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Acclimatization
Acclimatization
Acclimatization
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Acclimatization or acclimatisation (also called acclimation or acclimatation) is the process in which an individual organism adjusts to a change in its environment (such as a change in altitude, temperature, humidity, photoperiod, or pH), allowing it to maintain fitness across a range of environmental conditions. Acclimatization occurs in a short period of time (hours to weeks), and within the organism's lifetime (compared to adaptation, which is evolution, taking place over many generations). This may be a discrete occurrence (for example, when mountaineers acclimate to high altitude over hours or days) or may instead represent part of a periodic cycle, such as a mammal shedding heavy winter fur in favor of a lighter summer coat. Organisms can adjust their morphological, behavioral, physical, and/or biochemical traits in response to changes in their environment.[1] While the capacity to acclimate to novel environments has been well documented in thousands of species, researchers still know very little about how and why organisms acclimate the way that they do.

Names

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The nouns acclimatization and acclimation (and the corresponding verbs acclimatize and acclimate) are widely regarded as synonymous,[2][3][4][5][6][7] both in general vocabulary[2][3][4][5] and in medical vocabulary.[6][7] The synonym acclimation[4][6] is less commonly encountered, and fewer dictionaries enter it.

Methods

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Biochemical

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In order to maintain performance across a range of environmental conditions, there are several strategies organisms use to acclimate. In response to changes in temperature, organisms can change the biochemistry of cell membranes making them more fluid in cold temperatures and less fluid in warm temperatures by increasing the number of membrane proteins.[8] In response to certain stressors, some organisms express so-called heat shock proteins that act as molecular chaperones and reduce denaturation by guiding the folding and refolding of proteins. It has been shown that organisms which are acclimated to high or low temperatures display relatively high resting levels of heat shock proteins so that when they are exposed to even more extreme temperatures the proteins are readily available. Expression of heat shock proteins and regulation of membrane fluidity are just two of many biochemical methods organisms use to acclimate to novel environments.

Morphological

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Organisms are able to change several characteristics relating to their morphology in order to maintain performance in novel environments. For example, birds often increase their organ size to increase their metabolism. This can take the form of an increase in the mass of nutritional organs or heat-producing organs, like the pectorals (with the latter being more consistent across species[9]).[10]

The theory

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While the capacity for acclimatization has been documented in thousands of species, researchers still know very little about how and why organisms acclimate in the way that they do. Since researchers first began to study acclimation, the overwhelming hypothesis has been that all acclimation serves to enhance the performance of the organism. This idea has come to be known as the beneficial acclimation hypothesis. Despite such widespread support for the beneficial acclimation hypothesis, not all studies show that acclimation always serves to enhance performance (See beneficial acclimation hypothesis). One of the major objections to the beneficial acclimation hypothesis is that it assumes that there are no costs associated with acclimation.[11] However, there are likely to be costs associated with acclimation. These include the cost of sensing the environmental conditions and regulating responses, producing structures required for plasticity (such as the energetic costs in expressing heat shock proteins), and genetic costs (such as linkage of plasticity-related genes with harmful genes).[12]

Given the shortcomings of the beneficial acclimation hypothesis, researchers are continuing to search for a theory that will be supported by empirical data.

The degree to which organisms are able to acclimate is dictated by their phenotypic plasticity or the ability of an organism to change certain traits. Recent research in the study of acclimation capacity has focused more heavily on the evolution of phenotypic plasticity rather than acclimation responses. Scientists believe that when they understand more about how organisms evolved the capacity to acclimate, they will better understand acclimation.

