Aestivation

Aestivation (also spelled estivation in American English, from the Latin *aestas* for summer) is a dormant state in animals and plants that resembles hibernation but occurs during summer. It involves inactivity and a reduced metabolic rate, triggered by high temperatures and dry conditions, typically during the hottest months.

Both invertebrates and vertebrates enter aestivation to avoid heat damage and the risk of drying out. The state occurs in terrestrial and aquatic animals, as well as in many plants—for instance, cyclamens produce foliage and flowers in winter but become dormant in summer. Fossil evidence indicates aestivation may have evolved hundreds of millions of years ago.

**Physiology**

Aestivating organisms are in a relatively light dormancy, as their physiological state can reverse quickly, allowing a rapid return to normal activity. The main challenges for an aestivating animal are conserving energy, retaining water, rationing stored energy, managing nitrogen waste, and stabilizing organs, cells, and macromolecules. These tasks can be demanding when hot, arid conditions last months or even years. The drop in metabolic rate reduces the synthesis and breakdown of macromolecules; to stabilize them, aestivators boost antioxidant defenses and increase chaperone proteins. This strategy is common across all forms of hypometabolism, and the physiological processes in aestivating animals closely mirror those in hibernating animals.

**Plants**

Aestivation is widespread in plants, especially among autumn-, winter-, or early-spring-flowering bulbs (like snowdrops and daffodils) and tuberous plants (such as cyclamens) in Mediterranean climates. These plants become dormant during the hot, dry summers typical of such regions.

**Invertebrates**

*Mollusca*: Some air-breathing land snails—including species in the genera *Helix*, *Cernuella*, *Theba*, *Helicella*, *Achatina*, and *Otala*—commonly aestivate during heat. Certain species move to shaded vegetation or rubble, while others climb tall plants (including crops, bushes, and trees) or human-made structures like posts and fences. This climbing habit has led some introduced snail species to be declared agricultural nuisances. To seal their shell opening and prevent water loss, pulmonate land snails secrete a dried mucus membrane called an epiphragm. In species like *Helix pomatia*, this barrier is reinforced with calcium carbonate, resembling an operculum but with a tiny hole for oxygen exchange. Aestivating snails such as *Rhagada tescorum* and *Sphincterochila boissieri* show decreased metabolic rates and reduced water loss.

*Arthropoda*: Lady beetles (Coccinellidae) and another beetle, *Blepharida rhois*, aestivate when temperatures rise, re-emerging in late summer or early fall. Mosquitoes also reportedly aestivate. False honey ants are winter-active and aestivate in temperate climates. Bogong moths aestivate over summer to avoid heat and food shortages. Adult alfalfa weevils (*Hypera postica*) aestivate during summer in the southeastern United States, with dampened metabolism, respiration, and nervous system activity. Among crustaceans, the Australian crab *Austrothelphusa transversa* aestivates underground during the dry season.

**Vertebrates**

*Reptiles and amphibians*: Non-mammalian vertebrates that aestivate include North American desert tortoises, crocodiles, and salamanders. Some amphibians, such as the cane toad and greater siren, move underground to cooler, more humid conditions during hot, dry periods. The California red-legged frog may aestivate to conserve energy when food and water are scarce. The water-holding frog has an aestivation cycle: it buries itself in sandy ground inside a secreted, watertight mucus cocoon during hot, dry weather. Australian Aboriginals discovered they could dig up one of these frogs and squeeze it to release its bladder—producing up to half a glass of dilute urine that can be drunk, though this kills the frog, which cannot survive until the next rainy season without its stored water. The western swamp turtle aestivates to survive hot summers in the ephemeral swamps it inhabits, burying itself in various media depending on location and available substrates.

*Fish*: African lungfish and salamanderfish can aestivate.

*Mammals*: Aestivation is relatively rare in mammals. In a 2004 *Nature* paper, animal physiologist Kathrin Dausmann and colleagues reported 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, four-toed hedgehogs are thought to aestivate during the dry season.