Hibernation

Energy is needed all the time.
We all need energy – all the time. Every living animal on Earth is burning energy all the time. Physical activities like walking and breathing burn energy. Pumping blood and digesting food burns energy. Even thinking burns energy. For warm-blooded animals, a lot of energy is burned just keeping our body temperature where we need it. Even when we're sleeping, we're burning energy.

What happens in Winters?
That's the whole reason animals eat -- to gain enough energy to fuel all those processes. The system works fine when there's plenty of fruit on the trees or rabbits to catch and eat . But what happens when winter comes and it becomes very difficult to find food? How do animals survive with few energy sources available?
Winters can be very harsh in many places. Human beings find warmth in heaters, woollen clothes etc., but what do animals do? Where do they get their food from when all the land is covered in snow ?
Animals have different, wonderful strategies to cope with winters. Some migrate, Some Adapt, and yet some hibernate. Smaller animals tend to be more likely to hibernate, because migration would require an enormous amount of energy relative to their body size. Larger animals are less apt to hibernate because of the additional energy required to warm up a large body.

What is Hibernation?
Some animals "hibernate" for part or all of the winter. This is a special, very deep sleep. The animal's body temperature drops, and its heartbeat and breathing slow down. It uses very little energy. In the fall, these animals get ready for winter by eating extra food and storing it as body fat. They use this fat for energy while hibernating. Some also store food like nuts or acorns to eat later in the winter. Bears, skunks, chipmunks, and some bats hibernate. Cold-blooded animals like fish, frogs, snakes and turtles have no way to keep warm during the winter. Snakes and many other reptiles find shelter in holes or burrows, and spend the winter inactive, or dormant. This is similar to hibernation.
Water makes a good shelter for many animals. When the weather gets cold, they move to the bottom of lakes and ponds. There, frogs, turtles and many fish hide under rocks, logs or fallen leaves. They may even bury themselves in the mud. They become dormant. Cold water holds more oxygen than warm water, and the frogs and turtles can breath by absorbing it through their skin.
When an animal enters a hibernationlike state during the summer, it's known as estivation. It's much less common than hibernation. Estivation is like hibernation in hot weather. Animals that live in deserts or tropical climates practice estivation. It may not occur solely because of food supply issues, as with hibernation, but because the conditions become too hot and dry for the animal to survive. The process typically involves burrowing into the ground, where the temperature stays cool, and reducing metabolic activity in a similar manner to hibernation.

How does hibernation work ?
Hibernating animals have a special substance in the blood called hibernationinducement trigger, or HIT. This substance becomes active in the fall, when the days becomecooler and shorter. When HIT becomes active, the animals start preparing for winter. Someanimals store food so that they can eat when they wake up, and some animals eat a lot in latesummer and fall to add excess fat to their bodies. This fat keeps them warmer and acts as asource of energy while they are sleeping. Some animals also make changes to the placeswhere they will sleep (dens). They add leaves and grasses to keep them warm.

Preparing for Hibernation
Preparation is required to hibernate successfully. Some animals prepare a den (also known as a hibernacula) and line it with insulating material, just as leaves or mud. Ground squirrels and lemurs do this. Polar bears dig tunnels in the snow. Other bears might spend the winter in a hollow beside a tree or a shallow cave, leaving them partly exposed to the weather. Bats are well-known for wintering in caves or attics.
Next comes food storage. Food can be kept in the den if it's nonperishable, but this requires the animal to wake up briefly during the winter to eat. Another option is to eat a large amount of food starting in late summer, building up a reserve of internal fat.
Several other things occur when an animal is hibernating:Heart rate drops to as little as 2.5 percent of its usual level. A chipmunk's heart rate slows to five beats per minute from the usual 200.Breathing rate drops by 50 percent to 100 percent. Yes, 100 percent. Some animals stop breathing entirely. A few reptiles go their entire hibernation period without breathing, and even mammals have shown the ability to survive with drastically reduced oxygen supplies. Consciousness is greatly diminished. This varies by species, but many hibernating animals are completely oblivious to their surroundings and are nearly impossible to wake up.
If you were to wake up a hibernating animal midwinter, you would be effectively killing it. It would use up so much energy warming itself up in order to awaken that it would have no chance of making it to spring even if it could re-enter hibernation.