Examples

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Plants

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Many plants, such as maple trees, irises, and tomatoes, can survive freezing temperatures if the temperature gradually drops lower and lower each night over a period of days or weeks. The same drop might kill them if it occurred suddenly. Studies have shown that tomato plants that were acclimated to higher temperature over several days were more efficient at photosynthesis at relatively high temperatures than were plants that were not allowed to acclimate.[13]

In the orchid Phalaenopsis, phenylpropanoid enzymes are enhanced in the process of plant acclimatisation at different levels of photosynthetic photon flux.[14]

Animals

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Animals acclimatize in many ways. Sheep grow very thick wool in cold, damp climates. Fish are able to adjust only gradually to changes in water temperature and quality. Tropical fish sold at pet stores are often kept in acclimatization bags until this process is complete.[15] Lowe & Vance (1995) were able to show that lizards acclimated to warm temperatures could maintain a higher running speed at warmer temperatures than lizards that were not acclimated to warm conditions.[16] Fruit flies that develop at relatively cooler or warmer temperatures have increased cold or heat tolerance as adults, respectively (See Developmental plasticity).[17]

Humans

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See also: Cold and heat adaptations in humans

The salt content of sweat and urine decreases as people acclimatize to hot conditions.[18] Plasma volume, heart rate, and capillary activation are also affected.[19]

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 to increase the oxygen carrying capacity of the blood, in order to compensate for lower levels of oxygen intake.[20][21]

See also

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References

[edit]
Revisions and contributorsEdit on WikipediaRead on Wikipedia

Acclimatization

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Acclimatization is the process by which an individual undergoes reversible physiological adjustments in response to gradual changes in its , such as variations in , altitude, or , enabling it to maintain , performance, and survival without genetic alterations. These adaptations typically develop over days to weeks through repeated exposure to the and are distinct from acclimation, which involves similar changes but in controlled or artificial settings, and from evolutionary , which occurs at the population level over generations via . In humans and other organisms, acclimatization manifests through coordinated responses tailored to specific environmental challenges; for instance, exposure to high altitudes triggers increased ventilation, elevated production via , and higher levels to enhance oxygen delivery, with partial effects emerging in 5-7 days and full acclimatization taking about three weeks at elevations above 3,000 meters. Similarly, heat acclimatization during repeated exposure to hot conditions leads to beneficial adaptations like earlier onset and greater volume of sweating, reduced , and improved plasma volume expansion, which collectively lower core body temperature and cardiovascular strain, often stabilizing within 7-14 days. Cold acclimatization, in contrast, enhances metabolic production, , and insulation through mechanisms like non-shivering , though these changes are generally less pronounced and more variable across individuals. These physiological shifts underscore acclimatization's role in short-term resilience to environmental stressors, with benefits that can diminish upon return to baseline conditions, typically within weeks to a month. Notable aspects of acclimatization include its phenotypic nature—altering traits like enzyme activity, hormone levels, or organ function without changing DNA—and its ecological significance in enabling organisms to cope with seasonal or migratory shifts in climate. For example, in plants, it involves adjustments in photosynthetic rates or stomatal conductance to drought or temperature fluctuations, while in animals, it supports activities like endurance exercise in extreme conditions. Research highlights variability influenced by factors such as age, fitness, and genetics, emphasizing the need for gradual exposure to avoid maladaptive responses like acute mountain sickness at high altitudes. Overall, acclimatization exemplifies the plasticity of living systems, bridging immediate survival strategies with long-term environmental interactions.

Definition and Overview

Core Definition

Acclimatization is the reversible, non-genetic adjustment of an organism's physiological, biochemical, or morphological traits in response to environmental changes, such as variations in temperature, altitude, or salinity, typically occurring over a timescale of hours to weeks. This process exemplifies phenotypic plasticity, involving changes in gene expression or cellular function without alterations to the underlying DNA sequence, and it is distinct from genetic adaptation due to its short-term, reversible nature upon return to the original environment. It is primarily triggered by environmental stressors that challenge homeostasis, enabling the organism to maintain performance and survival without heritable modifications. The term "acclimatization" originated in the early , derived from the French word "acclimatation," and was initially applied to the and of and animals to new climatic conditions during colonial expansions.

