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Acclimatization is the reversible process of 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 seasonal shedding of 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.


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 acclimation hypothesis, is that it assumes that there are no costs associated with acclimation.[3] 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.



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. 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.[4]


When humans move from a cool or temperate environment to a hot, dry desert environment or vice versa, they should spend up to seven days acclimatizing to the change in their environment. This lets the body make internal adjustments (see homeostasis) to compensate for the change in environmental conditions. If people do not acclimatize, then the person is at higher risk of heat related injuries (heat stroke, heat cramp, pneumonia). A heat acclimatized person will begin to sweat earlier and more intensely under heat, have a lower heart rate, and a lower skin temperature. The salt content of sweat also decreases as people acclimatize.[5]

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.[6][7]

See also


  1. (2009) “Acclimatization” (n.d.) The Unabridged Hutchinson Encyclopedia Retrieved November 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. Angilletta, M.J. (2009). Thermal Adaptation: A Theoretical and Empirical Synthesis. Oxford University Press, Oxford.
  4. Lowe, C.H.; Vance, V.J. (1955) Acclimation of the critical thermal maximum of the reptile Urosaurus ornatus. Science 122, 73-74.
  5. Heat acclimatization guide. US Army. URL accessed on 2009-07-02.
  6. 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).
  7. Kenneth Baillie and Alistair Simpson. Altitude oxygen calculator. Apex (Altitude Physiology EXpeditions). URL accessed on 2006-08-10. - Altitude physiology model

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