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Acclimatization, or acclimation in UK English, is an aspect of adaptation and is the process in an individual organism adjusting to a gradual change in its environment (such as a change in temperature, humidity, photoperiod, or pH), allowing it to maintain performance across a range of environmental conditions. Acclimatization occurs in a short period of time (days to weeks), and within the organism's lifetime (compare to adaptation). This may be a discrete occurrence 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.



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.[2] Organisms may also express specific proteins called heat shock proteins that may act as molecular chaperons and help the cell maintain function under periods of extreme stress. 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. Note: acclimation and acclimatization are two very different terms that are not interchangeable. Acclimation is used under laboratory conditions, while acclimatization is "in the field" or in nature.[3]


Organisms are able to change several characteristics relating to their morphology in order to maintain performance in novel environments. Examples may include changing of skin color or pattern to allow for efficient thermoregulation, or a change in body size of offspring as a result of low food levels in the ecosystem.


While the capacity for acclimation has been documented in thousands of species, researches 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 pitfalls of the beneficial ac.[4] However, there are costs associated with acclimation such as the energetic costs in expressing heat shock proteins.

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.



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.[5]

In the orchid Phalaenopsis, phenylpropanoid enzymes are enhanced in the process of plant acclimatisation at different levels of photosynthetic photon flux.[6]


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.[citation needed] 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.[7]


The salt content of sweat and urine decreases as people acclimatize to hot conditions.[8]

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 in the air.[9][10]

See also


  • World Book encyclopedia 1989


  1. (2009) “Acclimatisation” (n.d.) The Unabridged Hutchinson Encyclopedia RetrievedNovember 5 2009 from
  2. Los, D.A.; Murata, N. (2004). Membrane fluidity and its roles in the perception of environmental signals. Biochimica et biophysica acta-biomembranes 0666, 142-157.
  3. Gatten, Robert E. Jr, Arthur C Echternautch, and Mark A. Wilson (July 1988). Acclimatization versus acclimation of activity metabolism in a lizard. Physiological Zoology 61 (4): 322–329.
  4. Angilletta, M.J. (2009). Thermal Adaptation: A Theoretical and Empirical Synthesis. Oxford University Press, Oxford.
  5. Camejo, D.; Marti, M.; Nicolas, E.; Alarcon, J.; Jimenez, A.; Sevila, F. (2007) Response of superoxide dismutase isoenzymes in tomato plants (Lycopersicon esculentum) during thermo-acclimation of the photosynthetic apparatus. Physiologia Plantarum 131, 367-377.
  6. Enhancement of phenylpropanoid enzymes and lignin in Phalaenopsis orchid and their influence on plant acclimatisation at different levels of photosynthetic photon flux. Mohammad Babar Ali, Serida Khatun, Eun-Joo Hahn and Kee-Yoeup Paek, Plant Growth Regulation, 2006, Volume 49, Numbers 2-3, pages 137-146, DOI:10.1007/s10725-006-9003-z
  7. Lowe, C.H.; Vance, V.J. (1955) Acclimation of the critical thermal maximum of the reptile Urosaurus ornatus. Science 122, 73-74.
  8. Heat acclimatization guide. US Army. URL accessed on 2009-07-02.
  9. Muza, SR; Fulco, CS; Cymerman, A (2004). Altitude Acclimatization Guide.. US Army Research Inst. of Environmental Medicine Thermal and Mountain Medicine Division Technical Report (USARIEM–TN–04–05).
  10. Kenneth Baillie and Alistair Simpson. Altitude oxygen calculator. Apex (Altitude Physiology EXpeditions). URL accessed on 2006-08-10. - Altitude physiology model