Biological Significance

Acclimatization plays a crucial role in enhancing organismal fitness by enabling physiological adjustments that allow tolerance to environmental variability, thereby reducing mortality risks during seasonal shifts or abrupt changes. This process supports the beneficial acclimation hypothesis, which posits that prior exposure to specific conditions improves performance and survival in those environments compared to unacclimated states. For instance, studies on intertidal oysters have demonstrated that acclimation to environmental stressors can significantly boost survival rates by mitigating pathogen loads and physiological strain. Similarly, in bacterial models like , heat acclimation has been shown to enhance survival under extreme temperatures, with improvements exceeding 50% in lethal conditions, underscoring its adaptive value in fluctuating climates. Ecologically, acclimatization influences limits by broadening the range of tolerable conditions, thereby shaping biogeographic patterns and community dynamics. It fosters resilience to by permitting thermal niche adjustments that buffer against rapid environmental shifts, potentially reducing the "climatic debt" where lag behind changing habitats. This plasticity also facilitates migration and successful of novel habitats, as acclimatized individuals exhibit higher establishment rates in variable ecosystems, contributing to overall maintenance. In practical applications, acclimatization informs strategies across multiple fields. In , it guides breeding programs for crops adapted to variability, enhancing yield stability in stressed environments. In , exploits acclimatization to elevate athletes' aerobic capacity and oxygen utilization, leading to performance gains at . For conservation, pre-translocation acclimatization protocols improve survival outcomes for relocated , such as , by aligning physiological states with new habitats and minimizing stress-induced mortality.

Terminology and Distinctions

Synonyms and Variants

The term acclimatization originates from the French verb acclimater, coined in the late 18th century, which combines the preposition à (to) with climat (climate), drawing from Latin roots ad- (toward) and clima (inclination or region). This etymology reflects the concept of inclining or adapting toward a new climatic condition. The noun form acclimatization first appeared in English in the 1820s, initially in agricultural contexts. Linguistic variants appear across languages, such as Aklimatisierung in German and aclmatación in Spanish, maintaining the core idea of climatic adjustment while adapting to phonetic and orthographic conventions. In , primary synonyms include acclimation, which is often used interchangeably with acclimatization in , though some distinctions persist: acclimation typically denotes controlled, laboratory-induced adjustments, while acclimatization refers to natural environmental processes. serves as a in contexts emphasizing behavioral responses to environmental stimuli. The term's usage evolved significantly in the 19th century, centered on and the introduction of non-native , as noted in Darwin's discussions of animal acclimatization for breeding and transplantation. By the mid-20th century, particularly post-1950s, its application shifted toward physiological , focusing on organismal responses to environmental stressors like and altitude. Acclimatization differs fundamentally from , as the former involves reversible, non-heritable physiological adjustments occurring over days to months in response to environmental changes, whereas entails heritable genetic modifications shaped by across multiple generations. Unlike , which permanently alters population-level traits through evolutionary processes, acclimatization does not involve genetic changes or and is typically lost upon return to original conditions. In comparison to acclimation, acclimatization generally refers to phenotypic responses developed under natural, field-based environmental conditions, while acclimation denotes similar physiological adjustments induced in controlled or experimental settings. Both processes are non-genetic and reversible, but acclimatization encompasses a broader scope of unpredictable stressors in wild contexts, whereas acclimation allows for precise manipulation of variables to study specific mechanisms. Acclimatization represents a targeted form of , specifically the reversible physiological responses to environmental stressors, but it excludes broader developmental plasticity such as changes in body size or morphology during . in general allows organisms to produce variable phenotypes from the same in response to environmental cues, yet acclimatization is distinguished by its focus on adult-stage stress acclimation rather than lifelong or developmental shifts. Unlike tolerance, which involves passive of environmental stress without active physiological modification, acclimatization entails dynamic, coordinated adjustments that enhance an organism's capacity to cope with stressors. Tolerance relies on inherent, static resistance mechanisms that do not change in response to exposure, whereas acclimatization actively reprograms to improve performance under altered conditions.

