The 'big sleep' animals go into

The environment around us is constantly changing, and we humans have evolved ways to survive through such changes. But, have you ever wondered how our friends in the animal kingdom manage to survive in adverse environmental conditions such as extreme cold or heat, which often leads to shortages of food and water?

Some animals are naturally equipped to survive in such situations, and are able not merely to endure, but actually thrive in conditions which are unbearable for most other life forms. Some other animals do so by what is popularly known as hibernation.

You may have heard the word hibernation being used often in conversations because, when some people have not been seen around for a while and suddenly make an appearance, people sometimes tend to ask them whether they have been in 'hibernation'. Well, hibernation is actually something animals resort to, so, let's check out what exactly hibernation means.

What is hibernation?

Hibernation, also known as the 'big sleep', is not the same as the nightly sleep or occasional nap we humans or for that matter, animals have. It is much, much more than that. In fact, during hibernation, body temperature drops, the heartbeat slows and the animal seems to be scarcely breathing! The animal uses hardly any energy during this 'big sleep'.

How hibernation takes place

Animals have to prepare themselves for hibernation well in advance. They cannot suddenly go into hibernation when they encounter harsh climatic changes overnight, the way we suddenly decide we want to sleep. Animals need several weeks to condition their bodies to these changes. The process of hibernation involves a complex series of physiological changes.

During the weeks of preparation for hibernation, the animal eats more than normal, in order to build up substantial fatty layers. For example, bats increase their total body weight by as much as a quarter, during this fattening-up process.

Extreme hibernators such as hedgehogs undergo many changes. Their blood's potassium content increases, their reproductive organs decrease markedly in size, as they will not be needed during the big sleep, and also some endocrine glands including the thyroid, reduces considerably in size.

Well, what do you think happens when the animals enter this stage, especially after loading themselves with extra food to build up fat? The same thing most of us would undergo when we have loaded ourselves with food... Feel sluggish of course. By now, they are almost ready for hibernation in the winter season. Hibernation itself is triggered by a drop in the environmental temperature below a certain critical level, for the species concerned.

Once the animals enter hibernation, the breathing rate automatically decreases dramatically. Some bats take only a single breath every two hours! Their heart rate also reduces from over 1,000 beats per minute in a bat in flight, to 25 per minute during hibernation. And when the heartbeat slows down, the circulatory system too slows down, making its skin and extremities(hands and feet) cold to the touch. In fact, at such times, the animal seems as if it is dead.

However, the reality is very much different, to what's visible from outside. Even though the animal's body is cold outside, the body's internal regions, especially those nearest the heart, are still warm, and its vital physiological functions, although slower than normal, are sufficient for the animal's survival. The animal's body temperature is controlled by the brain, and does not fall to dangerous levels, even if the temperatures outside falls to abnormally cold levels.

Wake-up calls...

Ground squirrels, which relinquish(give up) their fat supplies during hibernation, can awaken from their deep sleep in less than three hours. During arousal, the anterior (frontal) skeletol muscles get over 16 times more blood than their counterparts in a fully awake animal, powering their shivering to produce heat for raising the body's temperature. Waking up from hibernation can be more dangerous than survival during extreme cold conditions. The danger occurs because waking up from such a deep sleep needs a significant rise in metabolic(chemical process) activity.

Fuel is used rapidly to heat the body during the awakening period. As a result, if a hibernating animal is disturbed and woken by some unexpected external stimulus (something that rouses to activity ) and the hibernating animal does not replenish ( fill up again ) its fat supplies before sinking back into hibernation once more, the animal is in danger.

There is a danger that in such a situation, the hibernating animal will prematurely (before time) exhaust ( use up ) its supplies and die. So, never disturb hibernating animals if you ever come across them anytime.

What are true hibernators?

True hibernators are animals that sleep or go into hibernation and never wake up in between, during the period the particular species generally hibernates. Dormice (ground squirrels and bats) may spend as much as half the year in hibernation, but they do wake up occasionally to answer calls of nature and sometimes to even replenish their fat layers by feeding on whatever suitable nutrients they can find during winter time.

Hedgehogs however, are considered to be true hibernators, taking their hibernation to extremes. They do not wake up to even answer a call of nature. During this long period of inactivity, their internal system needs to be protected from attacks by the bacteria that inhabit their guts.

How do you think this is done? By large numbers of white blood cells that accumulate around their blood vessels and in the stomach lining as barriers to bacterial invasion.

Are bears true hibernators?

Bears are often said to hibernate, but many scientists feel this is not strictly correct, because, the physiological process involved is far less extreme than that of true hibernation. Many scientists therefore prefer to call the long periods of sleep bears spend in their dens as 'winter sleeping' or 'winter denning' rather than hibernation.

Just like true hibernators, bears too prepare for the winter by consuming excess amounts of food, in the autumn, especially fruit, to build up layers of fat. Once they retire to their dens at the onset of winter, and go to sleep, their breathing and heartbeat rates too drop significantly.

Their consumption of oxygen too falls to little under 50 per cent of normal. However, the difference is in the body temperature. While in true hibernating animals, the body temperature plummets (falls) to only a little above the environmental temperature in bears, it falls by only a few degrees.

And what's more, bears often wake up during 'hibernation' and leave their dens for days at a time. Females are even known to give birth to cubs during this time, and as scientists and researchers claim, this would not happen if they were true hibernators.

It is not only in cold winter months that animals go into hibernation. Some animals hibernate during extremely hot weather. This process is known as 'Summer hibernation'. We'll check out what this form of hibernation is all about in our next issues.

Fact file

* Some species such as dormice (domine in French means 'to sleep') hibernate for up to six months. But they do wake up occasionally.

* Northern Alaska's ground squirrel spends as much as nine months in hibernation.

* A hibernator's body temperature can fall by as much as 36 degrees fahrenheit (20 degrees celsius).

* Large amounts of fat are burned as fuel to raise the body temperature. This is accompanied by intense shivering and muscle contraction, which also generate heat.

* Much of the heat is derived from a kind of fat known as brown fat, which contains many energy-producing cells. This substance acts like an electric blanket, releasing heat to the heart and major blood vessels.

* Ground squirrels have as much as 57 per cent of this brown fat around their shoulders, 14 per cent in their neck and most of the balance fat in their thorax.

* American red bat can survive even if its body tissues freeze, when the outside temperature falls as low as 23 degrees Fahrenheit.

* Some butterflies go into a state of hibernation during winter, awakening in spring when flowers start to bloom.