Poikilotherms are animals whose body temperature varies with that of the
environment. Hibernation is a dormant condition in which many poikilothermic (“cold-blooded”)
animals pass the winter in cold areas . It involves the dropping of an animal's body
temperature on a seasonal basis in order to conserve energy. Any mammal that remains
inactive for many weeks with a body temperature lower than normal may be said to be in
hibernation, but physiological changes that occur during hibernation differ greatly among
different species. Body Temperature or degree of body heat in cold-blooded animals is a result
of the process of metabolism, by which food sources are converted into protein, carbohydrates,
and fat, with the release of energy in the form of heat. Because active muscles metabolize
food faster than muscles at rest, giving off more heat in the process, physical activity increases
body temperature. Although the temperature of poikilothermic or “cold-blooded” animals, such
as insects, reptiles, amphibians, and fish, varies with the temperature of their surroundings.
Therefore their rate of metabolism declines with a drop in the outside temperature, so
cold-blooded animals become torpid or inactive in cold weather. Almost all animals that
hibernate prepare for it during summer by eating large amounts of food, which they convert to
thick layers of fat. The temperature of cold-blooded animals is always maintained slightly
below the outside temperature to prevent the loss of body moisture through evaporation.
Poikilotherms avoid excessive body temperatures by favoring cool, dark places. Therefore
poikilotherms rely upon behavioral mechanisms to regulate body temperature. The rate of
heating and cooling in poikilotherms decreases as body size increases. Many insects
hibernate as larvae or grubs. These hide under dead leaves, lie in rotting wood, or
burrow into the ground. Most toads push their way down into the ground or hideout
under stones in ponds and streams. Their tadpoles lie in soft mud. Crocodiles also bury
themselves in the mud to hibernate. Snakes find shelter in holes and rocky dens. Being
constrained by the temperature of their environment, reptiles hibernate in regions where the
winter is cold. Reptiles regulate their body temperatures by taking advantage of different
sources of outside warmth, such as direct sunlight, warm stones and logs, and the heated
ground. By using such heat sources to varying degrees, individual species of animals can said
to be able to regulate their body temperature. Although heat gained by these organisms must
be exactly balanced by heat lost to its environment.
FIG#1 Thermal energy budget for a poikilotherm

A possible common mistake that may arise when researching hibernation is to
immediately relate it to a form of deep sleep. This is probably brought about by the cartoons or
idea of bears going into caves to sleep it out for the winter. Although this is not the case,
research has shown that sleeping is very different from hibernation. While sleeping, animals
relax, but their way of living does not change. Hibernators, however, almost stop living. Some
poikilothermic animals such as some frogs and northern fish can become almost partly frozen
while many insects are frozen solid. During hibernation the body processes slow down.
Digestion ceases, circulation is reduced, and the immune and other defense systems slow
down or stop. When poikilothermic animals enter hibernation their breathing slows down as
well as the beating of their hearts. One possible source of error between the distinction of
hibernating and sleeping is the research that showed if the weather surrounding a
poikilothermic animal becomes dangerously cold then that animal would “awaken” out of
hibernation, move about in order to raise the temperature of its body, and then reenter
hibernation. It does this to avoid freezing to death. It is a behavioral mechanism for survival.
FIG#2 The arousal effects during the time of hibernation versus temperature

Other interesting research done concerning poikilothermic hibernation is in
determining how the animal knows when to begin hibernation and when to end. For mammals,
its known that hibernation is triggered by the SCN cells of the hypothalamus which are
temperature sensitive, but its different for poikilothermic animals. Some scientists believe that
they inherit this tendency to hibernate, but this tendency is influenced by other factors such as
cold, the animal’s fat storage, and light. That hibernation is a response to the onset of cooler