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Estivation is a state of dormancy that is assumed by animals during the summer and is similar in many respects to hibernation. The primary difference is that estivation is experienced during summer as opposed to hibernation winter. Lowered metabolic rate and inactivity are some of the characteristics of animals in estivation, and this is in response to high temperatures, particularly in arid areas. Estivation takes place during periods of dryness and heat, dry and hot seasons that are characteristics of the summer season. Some animals, both vertebrates and invertebrates, estivate to avoid the danger of desiccation and damage due to high temperatures. Also, aquatic and terrestrial animals enter into estivation. The mechanism of estivation is possible to have evolved more than a hundred million years ago.Ìý

Estivation And Evolution

Estivation or aestivation is derived from the Latin word for summer (Aestas) or heat (Aestus). Arid conditions that limit the availability of food and water are some of the fundamental triggers of estivation. Scientists believe that estivation is one of the ancient traits, and records from fossils give evidence of structures indicating estivation. According to scientists, they have discovered earthworm chambers of the Pleistocene period and burrows of cretaceous lungfish of the Devonian era. There are discoveries also of burrows of Permian lysorophid that dates back to hundreds of millions of years ago. Estivation is undoubtedly one of the oldest phenomena because present organisms, which are believed to be the oldest life forms of metazoans such as nematodes and sponges, utilize hypometabolism (Dauer, and Diapause among others.)Ìý

Hypometabolism

Scientists believe that all organisms are driven to develop, grow, and reproduce by some selfish genes. The basic inputs required for this are energy (primarily ATP and their reducing equivalents, mainly derived from animal’s oxygen-based respiration), nutrients (both fuels required for the production of energy and building blocks needed for biosynthesis), and water (that is the universal solvent). If one of these basic inputs for life is limited or not available, the organism develops a strategy of self-preservation to escape death. The strategy involves a significant suppression of metabolic rates and changing to a hypometabolic state. Through strong suppression of universal demands of metabolism together with reprioritization of utilizing energy to maintain minimum critical functions, the organisms gain an edge or advantage in extending the time they can remain alive using energy reserves in their bodies. The duration is sufficient for survival until such time when conditions become favorable again for an active life. Hypometabolism is the critical feature in estivation among other conditions such as diapause, dormancy, torpor, hibernation, anaerobiosis, anhydrobiosis, and freeze tolerance.ÌýÌý

Physiological Condition Of Estivating Animals

Estivating organisms appear to be in a light state of dormancy because their physiological conditions can be reversed quickly, and the organisms can rapidly return to a normal condition within a short time. Studies on milk snailÌý(Otala lactea),Ìýwhich is native to North America and other parts of Europe, have shown that they can awaken from estivation within ten minutes following introduction to a wet environment. The biochemical and physiological concerns of an estivating animal are mainly conservation of energy, rationing the use of stored energy, retention of body water, handling of nitrogenous waste products, and stabilizing of body organs, macromolecules, and cells. This can be a challenging task because arid conditions and hot temperatures can last for several months. During estivation, metabolic rates are significantly depressed, causing a reduction in degradation and macromolecule synthesis. The estivating animals have to elevate chaperone proteins and enhance antioxidant defenses to stabilize the macromolecules. This strategy is utilized across all types of hypometabolism. This unique biochemical and physiological adaptation are the core attributes of hypometabolism in the animal kingdom. This means that all estivating animals appear to go through similar physiological processes as hibernating animals.ÌýÌý

The Problem Of Urea Storage

During estivation, urea is the nitrogenous end-product after metabolizing amino acids. Typically, urea accumulates in the body tissues of the organism because there is no excretion taking place. A similar occurrence of elevated concentration of urea in body tissues is experienced in several amphibians during dehydration. South Asian frogÌý(Rana cancrivora)Ìýthat lives in brackish water also concentrates urea in its body tissues to maintain osmotic balance with its environment. Estivation among the lungfish is similar to dehydration in amphibians because they are unable to excrete the nitrogenous wastes. The lungfish, however, is different compared to other species because its nitrogenous by-product is mainly ammonium (NH4+). Therefore the organism has to change the form of nitrogenous waste products and store them in the tissues as well. When the organism goes into estivation, there is a change in relative activities of different enzymes responsible for nitrogen metabolism. However, because the biosynthesis rate of urea does not change significantly during estivation, then the changeover is believed to be related to the control of the production of ammonium.Ìý Ìý

