● One would expect that during the `color{violet}("course of millions of years")` of their existence, many species would have evolved a relatively constant internal (within the body) environment that permits all `color{violet}("biochemical")` `color{violet}("reactions and physiological functions")` to proceed with `color{violet}("maximal efficiency")` and thus, enhance the overall ‘fitness’ of the species.

● This constancy, for example, could be in terms of `color{violet}("optimal temperature")` and `color{violet}("osmotic concentration ")` of
body fluids.

● Ideally then, the `color{violet}("organism")` should try to maintain the constancy of its `color{violet}("internal environment")` (a process called `color{violet}("homeostasis")` ) despite varying `color{violet}("external environmental conditions")` that tend to upset its homeostasis.

● `color{violet}("An analogy")` can be used to clarify this important concept.

● Suppose a person is able to perform his/ her best when the temperature is `25^0C` and wishes to maintain it so, even when it is `color{violet}("scorchingly hot")` or `color{violet}("freezingly cold outside")`.

● It could be achieved at home, in the car while travelling, and at workplace by using an `color{violet}("air conditioner in summer")` and `color{violet}("heater in winter")`.

● Then his/ her performance would be always `color{violet}("maximal regardless")` of the weather around him/her.

● Here the person’s homeostasis is accomplished, not through `color{violet}("physiological,")` but artificial means.

● Similarly other `color{violet}("living organisms cope")` with the situation using various means

● Some organisms are able to maintain homeostasis by `color{violet}("physiological")` (sometimes behavioural also) means which ensures `color{violet}("constant body temperature, constant osmotic concentration")`, etc.

● `color{violet}("All birds and mammals")`, and a very few `color{violet}("lower vertebrate and invertebrate species")` are indeed capable of such regulation (`color{violet}("thermoregulation")` and `color{violet}("osmoregulation")`).

● `color{violet}("Evolutionary biologists")` believe that the `color{violet}("‘success’ of mammals")` is largely due to their `color{violet}("ability to maintain a constant body temperature")` and thrive whether they `color{violet}("live in Antarctica")` or in the `color{violet}("Sahara desert.")`

● The `color{violet}("mechanisms")` used by most `color{violet}("mammals")` to regulate their `color{violet}("body temperature")` are similar to the ones that we humans use.

● We maintain a `color{violet}("constant body temperature")` of `– 37^0C.`

● In summer, when `color{violet}("outside temperature")` is more than `color{violet}("our body temperature")`, we sweat profusely.

● The resulting `color{violet}("evaporative cooling")`, similar to what happens with a `color{violet}("desert cooler")` in operation, brings down the `color{violet}("body temperature")`.

● In winter when the `color{violet}("temperature is much lower")` than `37^0C`, we start to shiver, a kind of exercise which `color{violet}("produces heat")` and `color{violet}("raises the body temperature")`.

● `color{violet}("Plants,")` on the other hand, do not have such `color{violet}("mechanisms")` to maintain `color{violet}("internal temperatures.")`

● `color{violet}("An overwhelming majority")` (99%) of `color{violet}("animals")` and nearly all `color{violet}("plants")` cannot maintain a `color{violet}("constant internal environment")`.

● Their `color{violet}("body temperature")` changes with the `color{violet}("ambient temperature")`.

● `color{violet}("In aquatic animals")`, the `color{violet}("osmotic concentration")` of the body `color{violet}("fluids change")` with that of the `color{violet}("ambient water osmotic concentration.")`

● These `color{violet}("animals")` and `color{violet}("plants")` are simply conformers.


● Considering the benefits of a `color{violet}("constant internal environment to the organism")`, we must ask why these conformers had not evolved to become `color{violet}("regulators")`.

● Compared to the `color{violet}("human analogy")` we used above; much as they like, how many people can really afford an air conditioner? Many simply ‘sweat it out’ and resign themselves to suboptimal performance in hot summer months.

● Similarly, `color{violet}("thermoregulation")` is `color{violet}("energetically expensive")` for `color{violet}("many organisms")`.

● This is particularly true for `color{violet}("small animals")` like `color{violet}("shrews")` and `color{violet}("humming birds")`.

● `color{violet}("Heat loss or heat gain")` is a function of surface area.

● Since `color{violet}("small animals")` have a larger surface area relative to their volume, they `color{violet}("tend to lose body heat")` very fast when it is `color{violet}("cold outside")`; then they have to `color{violet}("expend")` much `color{violet}("energy to generate body heat")` through `color{violet}("metabolism.")`



● This is the `color{violet}("main reason")` why `color{violet}("very small animals")` are rarely found in `color{violet}("polar regions")`.

● During the course of evolution, the costs and benefits of `color{violet}("maintaining a constant internal environment")` are taken into `color{violet}("consideration.")`

● Some species have evolved the `color{violet}("ability to regulate")`, but only over a `color{violet}("limited range of environmental")` conditions, beyond which they simply conform.

● If the `color{violet}("stressful external conditions")` are `color{violet}("localised")` or remain only for a short duration, the `color{violet}("organism")` has two other alternatives

● The `color{violet}("organism")` can move away temporarily from the `color{violet}("stressful habitat")` to a more `color{violet}("hospitable")` area and return when `color{violet}("stressful period")` is over.

● In `color{violet}("human analogy")`, this strategy is like a person moving from Delhi to Shimla for the duration of summer.

● Many animals, particularly birds, during winter undertake long-distance `color{violet}("migrations")` to more `color{violet}("hospitable areas")`.

● Every winter the famous `color{violet}("Keolado National Park (Bhartpur)")` in `color{violet}("Rajasthan")` host thousands of `color{violet}("migratory birds")` coming from `color{violet}("Siberia")` and other `color{violet}("extremely cold northern regions.")`

● In `color{violet}("bacteria, fungi")` and `color{violet}("lower plants,")` various kinds of thick walled spores are formed which help them to `color{violet}("survive unfavourable conditions")` – these `color{violet}("germinate")` on `color{violet}("availability of suitable environment")`.

● `color{violet}("In higher plants, seeds")` and some other vegetative reproductive structures serve as means to tide over periods of stress besides helping in dispersal – they `color{violet}("germinate")` to form new plants under favourable `color{violet}("moisture")` and `color{violet}("temperature conditions.")`

● They do so by reducing their metabolic activity and going into a state of `color{violet}("‘dormancy’.")`

● `color{violet}("In animals")`, the `color{violet}("organism")`, if unable to migrate, might avoid the stress by escaping in time.

● The familiar case of `color{violet}("bears going into hibernation during winter")` is an example of escape in time.

● Some `color{violet}("snails and fish")` go into aestivation to avoid summer–related problems-heat and desiccation.

● Under unfavourable conditions many `color{violet}("zooplankton")` species in `color{violet}("lakes")` and `color{violet}("ponds")` are known to enter diapause, a stage of suspended development.