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Aestivation (Latin:aestas (summer); also spelledestivation inAmerican English) is a state of animaldormancy, similar tohibernation, although taking place in the summer rather than the winter. Aestivation is characterized by inactivity and a loweredmetabolic rate, that is entered in response to high temperatures and arid conditions.[1] It takes place during times of heat and dryness, which are often the summer months.
Invertebrate and vertebrate animals are known to enter this state to avoid damage from high temperatures and the risk ofdesiccation. Both terrestrial and aquatic animals undergo aestivation. Fossil records suggest that aestivation may have evolved several hundred million years ago.
Organisms that aestivate appear to be in a fairly "light" state of dormancy, as their physiological state can be rapidly reversed, and the organism can quickly return to a normal state. A study done onOtala lactea, a snail native to parts of Europe and Northern Africa, shows that they can wake from their dormant state within ten minutes of being introduced to a wetter environment.
The primary physiological and biochemical concerns for an aestivating animal are to conserve energy, retain water in the body, ration the use of stored energy, handle the nitrogenous end products, and stabilize bodily organs, cells, andmacromolecules. This can be quite a task as hot temperatures and arid conditions may last for months, in some cases for years. The depression of metabolic rate during aestivation causes a reduction in macromolecule synthesis and degradation. To stabilise the macromolecules, aestivators will enhance antioxidant defenses and elevatechaperone proteins. This is a widely used strategy across all forms of hypometabolism. These physiological and biochemical concerns appear to be the core elements of hypometabolism throughout the animal kingdom. In other words, animals which aestivate appear to go through nearly the same physiological processes as animals that hibernate.[2]
Gastropoda: some air-breathingland snails, including species in the generaHelix,Cernuella,Theba,Helicella,Achatina andOtala, 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 human-made structures such as posts, fences, etc.
Their 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 anepiphragm. In certain species, such asHelix pomatia, this barrier is reinforced with calcium carbonate, and thus it superficially resembles anoperculum, except that it has a tiny hole to allow some oxygen exchange.[citation needed]
There is a decrease inmetabolic rate andreduced rate of water loss in aestivating snails likeRhagada tescorum,[3]Sphincterochila boissieri and others.
Insecta: Lady beetles (Coccinellidae) have been reported to aestivate.[4] Another type of beetle (Blepharida rhois) also chooses to aestivate. They usually do so when the temperature is warmer and will re-emerge in the late summer or early fall.[5] Mosquitoes also are reported to undergo aestivation.[6] False honey ants are well known for beingwinter active and aestivate in temperate climates.Bogong moths will aestivate over the summer to avoid the heat and lack of food sources.[7] Adult alfalfa weevils (Hypera postica) aestivate during the summer in the southeastern United States, during which their metabolism, respiration, and nervous systems show a dampening of activity.[8][9]
Crustacea: An example of a crustacean undergoing aestivation is with the Australian crabAustrothelphusa transversa, which undergoes aestivation underground during the dry season.[10]
Non-mammalian animals that aestivate includeNorth American desert tortoises,crocodiles, andsalamanders. Some amphibians (e.g. thecane toad andgreater siren) aestivate during the hot dry season by moving underground where it is cooler and more humid. TheCalifornia red-legged frog may aestivate to conserve energy when its food and water supply is low.[12]
Thewater-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 Aboriginals discovered a means to take advantage of this by digging up one of these frogs and squeezing it, causing the frog to empty its bladder. This dilute urine—up to half a glassful—can be drunk. However, this will cause the death of the frog which will be unable to survive until the next rainy season without the water it had stored.[13]
Thewestern swamp turtle aestivates to survive hot summers in the ephemeral swamps it lives in. It buries itself in various media which change depending on location and available substrates.[14] Because the species is critically endangered, thePerth Zoo began a conservation and breeding program for it. However, zookeepers were unaware of the importance of their aestivation cycle and during the first summer period would perform weekly checks on the animals. This repeated disturbance was detrimental to the health of the animals, with many losing significant weight and some dying. The zookeepers quickly changed their procedures and now leave their captive turtles undisturbed during their aestivation period.[citation needed]
African lungfish also aestivate[15][16] as cansalamanderfish.
Although relatively uncommon, a small number of mammals aestivate.[17] Animal physiologist Kathrin Dausmann ofPhilipps University of Marburg, Germany, and coworkers presented evidence in a 2004 edition ofNature that theMalagasyfat-tailed dwarf lemur hibernates or aestivates in a small tree hole for seven months of the year.[18] According to the Oakland Zoo in California,four-toed hedgehogs are thought to aestivate during the dry season.[19]
