Aestivation is similar to hibernation, if, indeed, it is not identical with it. If hibernation is to be called "winter sleep," aestivation may be with equal propriety, called "summer sleep." In zoology, it is defined as a state of reduced metabolic activity in which certain animals become quiescent. It is a resting interval associated with warm, dry periods in areas that have alternating wet and dry seasons. Animals are induced to aestivate when drought and heat interfere with their activities. With their bent for pathological interpretations, biologists also define aestivation as "the state of torpidity induced in animals by excessive dry heat." Physiological and physical quiescence should not be mistaken for a state of torpor. The same objections to calling it sleep that we made in the case of hibernation are also valid with reference to aestivation.

Aestivation is seen chiefly in the tropics during the long, hot, dry season, when food is scarce and vegetation is taking a rest. A few animals in the temperate zones, especially in the desert regions, also aestivate. Alligators, snakes, certain mammals, as taurec, insects and land snails become dormant.

During the dry season in the tropics the pools and streams dry up. The crocodiles aestivate in Summer, "sleeping" through the dry season without feeding or emerging from the mud in which they have buried themselves. It is said that they are able to "sleep" in this almost "lifeless" state for a whole year. The alligators, the American division of the crocodile family, hibernate in this country very much like frogs, but in the tropics they aestivate. When water is no longer obtainable the South American alligator, and some other animals, bury themselves in the mud, reduce their physiological activities to a bare minimum, while the earth above them is baked into a hard crust. When the rains come again, they resume activity, and come forward renewed by their long fast and rest.

Certain fish are able, when the pools and streams dry up, to burrow deep down into the mud and lie there until the coming of the rainy season. The mud-fish of Australia is an example of these fish, but many examples exist in the dry arid countries where summer, rather than winter, is the "hard time." Indeed, if we are to judge by the fish that may be found in a dry pond after a heavy rain, we may have such fish in this country. The lungfishes, Protoperis of Africa and Lipidosiren of South America, live in mud cocoons during the dry season. When the rice fields which it inhabits dry up during the drought, the spearhead fish, Opiocephidae, buries itself in the mud. Natives of Indo-Malaya "fish" for these animals with digging implements. The African lungfish digs into mud almost two feet, curls its tail around its body which becomes covered with mucus, and there exists, drawing air through a long tube and living on the breakdown of body fat and tail.

During droughts, planarians (flatworms) and leeches (annelids} bury themselves in mud. Small crustaceans, mollusks, etc., that are found in the pools and patches of water that frequently form in the desert, bury themselves deep in the clay or baked mud, when these pools dry up, and activate for long periods. Turtles activate in mud, while lizards and snakes retire to crevices. The Iberian water turtle hides under rocks.

Frogs burrow into mud and exist for months in its sunbaked hardness. During periods of aestivation frogs can survive the loss of half their body moisture. Certain Australian frogs become distended with water during the wet season and use this stored water during the aestivating period. This storing of water by these frogs is similar to the storage of fat by hibernating animals.

Birds are not known to activate, but a number of mammals, such as aardvark, Orycteropus, and some lemurs, Chirogale millii and Microcebus, undergo periods of quiescence.

Most prominent among aestivating animals of America are the land snails, although frogs, slugs, some fishes and other aquatic and semi-aquatic animals also aestivate. When the dry season comes, land snails secrete a membrane-like substance (epiphram), across the openings of their shells, leaving a small opening for the admission of air in breathing. Some snails secrete several diaphragms across the opening of their shells. There is an Australian snail that plugs the mouth of its shell with a morsel of clay before entering upon its period of aestivation. After a prolonged shower snails become active. Aestivating desert snails have been known to revive and crawl about after years in the dormant state. Records show that the African snail, Helix desertorum, may remain in aestivation as long as five years; the California desert snail, Helix veatchii, has become active after a six year aestivation period.

In the deserts of the world there are many plant-eating animals that lie dormant in times of drought, when vegetation is more scarce than during those periods when there is rainfall. There are many desert plants that also lie dormant during periods of drought. Both plants and animals fast during this period of dormancy.

In Australia the nymphs of a species of dragonflies aestivate in dry land. Slugs bury themselves in the ground and bivalve mollusks in the mud. Small crustaceans, mollusks, etc., that are found in the pools and patches of water that frequently form in the deserts, bury themselves deeply in the clay or baked mud, when these pools dry up and estivate for long periods.

While it seems that heat, dryness, and lack of food are the factors that induce aestivation, as cold and famine seem to induce hibernation, there is reason to believe that there is more to the practice than the mere existence of certain external factors. For example, the persistence of the aestivating habit is illustrated by the tenree, which in temperate zoological gardens, where food and water are abundant, aestivates at the time of their scarcity in its native Madagascar. This would seem to indicate that something other than scarcity of food and temperature is at work in aestivation, and, perhaps, also in hibernation.

A peculiar example of an animal that behaves opposite to aestivation is the Egyptian jerboa. It is said to be so closely adapted to dry conditions (of the desert) that rain or damp atmosphere induce it to pass into a dormant condition, in which state it does not eat.