The hibernation is a state of controlled Hypothermie, during several days or weeks which allows the animal to preserve their energy during the Hiver. During the hibernation the animals slow down to them Métabolisme up to very low levels, with the Température their bodies and rates respiratory which drop gradually, and by using the reserves of grease body which was stored during the active months.

An animal that some consider wrongly as one hibernating is the Ours. Indeed, although its heart rates slow down, the body temperature of the bear remains relatively stable and it can be easily awaked. It is the same for the Blaireau X, the raccoons and the Opossum S. the bears are semi-hibernating.

The animals considered as hibernating are: the Marmot S, the dormouse S, the Lérot S, the Spermophile S, the Hedgehog S, the Tenrec, the Setifer, the Goat-sucker of Nuttall, the Frog S, the Lizard S, like some Hamster S, Mouse, Poisson S and Bat.

Various types of temporary suspension of activities at the Homéotherme S

  • the torpor is a physiological state which stops as soon as the Air is heated, i.e. when the outside temperature increases, the animal readjusts its internal temperature by slightly decreasing it in order not to waste energy to be heated. The Bird X and some Chauves-souris know this state. Certain birds or some Chiroptère S can thus enter a state of daily torpor.

  • the somnolence winter (or Hivernation) of the carnivorous like the Bear and the Badger, intersected with many alarm clocks and accompanied by a moderate Hypothermia, does not involve an interruption of all the physiological activities. Thus, the bear gives rise to small during the winter. The vital bodies remain at a normal temperature to react in the event of danger.

  • the hibernation is true a Léthargie and a major reduction in the Température of the animal. This temperature is always positive but it can approach 0°C. The animals really hibernating are for example the dormouse S, the Marmotte S and some Chauves-souris.

The great majority of the mammals is obliged to maintain a temperature constant in a cold environment by controlling its temperature by named physiological processes Thermogenèse.

A similar state in summer is the Estivation (sleep of summer), adapted for example by the Crocodile of the Nile which is buried in mud for the hot period.

Categories of hibernating

  • obligatory hibernating as the Lérot hibernate any time as soon as the outside temperature is lower than 6°C during 48 hours. Its aptitude to hibernate is better in Hiver, i.e. as from September. It is enough 12 midnight to less 20°C so that the lérot returns in hibernation.

  • the seasonal hibernating , for example the Spermophile (American squirrel), are able to only hibernate enters mid-November and mid-February. Apart from this period, even if the conditions are unfavourable, the animal does not hibernate.

  • the optional hibernating as the hamster gilded have their capacity to hibernate conditioned by various factors. These factors can be the temperature, the reserves in Nutriment S, the Photopériode, etc So that these animals hibernate, it is necessary that all the conditions are met.

The entry in hibernation

Several months before the period of hibernation, hibernating them store and consume enormously Nourriture. For example, the spermophile passes from 150 grams of body mass to 350 grams. The reserves are primarily lipidic reserves stored under the skin.

Hibernating arrange then their burrow which one names a hibernaculum . The hibernaculum is selected to avoid important thermal variations. The animals put themselves in a position which keeps the maximum of heat, generally in ball.

The body temperature of the animal falls then in a spectacular way until the internal temperature approaches 1 or 2°C. Thermoregulation does not stop and thermogenesis recovers on the way to maintain the temperature interior of the animal at an acceptable temperature. The hibernation is not a passive state.

Physiology during the hibernation

The reduction in the internal temperature involves an adjustment of the various functions. The Métabolisme decreases by 98%.

There is a reduction:

  • of the consumption of Oxygen,
  • of the rate of respiration,
  • of the Cardiac rhythm (from 350 to 3 beats per minute for the spermophile, from 500 to 5 for the lérot),
  • of blood flow (there are an irrigation particular to the level of the Cerveau, fat Cœur and fabric),
  • of the rate of growth hormones.

The Nervous system is reactional. However, only the cerebral surfaces playing a part in the autonomous vegetative functions (as the Breathing) remain truly active. The other areas do not show a spontaneous cortical activity. But the animal reacts to the noises, the touch, etc

The periods of sleep are characterized from the respiratory point of view, by respiratory puffs of cycles intersected with Apnée S prolonged (up to one hour at the hedgehog or the lérot). The weak gaseous exchange on the level of the Poumon S contributes to accumulation in the organization of Carbon dioxide dissolves, which acidifies the Sang (one speaks about respiratory acidosis).

During the hibernation, there are periodic alarm clocks at variable, and increasingly frequent times when one arrives at the end of the hibernation. The alarm clock lasts a few hours and corresponds to an increase of fast temperature. This for all hibernating them, at variable intervals. For example the gilded hamster awakes all the 3 to 5 days whereas the spermophile awakes every 15 days. During these alarm clocks, the animal turns in the hibernaculum, eats, urine and rendort. Experiments of ablation of the Neuron S of the Hypothalamus showed a removal of these alarm clocks and a death of the animal. These alarm clocks are thus fundamental, they in particular make it possible to eliminate waste from the metabolism whose accumulation is very toxic. These alarm clocks utilize the Thermogenèse known as without shiver, i.e. by using brown fat fabric. 90% of the weight loss during the hibernation are due to these phases of alarm clock.

