We have all heard of certain animals sleeping through the long winter months and most of us have probably wondered what happens to them when they do this.

This hiding away for a long sleep, or hibernation, as it is called, commences when the food of the animal begins to get scarce, and the length and depth of the sleep depends on the habit and constitution of the animal.

Bats, bears, some animals of the rodent order, such as the porcupine, the dormouse, some squirrels, etc., all the animals belonging to the classes of Amphibia and Reptilia, such as tortoises, lizards, snakes, frogs, etc., and many species of mollusks and insects, hibernate more or less completely, retiring to suitable places of concealment - the bat to dark caves, the hedgehog to fern-brakes, snakes to holes in trees, etc.

During hibernation there is a great decrease of heat in the bodies of the animals, the temperature sometimes sinking to 40° or even 20° F., or in general to a point a little above that of the surrounding atmosphere. The respiration as well as the pulsation of the heart is exceedingly slow, and the irritability of the animal often so low that in some cases it can be awakened only by strong electric shocks.

With frogs and amphibious reptiles "the dormant state is very common, and if the temperature is kept low by artificial means they may remain dormant for years.

The term "aestivation" has been used to describe a similar condition into which certain animals, such as serpents and crocodiles, in tropical countries pass during the hottest months of the year.