‘Success’ of Mammals'

Why a highly variable external environment should bother organisms after all ?

Relatively constant internal (within the body) environment permits all biochemical reactions and physiological functions to proceed with maximal efficiency and thus, enhance the overall ‘fitness’ of the species. Evolutionary biologists believe that the ‘success’ of mammals is largely due to their ability to maintain a constant body temperature and thrive whether they live in Antarctica or in the Sahara desert

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‘No organism is a perfect regulator or conformer.'

Regulating and conforming are two extremes in how animals deal with environmental fluctuations.

An animal may maintain homeostasis while regulating some internal conditions and allowing others to conform to the environment. For example, most freshwater fishes regulate their internal solute concentration but allow their internal temperature to conform to external water temperature.

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‘No organism is a perfect regulator or conformer.'

If the stressful external conditions are localised or remain only for a short duration, the organism has two other alternatives for survival.

Migrate : The organism can move away temporarily from the stressful habitat to a more hospitable area and return when stressful period is over. Every winter the famous Keolado National Park (Bharatpur) in Rajasthan host thousands of migratory birds coming from Siberia and other extremely cold northern regions.

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‘No organism is a perfect regulator or conformer.'

Suspend: In bacteria, fungi and lower plants, various kinds of thickwalled spores are formed which help them to survive unfavourable conditions – these germinate on availability of suitable environment.

In higher plants, seeds and some other vegetative reproductive structures serve as means to tide over periods of stress besides helping in dispersal – they germinate to form new plants under favourable moisture and temperature conditions. They do so by reducing their metabolic activity and going into a state of ‘dormancy’.

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‘Hibernation'

long-term torpor that is an adaptation to winter cold and food scarcity.

When vertebrate endotherms enter torpor or hibernation, their body temperatures decline. Some hibernating mammals cool to 1–2°C, and a few drop slightly below 0°C in a supercooled, unfrozen state. Metabolic rates during hibernation may be several hundred times lower than if animals tried to maintain normal body temperatures. Hibernators can survive for very long periods on limited supplies of energy stored in body tissues or as food cached in a burrow.

Some hibernators prepare for winter by storing food in their burrows or by eating huge quantities of food. Ground squirrels double their weight prior to hibernation.

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‘Estivation'

summer torpor, is also characterized by slow metabolism or inactivity.

Estivation allows animals to survive long periods of high temperatures and scarce water supplies..

Some snails and fishes

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‘Daily torpor'

Many small mammals and birds exhibit a daily torpor that is adapted to their feeding patterns.

• 1: Most bats and shrews feed at night and go into torpor during daylight hours.
• 2:hummingbirds feed during the day and go into torpor on cold nights. The body temperature of a hummingbird may drop by 25–30°C at night.
• 3.: freshwater fish maintains a stable internal concentration of solutes in its blood that is higher than the water in which it lives.

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“Antifreeze” Compounds

Many fish thrive in Antarctic waters where the temperature is always below zero. How do they manage to prevent their body fluids from freezing?

Acclimatization in ectotherms involves compensating for temperature changes. Acclimatization responses in ectotherms often include adjustments at the cellular level. Cells may increase the production of certain enzymes or produce enzyme variants with different temperature optima. Membranes also change the proportions of saturated and unsaturated lipids to keep membranes fluid at different temperatures. Some ectotherms produce “antifreeze” compounds, or cryoprotectants, to prevent ice formation in body cells.

The extra solutes which prevent freezing are glycerol and antifreeze proteins. They lower the freezing point of body fluids. Antarctic Fish (Trematomus) remains active even in extremely cold sea water due to this hardiness.

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“Piezolytes”

A large variety of marine invertebrates and fish live at great depths in the ocean where the pressure could be >100 times the normal atmospheric pressure that we experience. How do they live under such high pressures and do they have any special enzymes?

Creatures that live in the deepest parts of the ocean have special features, which help them deal with these tough conditions – including the crushing pressure To help with this, deep sea creatures have “piezolytes” – small, organic molecules which have only recently been discovered. These piezolytes stop the other molecules in the creatures’ bodies, such as membranes and proteins, from being crushed by the pressure Microbes have been recovered from the very bottom of the deep sea, and they have peizolytes to help protect them, too.

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