Having realized that the a biotic conditions of many habitats may vary drastically in time, we now ask–how do the organisms living in such habitats cope or manage with stressful conditions? But before attempting to answer this question, we should perhaps ask first why a highly variable external environment should bother organisms after all. One would expect that during the course of millions of years of their existence, many species would have evolved a relatively constant internal (within the body) environment that permits all biochemical reactions and physiological functions to proceed with maximal efficiency and thus, enhance the overall „fitness‟ of the species. This constancy, for example, could be in terms of optimal temperature and osmotic concentration of body fluids. Ideally then, the organism should try to maintain the constancy of its internal environment (a process called homeostasis) despite varying external environmental conditions that tend to upset its homeostasis. Let us take an analogy to clarify this important concept. Suppose a person is able to perform his/ her best when the temperature is 250C and wishes to maintain it so, even when it is sportingly hot or feelingly cold outside. It could be achieved at home, in the car while travelling, and at workplace by using an air conditioner in summer and heater in winter. Then his/ her performance would be always maximal regardless of the weather around him/her. Here the person‟s homeostasis is accomplished, not through physiological, but artificial means. How do other living organisms cope with the situation? Let us look at various possibilities. Mammals from colder climates generally have shorter ears.