Acclimatization

Acclimatization is the process of an organism adjusting to changes in its environment, often involving temperature or climate (In laboratory conditions, this same process is termed "Acclimation"). Acclimatization usually occurs in a short time, and within one organism's lifetime (compare 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. Where acclimatization occurs naturally, some authors have used acclimation to describe the process of an organism being forced to adjust to changes in their environment by artificial means, such as in a laboratory setting.

Plants
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. This process is called hardening and involves several changes, such as a decrease in the water content and an increase in the sugar content of the plant, lowering the freezing point of sap.

Animals
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 acclimation bags until this process is complete.

Humans
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 environment conditions. If the acclimatization process is ignored, then the person is at higher risk for heat related injuries (heat stroke, heat cramp, pneumonia).

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.