Estivation In Insects

Estivation in insects is one of the critical stages to go through summer in diapause or quiescence. For instance, the African chironomid midgeÌý(Polypedilum vanderplanki)Ìýlarva that is found in temporary pools can go into estivation when water evaporates. The larvae of the midge can come out of estivation if they are immersed in water, even if they have been dormant for years. The larvae go into a state of cryptobiosis and can withstand the loss of water content in their bodies to about 4% only, and they can survive brief exposure to the high temperature of between -454oF (−270°C) and 215.6oF (102°C). Similarly, the brown locustÌý(Locustana pardalina)Ìýfound mainly in the arid regions of South Africa can remain in dry soil for several years and can lose water content to about 40%. When it rains, they absorb water and development resumes instantly. When they hatch, it results in outbursts of huge populations of locusts. The phenomenon is dramatic, and information is scanty on the number of insect species that estivate in the arid tropics. Other insects that have been observed to estivate include lady beetlesÌý(°ä´Ç³¦³¦¾±²Ô±ð±ô±ô¾±»å²¹±ð)Ìýand mosquitoes. False honey ants are typically active during winter and estivate in temperate climates. Others include Bogong moths that estivate during summer to escape the heat and cope with lack of food sources

ÌýEstivation In fish

African lungfish (Protopterus annectens)Ìýis one of the fascinating animals that have puzzled scientists for a long time. The African lungfish’s brain is capable of coordinating the whole-body response to induce estivation and awaken from estivation. The African lungfish occupy a critical position in the evolution path of vertebrates, particularly regarding the transition from water to land. The fish has six extant species, out of which four are found in Africa. African lungfish can estivate during desiccation when they enter into a state of torpor in mud cocoon, where they survive without water or food for a period of up to 5 years.ÌýÌý

Cells Response To dehydrationÌý

One of the most critical issues for the estivating organisms that have to spend several weeks, months, and even years in an arid environment while in dormancy, is the control of body hydration together with the related osmolarity of body fluids and ionic strength. Scientists have studied the response of African clawed frogÌý(Xenopus laevis)Ìýto dehydration. The species is an exclusive aquatic amphibian found mainly in the southern part of Africa. The frog faces the seasonal drying of ponds, its typical habitat. In response, the frog either migrates overnight to other water sources or by digging into the damp subsoil of the drying pond and entering into a state of estivation. They can endure significant desiccation by losing up to 32% to 35% of their entire body water. When they are under dehydration stress, they increase the production of nitrogenous osmolytes. They also increase the amino acids and ammonia that increase up to twofold or threefold. Production of Urea also increases up to 15-fold or 20-fold, and this is to enhance retention of water as well as the uptake of water from the damp soil.Ìý

Different Phases Of EstivationÌý

Estivation, particularly among the African lungfish, has three phases that include induction, maintenance, and awakening. At the induction stage, the estivating lungfish can detect environmental cues and transforms into internal signals that trigger the necessary biochemical, physiological, structural, and behavioral changes to prepare the lungfish for estivation. During this phase, the lungfish hyperventilates and secretes mucus-like substance, which turns into a dried cocoon between 6 to 8 days. The second phase of estivation is maintenance, which starts when cocoon completely encases the lungfish. During this stage, all locomotor activities and feeding cease completely. The lungfish in this stage have to preserve biological structures, prevent cell death, and maintain a slow production of waste to avoid polluting its internal environment. The third stage is arousal or awakening that happens almost instantly, water returns. Following awakening, the fish has to excrete all the accumulated wastes and has to feed for growth and repair. During desiccation, the brain of the lungfish processes all internal cues and coordinates the response of the body to induce estivation. Similarly, the same procedure is used o awaken the body from estivation as soon as water becomes available.ÌýÌý