hibernation

Hibernation is a time when animals ‘sleep’ through cold weather.  This sleep is not like human sleep where loud noises can wake you up.  With true hibernation, the animal can be moved around or touched and not know it.  [Don’t you do this, though.  Some animals only go into a torpor or temporary sleep time and can wake up quickly.  Like BEARS.]  We are going to use the word 'sleep' sometimes but hibernation is different from regular sleep.  With normal sleep, the animal moves a little, has an active brain, and can wake up very quickly.  With true hibernation, the animal appears dead.  There is no movement and it takes a long time for it to wake up enough to even walk around.
    We will show you how animals get ready to sleep the winter away, what it is like, and who does it.
    GETTING READY:  During the fall, hibernating animals eat more food than usual.  Their bodies will live off their body fat as they ‘sleep’ through winter.  The animal will use up the body fat it stores and not lose any muscle.  This causes the animal to come out of hibernation thinner but still as strong as it was in the fall.
    The animals get their winter nests, dens and burrows ready.  Different kinds of animals hibernate in different kinds of safe spots.  When they go into hibernation and their bodies slow down, enemies can get them easier.  They try to pick the safest place to spend the winter away from these enemies.
    WHAT IT IS AND WHO DOES IT: Hibernation is the way that animals adapt to the climate and land around them.  Animals must be able to live through extreme cold…. or die.  Animals hibernate—or deep sleep—to escape that cold.  They also do this because it is really hard to find food during the winter.
    We don’t think about body energy too often.  Our bodies are like machines that need power to work right.  Food gives animals the energy they need to walk, run, hunt for food, and lots of other things.  Hibernating animals store food as body fat during the end of summer and during fall.  This body fat runs their bodies all winter.  This would be hard to do if they stayed awake, moved around a lot, or ran around because those things would use up the body fat before winter was over.   A hibernating animal’s body saves energy by doing a couple of cool things.
    When an animal begins to hibernate, its body temperature drops very low so that it almost matches the temperature outside.   Your temperature is normally about 98.6 degrees Fahrenheit.  If you were a hibernator and it was 30 degrees outside, your body temperature would drop from 98.6 down to about 30-40 degrees. THAT’S cold!
    The animal’s heartbeat and breathing slow down, too.  This is when that stored fat that the animal packed on in the fall comes in handy.  This stored fat lasts longer because their bodies are slowed down so much that they don’t need much energy.  This is how the animal makes it through the whole winter on the fat it has stored in its body.  This is why it's important for animals to get enough food stored in the fall.  If there is a shortage of food at that time, the animal might not live until spring when it can find its food again.
    Some of these hibernators also store food in their caves and burrows.  The ones that do this do not sleep straight through the winter.  They wake up once in awhile, walk around a little, and eat before they go back to sleep.  Some warm-blooded hibernators are:

Badgers, Hedgehogs, Bats, Nighthawks, Chipmunks, Poor-Wills, Dormouse, Prairie Dogs, Fat-tailed lemurs, Raccoon, Ground squirrels, Skunks, Hamsters, Swifts, Marmots, Groundhogs, Woodchucks And bears

Cold-blooded hibernators begin hibernation when the cold weather causes their body temperatures to drop.  Cold-blooded animals do not have a body temperature like humans do.  Our temperature stays about 98.6 degrees Fahrenheit all the time.  Cold-blooded animal temperatures stay the same as the air temperature around them.  If it is 50 degrees outside, the lizard is around 50 degrees.  If it is 110 outside, then they are about 110, too.  Since we already said that hibernators adapt to their environments, you can see why these animals would try to escape extreme cold AND heat by hibernating.  Hibernation is sleeping through cold and estivation is sleeping through heat.  Cold-blooded hibernators will wake up when the air outside warms or cools enough for them to be comfortable.  Some cold-blooded hibernators are:

Bees, Earthworms, Frogs and toads, Lizards, Mud Turtles, Snails, Snakes

Hibernating mammals

Rodents

Hibernation among rodents has been extensively studied for decades. Species of ground squirrel, marmot, prairie dog, dormouse, and hamster have all been shown to demonstrate hibernation. These animals all exhibit the classic hibernation pattern where body temperature remains at ambient temperature for days to weeks, followed by a brief (<24hr) return to high body temperature.

