Estivation (or aestivation) is a survival mechanism. It is the process by which an organism becomes dormant during certain adverse circumstances. What differentiates estivation from hibernation is, in large part, the season or conditions. Estivation occurs in many large animals. Climates that encourage estivation include both desert climes, temperate climates, and seasonal floodplains.
When an animal estivates, it slows down its breathing, heart rate, and other metabolic processes. This decreases its body's need for water. Organisms can thereby survive hot or dry periods in which they might otherwise die.
Desert organisms are apt to estivate during the long hot dry spells, and come to life in the brief periods of moisture after rainfall. Desert plants and insects are the usual estivators - some plants dry up into brittle brown shells that appear completely dead, yet spring to life with the addition of water. Other desert organisms that estivate include reptiles, snails, tortoises, and the African hedgehog. One unusual find in the southern Sahara is a species of crocodile which estivates during the dry hot season. It essentially sinks into a state of torpor, not eating, and only minimally active. They occasionally come out of the cool burrows at night to lie on the rocks.
In more temperate climates, reptiles and amphibians are the most likely to estivate. Worms also estivate, crawling down into the soil and rolling up in a ball to preserve moisture during dry spells. Some species of fish burrow into the mud as the ponds, streams, or lakes they live in dry up. When water returns they come out of dormancy and repopulate the waters. Some fish (such as lungfish), frogs, and salamanders actually form a cocoon to prevent transdermal water loss. Turtles, salamanders, frogs, snails, slugs, and many insects are common temperate-climate estivators.
Vast floodplains such as those in Africa and South America also host organisms which estivate. Reptiles and amphibians are the most common types in these environs, although many water insects, fish, and plants also estivate.
By far the most numerous estivating organisms are microbes. Many bacteria, amoebas, and fungi have the ability to estivate either by encapsulating themselves, or by alteration of their biochemistry to survive in a dry state. Pathogenic bacteria can often survive for years in dried bodily fluids. Mycobacterium tuberculosis is an excellent example of the staying power some bacteria can achieve. (Wash your hands!)