It is remarkable that zone CA3 of the brain of an animal in hibernation undergoes same the synaptic regressions as the brain of a person reached of the Maladie of Alzheimer. But at hibernating, these regressions are reversible and would be the product of hyperphosphorylations of Protéine S tau produced by the cells of the brain (irreversible at the patients of Alzheimer or progressive Paralysie supranucléaire), and of déphosphorylations when connections are reformed.

Control hibernation

Internal clock

For hibernating seasonal, even if the outside temperature remains high, the animal enters in hibernation. In captivity and absence of external Stimulus, there is always a phenomenon of hibernation but the cycle starts more and more early in the year. It is an endogenous rate/rhythm of hibernation but under the natural conditions, the entry in hibernation is resynchronisée by the external conditions to begin and finish at the strategic times.

External factors

The factors like the Photoperiod and the Température synchronize these rates/rhythms. For the same species, the entry in hibernation is earlier when the population is more Scandinavian or higher in Altitude.

Internal factors

The internal factors were highlighted with the spermophile. By injecting blood of a spermophile hibernating in a spermophile not hibernating, one notes that the spermophile not hibernating becomes hibernating. The internal factors of hibernation thus circulate in blood (these factors are still badly known). According to recent studies, the surface pre-optics of the Hypothalamus allows the fall of the Point instruction of the organization until 2°C at certain species.

The reproductive Système would be also implied in the inhibition of the hibernation. In experiments, the injection of Testostérone causes the end of the hibernation.

On the level of the Foie are produced Protéine S forming a complex: HPc (complex of proteins of hibernation). This complex is composed of proteins HP20, HP22, HP27 and HP55. A reduction in the blood rate of these HPc precedes the hibernation. The cycle is reversed on the level of the céphalo-rachidian Liquide (LCR): indeed, the maximum of the rate of HPc is reached there during the hibernation. Also let us note that protein HP50 is never present in the céphalo-rachidian liquid, but this one contains the HP20. This protein would pass from blood towards the LCR on the level of the Plexus choroid, the area of the brain where this liquid is produced.

Hibernation at the cellular level

The cellular processes are stopped or at least strongly slowed down several manners:

Phosphorylation of certain components

Phosphoryls groups are fixed on the pumps at Sodium and on the pumps at Potassium, thus preventing the exchanges of these Ion S between the intracellular and extracellular compartments. Moreover, groupings phosphoryls stick to the Ribosome S, which blocks the proteinic Synthèse.

Cellular energy source during the hibernation

Whereas cellular energy is in normal weather mainly drawn from the Oxydation of molecules of Glucose, in fact the Lipide S become the priority energy source during the hibernation.

Deceleration of the transcription of the DNA

A Acétylase supports the transition from the Histone S of their state acetylated towards the désacétylé state. This causes an increased condensation of the DNA, which is rolled up then more narrowly around the histones, and returns the Gène S much less accessible.

Moreover, the ARN polymerases are not active any more, which still reduces the possibilities of transcription.

Cellular origin of the reduction in body temperature

In the fat fabrics brown, the internal membrane of the Mitochondrie S has uncoupling proteins which let pass easily the Proton S, thus making it possible to decrease the gradient of concentration between the two compartments located on both sides of this membrane. A less great quantity of ATP is thus produced by ATPase. The flow of protons thus feeds mainly the rise in the temperature by uncoupling proteins. During the hibernation, the activity of these uncoupling proteins is decreased.

Exit of the hibernation

The exit of the hibernation is characterized by a fast warming of the various parts of the body, heart rate, etc These mechanisms are faster than those of the entry in hibernation. All is restored in a few hours.

Membrane adaptations during the hibernation

The membrane of the animal cells is made of double-layered a lipidic fluid at normal temperature. The cold when the temperature approaches 0°C involves a disappearance of the fluidity of the membrane safe at hibernating because the lipids of their membranes have unsaturated fatty-acids in concentration higher than that of not-hibernating. Moreover the latter have proteins “hood” protecting the lipids from a modification of their phase (the fatty-acids keep their fluidity in the membrane).

Energy characteristics of hibernating

Hibernating are generally animals of intermediate size.

If they are too small, they have a very high metabolism which prevents long periods of hibernation, because even with a weaker cardiac rhythm, the reserves would be insufficient.

If they are of big size, the metabolism is relatively low, therefore the increase of temperature would take several days, which is not easily possible after one period of hibernation. The scientists think that so that the hibernation is a profit for the survival of the animal, it should not exceed 7 kg. Beyond, energy necessary during the periods of alarm clock would be too consequent.

For hibernating, the hibernation is always profitable from the energy point of view and corresponds to an energy saving. For example, for an American mouse Perognathus, if it returns in hibernation for 100 hours, it consumes 7.7 ml of oxygen per gram of its weight, whereas for the same period by preventing the entry in hibernation, it consumes 40 ml of oxygen per gram during 100 hours to be maintained with 37°C.

See too

Simple: Hibernation

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