Mechanisms

Physiological Mechanisms

Acclimatization involves coordinated physiological adjustments at the organ and systemic levels to mitigate the impacts of environmental stressors, enabling organisms to maintain without permanent genetic changes. These mechanisms primarily encompass modifications in cardiovascular, respiratory, and osmoregulatory functions, as well as thermal regulatory processes, which collectively enhance survival and performance under altered conditions such as hypoxia, salinity fluctuations, or temperature extremes. In hypoxic environments, like high altitudes, peripheral chemoreceptors in the carotid bodies detect reduced arterial oxygen tension and trigger an immediate increase in ventilation to improve alveolar oxygen exchange. This ventilatory acclimatization elevates resting from 5-7 liters per minute at to approximately 15 liters per minute at 4,300 meters over about one week, thereby raising arterial . Cardiovascular responses complement this by initially increasing to elevate and oxygen delivery to tissues, with later decreasing due to plasma volume reduction, stabilizing overall output after several days at altitude. Osmoregulatory acclimatization occurs through modifications in ion transport across epithelial barriers, particularly in aquatic organisms facing salinity shifts. For instance, in fish transferred to , Na⁺/K⁺-ATPase activity surges to drive active sodium extrusion and secretion, preventing osmotic influx. In the fourspine (Apeltes quadracus), this activity increases 2.6-fold, peaking on days 7 and 14 post-transfer to maintain . Renal adjustments, such as enhanced reabsorption or ion excretion, further support ionic balance during these transitions. Thermal acclimatization entails vascular and hormonal shifts to regulate body temperature amid extremes. Heat exposure induces peripheral , increasing cutaneous blood flow for loss, alongside respiratory panting to boost evaporative cooling when ambient humidity limits sweating. In cold conditions, minimizes conductive heat loss, while elevated levels signal metabolic reprogramming to prioritize energy allocation for . Over longer periods, insulation enhances via physiological cues promoting fat deposition or pelage thickening. These adaptations operate across distinct time scales, with acute responses—such as in hypoxia or in —emerging within minutes to hours for rapid stabilization. Chronic changes, including increased production via stimulation in hypoxia or sustained enzyme upregulation in , develop over days to weeks, reflecting progressive systemic integration.

Biochemical Mechanisms

Biochemical mechanisms of acclimatization involve dynamic molecular pathways that enable cells to adapt to environmental stressors without altering the DNA sequence. These processes primarily occur through enzyme induction, metabolic reprogramming, and epigenetic regulation, allowing rapid and reversible responses at the cellular level. Enzyme induction and regulation play a central role in acclimatization by upregulating protective proteins in response to specific stressors. In thermal stress, heat shock proteins (HSPs) such as HSP70 are rapidly induced to prevent protein misfolding and aggregation; for instance, in heat-shocked fish, branchial HSP70 expression increases approximately 10-fold relative to controls during recovery. Similarly, cytochrome P450 enzymes are upregulated during exposure to toxins or xenobiotics, facilitating their detoxification; in specialist herbivores adapting to plant defenses, multiple P450 isoforms show significant upregulation to metabolize these compounds. Metabolic shifts further support acclimatization by reallocating energy pathways to cope with stressors like hypoxia. The exemplifies this, where anaerobic glycolysis is enhanced to compensate for reduced , leading to increased lactate production; in hypoxic conditions, this involves boosted activity of (LDH), which converts pyruvate to lactate, sustaining ATP generation. In high-altitude acclimatization, this glycolytic upregulation, coupled with decreased mitochondrial density, actively promotes lactate export to maintain cellular energy balance. Gene expression during acclimatization is modulated without DNA sequence changes, relying on epigenetic mechanisms for swift, heritable responses. acetylation, for example, loosens structure to enable rapid transcription of stress-response genes; in exposed to abiotic stressors like or , increased acetylation at key loci promotes defense gene activation and establishes long-lasting epigenetic memory for enhanced tolerance. Complementing this, antioxidant enzymes such as (SOD) are activated to mitigate ; during cold acclimatization, SOD activity rises alongside other ROS scavengers to protect against elevated from heightened metabolism. These modifications ensure cellular resilience by fine-tuning gene availability in real-time.

