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Acclimatization (/ə.ˌklaɪ.mə.tə.ˈzeɪ.ʃən/) is the biological process by which an organism adjusts to a change in its environmental conditions in order to maintain performance. Organisms can acclimatize to a variety of environmental changes — altitude, temperature, humidity, photoperiod, pH, and salinity —using biochemical, morphological, and behavioural methods. Acclimatization occurs over a short period of time (hours to weeks) and also occurs within an organism's lifespan (this is in contrast to adaptation, which is an evolutionary change that takes place over many generations). Furthermore, acclimatization may be a discrete occurrence (like when mountaineers acclimatize to high altitudes) or may instead be part of a periodic cycle (such as mammals shedding heavy winter fur in favor of a lighter summer coat). While the capacity to acclimatize to novel environments is well documented in thousands of species, researchers still know very little about how and why organisms acclimatize the way that they do.

Terminology
In general vocabulary, the nouns acclimatization and acclimation — and the corresponding verbs acclimatize and acclimate — are widely regarded as synonymous. In some discipines, it has been asserted that they should be differentiated by reserving acclimatization for natural processes and acclimation for changes occurring in response to an artificial or controlled situation. However, this assertion is not widely known or followed (as the foregoing citations of 6 major dictionaries show). Writers who intend to use "acclimatization" must explicitly state their usage if they expect their intended meaning to be received by their audience.

Medical usage
In a medical context — as was the case with general vocabulary — acclimatization and acclimation are used synonymously. Distinction between the two terms is insignificant in medical settings due to an application-based and organism-scale focus.

Physiological usage
In physiology, the aforementioned division between acclimatization and acclimation is emphasized. Acclimatization is reserved for reversible responses to multi-faceted environmental changes (e.g., animals shedding heavy winter fur with natural seasonal change), while acclimation is designated for reversible responses to isolated changes in a single variable (e.g., changes resulting from experimental manipulation of temperature).undefined This distinction is important because physiology involves multiple levels of biological organization and has a research-based orientation.

Ecological usage
In ecology, acclimatization (or species acclimatization) describes the systematic introduction of organisms to a new region. This definition dates back to 1857, when Edward Wilson (the founder of the acclimatisation society) pushed for the introduction of non-native livestock in Australia. Wilson's motive in doing so revolved around the numerous economic, political, and cultural benefits that came with non-native species introduction. Contemporary species acclimatization efforts have focused more on the conservational value of species introduction. These endeavors are commonly used to fill the ecological niches left by extinct or extirpated species. For example, when introduced to urban regions, the Norway maple will replace the air-purifying and habitat-providing roles of less hardy trees that were unable to survive in the area.

Altitude
Acclimatization in response to high altitudes is a "truer" form of acclimatization in the sense that it fits both the general and physiological definitions. Increased altitude imposes a number of stressors — low temperature, less humidity, shifts in pH, and decreased oxygen concentration — that an organism must deal with in order to maintain function. This change in response to a complex environmental shift (involving many factors) is the epitome of physiological acclimatization. The process generally involves a change in metabolic pathways and respiratory function.

Acclimatization to high altitude continues for months or even years after initial ascent, and ultimately enables humans to survive in an environment that — without acclimatization — would kill them. Humans who migrate permanently to a higher altitude naturally acclimatize to their new environment by developing an increase in the number of red blood cells. This enhances the oxygen carrying capacity of the blood and helps to compensate for lower levels of oxygen intake.

Temperature
Acclimatization in response to temperature is critical for a number of reasons; the most important of these is the maintenance of enzymatic reactions. A variety of physiological processes can be altered in order to maintain suitable temperature. When heat is elevated, humans produce a larger volume of sweat at more dilute concentrations to facilitate evaporative cooling (an unacclimatized person produces sweat with a salinity of up to 60 mEq/L, while an acclimatized person would produce sweat at around 5 mEq/L). In addition, plasma volume, heart rate, and capillary activation are also affected. Most organisms can adjust the chemical composition of their cell membranes to allow for more fluidity when it is cold and greater viscosity when it is hot, or code for heat shock proteins that may act as molecular chaperones and help the cell maintain function under periods of extreme stress.

Organisms who allow their temperature to fluctuate, like many temperate lizards, have shown some ability to use behavioural processes for acclimatization. These lizards showed superior functioning in their environments than lizards that lacked this capacity to acclimatize; these lizards were able to run and maintain activity levels that exceeded those of their non-acclimatized counterparts.

Humidity
Humidity is one of the fundamental abiotic factors that determine which animals and plants can survive in a given environment. While stomata and vein density is largely fixed based on adaptive evolution and natural selection, acclimatization to variable humidities can be achieved through differential epidermal cell expansion in many plants.

Photoperiod
Photoperiodism is a form of acclimatization relative to the length of nights or dark periods. It is primarily observed in plants but can also be observed in animals. Many angiosperms use photoreceptor proteins — like cytochromes or phytochromes — to detect changes in photoperiod and acclimatize accordingly. Plants can be classified into three groups relative to their photoperiodic changes and flowering tendencies: short-day plants (for example, rice), long-day plants (like carnations) , and day-neutral plants (like roses).

pH
pH is the measure of the acidity or basicity of an aqueous solution. pH levels can have immense effects on an organism; from organ-level operation all the way down to cellular and enzymatic functioning. A notable example of pH acclimatization is the process by which tropical fish are attuned to domestic fish tanks through the use of acclimatization bags. In incremental steps, the fish are exposed to bags with more neutral pH until they are ready to be transferred to the tank.

Salinity
The regulation of salinity (the concentration of salt dissolved in water) and the correlated process of osmoregulation are critical to ensuring proper organism function. The cells of organisms are bathed in aqueous mediums. If the medium has an excess proportion of salt (or other solutes) than cells may crenate; if the medium has too little salt (or other solutes) than cells may lyse. Migratory fish species may be anadromous (migrating from sea to freshwater) or catadromous (migrating from freshwater to sea). Regardless of their classification, migratory fish — like salmon and bass — are able to adjust to changes in salinity through gradual acclimatization as they move down gradients between habitats. However, if these fish were to be translocated directly between habitats with different salinity without gradual acclimatization, many would be unable to survive.

Beneficial acclimation hypothesis
Since researchers first began to study acclimatization, the overwhelming hypothesis has been that all acclimatization functions to enhance the performance of an organism. This idea has come to be known as the beneficial acclimation hypothesis. Despite early widespread support, studies have arose that show that acclimatization does not always serve to enhance performance. One of the major objections to the beneficial acclimation hypothesis is that it assumes that there are no costs associated with acclimatization. However, a number of costs have been identified. These include the costs of sensing environmental conditions and regulating responses, producing the structures required for plasticity (e.g., the energetic costs that come with expressing heat shock proteins), and a variety of genetic tradeoffs (e.g., linkage of plasticity-related genes with harmful genes).

Contemporary study
Given the shortcomings of the beneficial acclimation hypothesis, researchers are continuing to search for a theory that can be supported by empirical data. Contemporary research on acclimatization has focused more heavily on the evolution of phenotypic plasticity rather than acclimatization responses. Scientists believe that if they can comprehend how organisms evolved the capacity to acclimatize, they can better understand the mechanisms by which the process itself is carried out. Furthermore, current research is often oriented towards how anthropogenic influences may act on developed acclimatization responses.