Aestivation

Aestivation (from Latin aestas, summer, but also spelled "estivation" in the USA) is a state of animal dormancy, characterized by inactivity and a lowered metabolic rate, that is entered in response to high temperatures and arid conditions. It takes place during times of heat and dryness, the hot dry season, which is often but not necessarily the summer months.

Invertebrate and vertebrate animals are known to enter this state to avoid damage from high temperatures and the risk of desiccation. Both terrestrial and aquatic animals undergo aestivation.

Molluscs
Gastropoda: Some air-breathing land snails, including species in the genera Helix, Cernuella, Theba, Helicella, Achatina and Otala, commonly aestivate during periods of heat. Some species move into shaded vegetation or rubble. Others climb up tall plants, including crop species as well as bushes and trees, and will also climb man-made structures such as posts, fences, etc.

The habit of climbing vegetation to aestivate has caused more than one introduced snail species to be declared an agricultural nuisance.

To seal the opening to their shell to prevent water loss, pulmonate land snails secrete a membrane of dried mucus called an epiphragm. In certain species, such as Helix pomatia, this barrier is reinforced with calcium carbonate, and thus it superficially resembles an operculum, except that it has a tiny hole to allow some oxygen exchange.

There is decrease in metabolic rate and reducing rate of water loss in estivating snails like Rhagada tescorum, Sphincterochila boissieri and others.

Arthropods
Insecta: Lady beetles (Coccinellidae) have been reported to aestivate.

Crustacea: Many land crabs spend dry seasons in an inactive state at the bottom of their burrows.

Reptiles and amphibians
Non-mammalian animals that aestivate include North American desert tortoises, crocodiles, and salamanders. Some amphibians (e.g. the cane toad and greater siren) aestivate during the hot dry season by moving underground where it is cooler and more humid. The California red-legged frog may aestivate to conserve energy when its food and water supply is low.

The Water-holding Frog has an aestivation cycle. It buries itself in sandy ground in a secreted, water-tight mucus cocoon during periods of hot, dry weather. Australian Aborigines discovered a means to take advantage of this by digging up one of these frogs and gently squeezing it, causing the frog to release some of the fresh water it stores for itself in its bladder and skin pockets. This water can be drunk by the Aborigine, who then releases the frog.

African lungfish also aestivate.

Mammals
Although relatively uncommon, a small number of mammals aestivate. Animal physiologist Kathrin Dausmann of Philipps University of Marburg, Germany, and coworkers presented evidence in the 24 June 2004 edition of Nature that the Malagasy fat-tailed dwarf lemur hibernates or aestivates in a small tree hole for seven months of the year. According to the Oakland Zoo in California, East African Hedgehogs are thought to estivate during the dry season.