Our understanding of how reptiles and amphibians react to humidity or barometric changes is very limited. Some species (mainly tropical amphibians), breed during, or immediately after rain, irrespective of the time of the year. This is obviously and important consideration if an attempt is to be made to breed these species. Members of at least one family of aquatic amphibians are also stimulated to breed at the advent of the rainy season, but whether they are reacting to an increase in the water level or a sudden drop in temperature, or even to variations in the chemistry of the water, is unknown. Manipulations which vary the degree of humidity from to times are therefore bound to be of somewhat experimental nature, but periods of "drought" followed by a thorough spraying or raising of water levels are worth considering if it is intended to breed species which would normally experience these conditions. Fluctuations in humidity throughout the day will automatically be brought about by temperature changes, in captivity as in nature.
Notwithstanding the above, the importance of adequate ventilation should not be overlooked; a beautifully set-up and maintained vivarium can lose much of its appeal as soon as the lid is removed, releasing a build-up of stagnant and decidedly unsavory vapor. The problem most likely to arise where aquaria is used as accommodation because ventilation is restricted to the lid - carbon dioxide and the other gaseous products of metabolism are heavier than air and so accumulate in the bottom few inches of the container, exactly where the animals are living and breathing (or trying to breathe). A side effect is that it becomes very difficult to grow plant in such an atmosphere.
Where a hot, dry environment is required, for example in cages housing many Frogs and snakes, ventilation is easily provided: much of the lid can be covered with mesh, if a spotlight or similar heat source is used, a convection curent will be set up as the stale air in the tank warms up, rises and exits through the top. In cages with one or more sides made of wood it is relatively easy to incorporate a panel of perforated metal or plastic near the bottom to increase further the rate of air exchange.
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This leaves us with cages containing animals which require a moist or humid environment, such as amphibians and some of the small Frogs and snakes (especially those coming from rainforest habitats). Very often these cages are provided with almost no ventilation in order to maintain a high humidity, resulting in the conditions described above. It is important to realize, however, that in areas of high humidity air is constantly moving, and that in nature stagnant air rarely accumulates. several ways of creating a high humidity without sacrificing ventilation are available.
The simplest is to spray the tank frequently with tepid water in order to replace the moisture lost through evaporation; the disadvantage here is that the surface will dry out rapidly unless spraying is carried out very frequently indeed.
An alternative method is to arrange for a portion of the cage to contain standing water by dividing it into two parts or by placing a container of water in it. This will only need topping up occasionally and also has the advantage of creating a humidity gradient where the animas will find the part of the cage which suits them best. In fact, quite a large number of amphibians prefer an environment which is quite dry, many toads and a number of tree-frogs fall into this category. In either of the above arrangements air circulation can be improved still further by introducing air from outside the cage using a small diaphragm pump, with tubing leading either into the water or into a corner of the land area.