernating. However, other biologists regard bear as the "super hibernator" or extremely efficient hibernator for their special features. For example, a grizzly or black bear's heart rate may drop from a normal of 40-50 beats per minute down to 8-12, its body temperature remains relatively stable (depressed from 37°C to approximately 31°C) and it can be easily aroused. These bears are also unique because, unlike other hibernators, they do not eat, drink, or excrete at all while hibernating, which can be as long as six months.

Termination of hibernation

The hibernators wake up in spring due to the warming of the environment and are in very hungry state. In many mammals, a area rich in brown fat cell occurs near the thoracic blood vessels. Upon stimulation from sympathetic neurons, these cells conduct rapid aerobic oxidation of numerous fat droplets. The energy released in this way is particularly important to rapidly raise the core temperature during waking from hibernation.

Noise and vibration from snowmobiles, all-terrain vehicles and the like is said to sometimes awaken hibernating animals, who may suffer severely or die as a result of premature awakening in times of food shortage.^{[citation needed]}

Conclusion

Every organism exists for individual as well as for species survival and for contributing in the natural cycles of the ecosystem, where it belongs to. This dual purpose principle can be found working in hibernation too. Hibernation helps individual animals to overcome the adversity of the environment, but at the same time, it is also a kind of sharing of resources and the conservation of resources from unnecessary exploitation. Therefore, it should not be regarded as escape from having vigorous competition, rather a wise sharing of the time, space and resources with others.

See also

• Dormancy - a period when development is temporarily suspended

External links

• Hibernation on Demand
• Freeze avoidance in a Mammal: Body Temperatures Below 0°C in an Arctic Hibernator
• Prospects for Human Hibernation: ESA Advanced Concepts Team
• Hibernation

Credits

New World Encyclopedia writers and editors rewrote and completed the Wikipedia article in accordance with New World Encyclopedia standards. This article abides by terms of the Creative Commons CC-by-sa 3.0 License (CC-by-sa), which may be used and disseminated with proper attribution. Credit is due under the terms of this license that can reference both the New World Encyclopedia contributors and the selfless volunteer contributors of the Wikimedia Foundation. To cite this article click here for a list of acceptable citing formats.The history of earlier contributions by wikipedians is accessible to researchers here:

• Hibernation  ^history

The history of this article since it was imported to New World Encyclopedia:

• History of "Hibernation"

Note: Some restrictions may apply to use of individual images which are separately licensed.