Primates

While hibernation has long been studied in rodents, namely ground squirrels , no primate or tropical mammal was known to hibernate prior to animal physiologist Kathrin Dausmann of Philipps University of Marburg, Germany, and coworkers presenting evidence that the Fat-tailed Dwarf Lemur of Madagascar 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 to a low body temperature.

Bears

Bears hibernate very differently than either rodents or primates. They rely on active metabolic suppression, rather than a decreased body temperature to save energy over winter. Despite their lack of body temperature change, bears have an impressive hibernation physiology. They are able to recycle their proteins and urine, allowing them to stop urinating for months. This is not considered hibernation however, It is called "TORPOR".

Obligate hibernators

Obligate hibernators are defined as animals that spontaneously, and annually, enter hibernation regardless of ambient temperature and access to food. Obligate hibernators include many species of ground squirrels, other rodents, mouse lemurs, the European Hedgehog and other insectivores, monotremes and marsupials. These undergo what has been traditionally called "hibernation": the physiological state where the body temperature drops to near ambient (environmental) temperature, and heart and respiration rates slow drastically. The typical winter season for these hibernators is characterized by periods of torpor interrupted by periodic, euthermic arousals, wherein body temperatures and heart rates are restored to euthermic (more typical) levels. The cause and purpose of these arousals is still not clear.

The question of why hibernators experience the periodic arousals (returns to high body temperature) has plagued researchers for decades, and while there is still no clear cut explanation, there are a myriad of hypotheses on the topic. One favored hypothesis is that hibernators build a 'sleep debt' during hibernation, and so must occasionally warm up in order to sleep. This has been supported by evidence in the arctic ground squirrel. Another theory states that the brief periods of high body temperature during hibernation are used by the animal to restore its available energy sources. Yet another theory states that the frequent returns to high body temperature allow mammals to initiate an immune response.

Hibernating arctic ground squirrels may exhibit abdominal temperatures as low as -2.9 °C, 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 or above.

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 to what other hibernators undergo (32°C+), many researchers thought that their deep sleep was not comparable to true, deep hibernation. This theory has recently been refuted by recent research in captive black bears.

Facultative hibernation

Unlike obligate hibernators, facultative hibernators only enter hibernation when either cold stressed or food deprived, or both. A good example of the differences between the two types of hibernation can be seen among the prairie dogs; where the white-tailed prairie dog is an obligate hibernator and the closely related black-tailed prairie dog is a facultative hibernator.

Hibernating birds

Historically, Pliny the Elder believed swallows hibernated, and ornithologist Gilbert White pointed to anecdotal evidence in The Natural History of Selborne that indicated as much. Birds typically do not hibernate, instead utilizing torpor. One known exception is the Common Poorwill (Phalaenoptilus nuttallii), first documented by Edmund Jaeger.

Dormancy in fish

Fish are ectothermic, and so, by definition, cannot hibernate because they cannot actively down-regulate their body temperature nor their metabolic rate. However, they can experience decreased metabolic rates associated with colder environments and/or low oxygen availability (hypoxia) and can experience dormancy. For a couple of generations^[ during the 20th century it was thought that basking sharks settled to the floor of the North Sea and became dormant. 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 (79 °F). 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. However, the ability to survive hypoxic or anoxic conditions is not the same, nor closely related, to endotherm hibernation.

Hibernation induction trigger

Hibernation induction trigger (HIT) is a bit of misnomer. Although research in the 90'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 phenomena. Despite the inability to induce torpor, there are substances in hibernator blood that can lend protection to organs for possible transplant. Researchers were able to prolong the life of an isolated pig's heart with a HIT. This may have potentially important implications for organ transplant, as it could allow organs to survive for up to 18 or more hours, outside the human body. This would be a great improvement from the current 6 hours.

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

Artificial hibernation

See also: Suspended animation

There are many research projects currently investigating how to achieve "induced hibernation" in humans. The ability for humans 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 (compare induced coma). In addition, hibernation would be useful for humans during various proposed plans for interstellar travel in the future. Similar to human hibernation, suspended animation deals with the slowing of life processes in general, by external means and without termination.

Hibernation Research and Human Health

Hibernation, and the species that are able to utilize it, has become a fantastic model for many different human diseases. Hibernators make natural models for stroke, ischemia-reperfusion injury,diabetes, obesity and depression to name a few.