Abstract: A characteristic of an organism that makes it fit for its environment or for its particular way of life. Adaptation is a key biological process in organisms and can take many forms. Typically, adaptation refers to two mechanisms: (1) an adjustment to new or altered environmental conditions by changes in genotype (natural selection) or phenotype; and (2) the occurrence of physiological changes in an individual exposed to changed conditions. For example, the Arctic fox (Vulpes lagopus; see illustration) is well adapted for living in a very cold climate. Appropriately, it has much thicker fur than similar-sized mammals from warmer places; measurement of heat flow through fur samples demonstrates that the Arctic fox and other Arctic mammals have much better heat insulation than tropical species. Consequently, Arctic foxes do not have to raise their metabolic rates as much as tropical mammals do at low temperatures. This is demonstrated by the coati (Nasua narica), which lives in Panama and has a body mass similar to Arctic foxes and about the same metabolic rate at comfortable temperatures. When both animals are cooled, however, the coati's metabolic rate starts to rise steeply as soon as the temperature falls below 20?C (68?F), whereas that of the Arctic fox begins to rise only below ?30?C (?22?F). The insulation of Arctic foxes is so effective that they can maintain their normal deep-body temperatures of 38?C (100?F) even when the temperature of the environment falls to ?80?C (?112?F). Thus, thick fur is obviously an adaptation to life in a cold environment