Morphological Mechanisms

Morphological mechanisms of acclimatization involve structural adaptations in organs, tissues, and cells that enhance organismal fitness in response to environmental stressors such as hypoxia, , or altered mechanical loads. These changes are typically reversible and occur through processes like , , or remodeling, allowing organisms to maintain without genetic alterations. Unlike transient physiological shifts, morphological adjustments provide longer-term , often triggered by biochemical signals but manifesting as physical alterations. Organ enlargement is a key morphological response in aquatic and semi-terrestrial facing oxygen limitation. In exposed to prolonged hypoxia, gill remodeling increases surface area through interlamellar cell mass reduction and lamellar protrusion, enhancing oxygen uptake efficiency over periods of weeks. In tadpoles exposed to chronic hypoxia, the skin becomes thinner with increased vascularization, enhancing cutaneous . These enlargements enable sustained by expanding storage or exchange capacities without requiring immediate behavioral changes. Tissue remodeling contributes to resistance against and mechanical challenges. In , stress induces cuticle thickening via enhanced deposition and structural modifications, reducing and improving water barrier properties in leaves. For woody , increased bark thickness limits conductance, thereby minimizing stem during prolonged dry conditions. In animals, acclimating to arid environments exhibit cuticle thickening and altered composition, which lowers permeability and cuticular water loss rates. Mammals in microgravity, such as during , undergo adjustments through reduced mineralization in bones, adapting to diminished load-bearing demands and preventing overload in altered gravitational contexts. At the cellular level, morphological changes include proliferation to bolster aerobic performance. In , high-altitude acclimatization over 28 days elevates mitochondrial volume density, particularly in intermyofibrillar regions, enhancing oxidative capacity without necessitating . These proliferations are reversible and concentrated in metabolically active tissues, allowing efficient energy production under hypoxic stress. Such adaptations are often limited to plastic tissues like muscle or , where remodeling is feasible without permanent damage. Morphological mechanisms are constrained by tissue plasticity and temporal scales, typically requiring weeks to months for full development, slower than physiological responses. They are confined to responsive structures, such as gills or cuticles, where or can occur, but rigid tissues like mature show limited reversibility. This gradual pace ensures structural integrity but may delay full acclimatization in rapidly changing environments.

Theoretical Foundations

Core Principles

Acclimatization operates through homeostatic feedback loops that enable organisms to maintain internal stability amid environmental perturbations. These loops primarily involve mechanisms, where specialized sensors detect deviations from optimal conditions and initiate corrective responses via effectors to restore balance. For instance, in , peripheral and central thermoreceptors relay signals to the , which acts as a control center to adjust physiological processes like sweating or , thereby counteracting temperature shifts and preventing cellular damage. This reactive regulation ensures short-term survival but can be complemented by anticipatory adjustments. The principle of extends by emphasizing predictive regulation, where organisms proactively alter their internal set points in anticipation of environmental demands, achieving stability through change rather than rigid constancy. Unlike traditional , which focuses on returning to a fixed equilibrium, involves dynamic variability in parameters such as levels or metabolic rates to optimize allocation during prolonged stressors, such as seasonal fluctuations. This anticipatory framework underlies successful acclimatization by minimizing reactive costs and enhancing resilience to predictable changes. Acclimatization is bounded by plasticity thresholds, representing the viable environmental range within which phenotypic adjustments improve and fitness; exceeding these limits results in , reduced function, or mortality. These thresholds vary by and —for example, in thermal contexts, critical thermal minima and maxima define the scope for beneficial plasticity, beyond which compensatory mechanisms fail. Quantitative models assess acclimatization efficiency. Responses during acclimatization exhibit predictability, as they are environmentally induced yet modulated by underlying , leading to consistent patterns across individuals of similar genetic backgrounds. These adjustments often align with dose-response curves, where the extent of phenotypic change scales nonlinearly with intensity—initial mild exposures elicit modest shifts, while escalating demands amplify responses up to saturation points, reflecting efficient resource use without overcompensation.

