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Acclimatization (UK also acclimatisation; US also acclimation) is the process in which an individual organism adjusts 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. 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. When used as a technical term (such as in the study of physiology), acclimatization refers to a natural process (e.g., shedding heavy winter fur with natural seasonal change), whereas the term acclimation is reserved for changes occurring in response to an artificial or controlled situation, such as changes in temperature imposed in an experimental manipulation.
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. 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.
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, 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. 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.