Acclimatization, is the process by which an organism adjusts itself to living under climatic conditions differing from those of its native area. Acclimatization may involve only one part or function of the organism, or it may concern all of its parts and cover its entire life cycle. A plant or animal established in a foreign area, where it is able not only to develop but also to reproduce and maintain itself, generation after generation, is said to be “naturalized.” This would seem to be the ultimate, though relatively rare, step in acclimatization.
Preacclimatization. It can be said that plants and animals are more or less efficiently adapted to meteorological elements at the intensities, fluctuations, frequencies, and times at which they occur within their spontaneous range of dispersal. If conditions are closely duplicated in remote parts of the world, an organism would seem to stand a good chance of thriving under one of these climatic equivalents. Such is, indeed, the case of western European trees (for example, Norway maple and Scotch pine), so commonly planted along eastern North American highways or city streets. Some of these plants tend to leaf out earlier in the year and to retain their foliage almost as late as they would in their country of origin. They suffer little injury from the slightly different climate. However, although they leaf, flower, and fruit quite abundantly, they hardly ever manage to propagate themselves spontaneously. Ry contrast, reproduction in foreign surroundings is achieved widely not only by roadside herbs like mustard, noxious insects like the corn borer, and mammals like rats, but also by less injurious invaders of relatively undisturbed habitats. The North American muskrat (Ondatra zibethica), introduced to Czechoslovakia, is now diffused all over central Europe.
Climatic equivalence or near-equivalence, however, in no way ensures acclimatization (very much less, naturalization), and attempts at transplantation and domestication have often proved unsuccessful. The unpredictability of an organism’s capacity to acclimatize to a new location is another factor to be recorded in the case of organisms excellently acclimatized, and even naturalized, under a foreign climate quite different from their native one. Thus, the success of the California coastal Monterey pine (Pinus radiata) and of the European deer and rabbit in New Zealand reveal the great capacity of these organisms to adjust to and thrive in another climate. The rabbit, in fact, has a much wider climatic tolerance than to the climates of Europe and New Zealand, as witnessed by its spectacular invasion of even dry and hot regions of Australia.
Directed Acclimatization. Man has utilized these relative tolerances of plants and animals in two ways: he has established them usefully under new climatic conditions by affording them protection against adverse periods or seasons, and he has bred them consciously for the purpose of achieving greater tolerance. Thus, cereals, most of them of tropical or subtropical origin, generally are stored in the form of seed and do not undergo the rigors of winter, whereas perennial pasture grasses and orchard trees have been genetically improved with respect to coldresistance. Similarly, domestic cattle and pigs are sheltered in manmade constructions. Many races of fowl have been bred for the particularly trying conditions of both cold and tropical regions.
Evolution and Acclimatization. Response to climatic factors is only one of the many complex, interrelated features of adaptation; and it is not always readily separable from other responses. For instance, an animal may have to adapt in response to a change in climate over a period of time. But an animal may also have to adapt to a ne w location if it has migrated. Many species have become extinct in the face of such changes, but others have responded successfully and have maintained themselves. Viewed on a broad geological time scale, the species of oak have developed the deciduous habit in the moist-cool and cold-temperate regions, and have much reduced the size of their evergreen leaves in the dry and warm regions.
As for man, his acclimatization to the extremely dry, warm, or cold parts of the earth has been achieved through some hereditary shifts in metabolic activity and through the development of proper cultural habits of clothing and food. Studies of these habits in the Eskimo and the high Andean Indians have revealed the biological and cultural nature of the acclimatization of these groups.