Evolutionary and Ecological Context

Acclimatization, as a manifestation of , likely originated with the emergence of multicellular life around 1.6 billion years ago, providing early organisms with a mechanism to buffer against fluctuating environmental conditions such as and variations. This capacity arose as genetically uniform cells in developing multicellular structures coordinated their responses through plastic adjustments, enabling survival in unstable habitats before fixed genetic adaptations could evolve. The trait has been conserved across eukaryotic kingdoms, facilitated by shared molecular pathways like (HSP) responses and (ROS) signaling, which activate conserved stress mitigation mechanisms in , animals, and fungi. For instance, IPK2-type kinases, involved in transducing thermal cues, trace back to early land evolution but reflect broader ancestral conservation in stress acclimation. In ecological contexts, acclimatization expands an organism's niche breadth by allowing reversible phenotypic adjustments that enhance fitness across heterogeneous environments, thereby promoting species persistence and distribution. It interacts with evolutionary processes by alleviating immediate selection pressures, as outlined in the , where initial plastic responses to novel conditions facilitate subsequent genetic assimilation of adaptive traits, accelerating long-term evolution. This dynamic reduces the intensity of , enabling populations to explore broader adaptive landscapes without immediate risk. Amid rapid , however, the pace of environmental shifts often exceeds acclimatization capabilities, leading to lags in thermal tolerance that heighten for many . For example, some populations, such as Hawaiian corals, have shown acclimatization rates of approximately 0.5°C per decade, but these often lag behind accelerating global warming rates of about 0.27°C per decade as of 2025, heightening . As of 2025, corals have reportedly crossed a tipping point, with warming rates of 1.2–1.4°C above pre-industrial levels exacerbating acclimatization lags and leading to accelerated reef decline. In urban ectotherms, heat tolerance adjustments lag behind warming by about 0.84°C per 1°C increase, underscoring limits to plastic responses under accelerated change. Acclimatization entails significant trade-offs, including elevated energy costs that can increase metabolic rates by up to 20%, diverting resources from growth and . These costs vary with life-history strategies: r-selected species, adapted to unpredictable environments, may tolerate higher plasticity expenses for rapid adjustments, while K-selected species in stable niches prioritize efficiency, limiting plastic investments to minimize reproductive impacts. Such trade-offs constrain the of plasticity, balancing short-term against long-term fitness.

Examples Across Organisms

In Plants

In plants, acclimatization to temperature fluctuations often involves adjustments in leaf orientation and photosynthetic levels to optimize energy capture while minimizing . In colder environments, such as alpine habitats, may enhance insulation through various mechanisms to reduce the risk of damage. acclimatization in primarily manifests through rapid stomatal and the synthesis of protective osmolytes to preserve cellular hydration. Under deficit, stomata close within minutes to hours via signaling, limiting and conserving soil moisture, which can reduce loss by 50-90% depending on severity, though this temporarily curbs CO₂ uptake and . Concurrently, compatible osmolytes such as accumulate in the to lower osmotic potential and maintain ; these responses collectively enhance survival. Acclimatization to varying light and nutrient availability drives plasticity in systems and flowering timing, allowing to exploit heterogeneous environments. In nutrient-poor soils, shifts toward longer, deeper laterals to access subsoil resources; for example, wild ( spontaneum) shows greater plasticity under low conditions. further enables acclimatization to seasonal light cues, where short-day like chrysanthemum () initiate flowering upon detecting day lengths below 12-14 hours, synchronizing reproduction with favorable conditions and avoiding energy expenditure during suboptimal periods.

In Animals

Acclimatization in animals often integrates physiological adjustments with behavioral strategies, leveraging their mobility to exploit microhabitats or migrate through varying environmental gradients. Unlike sessile organisms, mobile animals like birds and reptiles can actively seek optimal conditions while undergoing internal changes to cope with stressors such as hypoxia, thermal extremes, and osmotic challenges. These responses enhance survival and performance without genetic alterations, typically occurring over days to weeks. In high-altitude species, hypoxia acclimatization involves enhanced oxygen transport mechanisms, particularly in migratory birds. For instance, bar-headed geese (Anser indicus) during high-altitude flights over the complement genetic traits like elevated hemoglobin-oxygen affinity with physiological responses such as to support sustained aerobic activity in severe hypoxia. Behavioral migration patterns expose them to progressive altitude gains, aiding acclimatization. lizards exemplify acclimatization by combining physiological shifts with burrow-seeking behavior to manage extreme heat. Horned lizards (Phrynosoma spp.), such as the (P. cornutum), use behaviors like burrowing for refuge, allowing physiological recovery and extension of activity windows beyond surface heat limits. Post-heat exposure, they increase evaporative cooling efficiency through enhanced panting rates, which acclimates critical thermal maxima and minimizes risks in arid environments. Marine invertebrates like barnacles demonstrate salinity acclimatization as osmoconformers, adjusting intracellular solute pools to match fluctuating external osmolarities. In species such as Balanus balanoides, free amino acid (FAA) pools in muscle tissue regulate over weeks to maintain cellular volume and osmotic balance during tidal salinity shifts from near-freshwater lows to full seawater. Total FAA levels peak in autumn and decline in spring, correlating with environmental salinity variations and supporting survival in estuarine habitats without active mobility. This biochemical adjustment prevents osmotic stress, enabling reproduction and growth across broad salinity gradients. A notable involves such as ( spp.), which remodel ionocytes to acclimatize to fluctuations, enhancing survival in acidic waters. In (), exposure to low (e.g., 4.0) triggers transcriptomic changes in mitochondria-rich cells (ionocytes), upregulating genes for and ion transport. This remodeling increases the density and functional specialization of ionocytes, improving proton excretion and sodium uptake to counteract over days to weeks. Such plasticity allows to thrive in fluctuating freshwater habitats, like acidic rivers, by integrating adjustments with schooling behavior for microhabitat selection.

In Humans

Acclimatization to high altitude in humans primarily involves the release of (EPO) from the kidneys in response to , which stimulates to produce more red blood cells, resulting in . This adaptive response enhances oxygen-carrying capacity in the blood and typically unfolds over 2-3 weeks at altitudes above 4,000 meters, during which red blood cell mass can increase by 20-30%, thereby improving tissue oxygenation and reducing the incidence and severity of acute mountain sickness. In hot environments, acclimatization improves thermoregulatory efficiency, particularly through enhanced sweating responses that begin earlier and at lower core temperatures, leading to better dissipation. Studies on undergoing acclimatization training demonstrate that after about 10 days of exposure, sweat rate can increase by up to 50% while loss decreases, resulting in approximately 20% greater evaporative cooling efficiency during exercise. In cold conditions, acclimatization lowers the threshold, allowing individuals to tolerate lower and core temperatures before onset, which reduces metabolic demand and enhances overall cold tolerance without compromising production. The human dive response, an innate reflex enhanced by apnea training, promotes and peripheral to conserve oxygen during breath-holding, while splenic contraction releases additional red blood cells into circulation to boost oxygen stores. In trained pearl divers, such as the Japanese ama or Korean , repeated practice strengthens these responses, enabling breath-hold durations of 2-3 minutes per dive for repetitive immersions, with elite practitioners achieving over 10 minutes in competitive settings through optimized splenic contraction and cardiovascular adjustments. Despite these benefits, acclimatization has physiological limits and risks, particularly when prolonged or excessive, as overproduction of red blood cells can lead to characterized by extreme , increased blood viscosity, and cardiorespiratory strain. (HAPE) remains a severe if ascent outpaces acclimatization, often exacerbated by rapid exposure without adequate rest days. Individual variability in acclimatization success is influenced by factors such as age, , and , with older individuals and those with lower aerobic capacity experiencing slower adaptations and higher susceptibility to altitude-related illnesses.

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