Adaptations Developing in Response to Extremes of Temperature

Both in plants as well as animals there has been the development of a number of morphological and physiological characteristics during the course of evolution, in order to meet the extremes of temperature conditions. These may be briefly summarised as follows:

1. Production of spores, cysts, eggs, pupae, seeds

Plants and animals, to resist the extreme (both minimal as well as maximal) of temperature, produce thick-walled structures as spores, cysts, eggs, pupae, seeds etc. Thus amoeba in encysted state may tolerate a temperature below 0°C.

2. Increase in osmotic concentration

In some plants, there take place such physiological changes as increase in osmotic concentration that prevents freezing. Increase in osmotic values enhances the amount of bound water in colloidal form. Thus, in winter rye at 0°C, leaves and stems are flexible, and not brittle.

3. Removal of water from tissues

Dried seeds, spores and cysts like structures avoid freezing because no liquid remains in them to freeze. Dry seeds are able to germinate, even after their exposure to -190°C for a period of three weeks.

4. Dormancy

This includes both, hibernation as well as aestivation. The former is overwintering in dormant state. This is common in poikilothemnic as well as homoiothermic animals, such as bats, hedgehogs, ground squirrels and jumping mice.

During this period, metabolic rate and heartbeat rate are reduced. Aestivation occurs during summer to resist higher extremes of temperature, which is very common among insects, some plants, lungfishes, amphibians and some mammals.

Lungfish, Protopterus, during summer, burrows, into the mud, secretes a cocoon of slime around it and lies dormant. In insects aestivation is a achieved by the formation of diapause, a stage during which morphological growth and development remain suspended.

Some insects undergo hibernation and aestivation during different months of the year, depending upon the environmental temperatures. One very well known example is the insect, Coccinella septempunctata, which has different hibernation and aestivation periods in different countries i.e. London, Moscow and Formosa, with no hibernation in the last country.

5. Homoiothermy

Birds and mammals are able to maintain their body temperature at a constant value and thus are not affected at all by variations in the environmental temperature extremes. This is achieved through evaporation of water from bodies during summer, and through insulating action of fur, feathers, fats, etc., during cold weather of winter months.

6. Thermal migrations

These are found only in animals. The journeys taken by animals that enable them to escape from extremely hot or cold situations are referred to as thermal migrations. This is very common in such locomotory forms as desert animals like amphibians, burrowing animals, deer, bear, insects, birds, mammals, etc. Fishes are also known to migrate.

7. Phenotypic changes

Temperature is known to affect the phenotype of some animals. For instance, in Drosophila melanogaster temperature affects the eye size and number of legs. Moreover, in some animals like Daphina, the body form undergoes different modifications in relation to seasonal changes in temperature, and this phenomenon is known as cyclomorphosis. During winter, the head of the animal is round, in spring there develops a helmet-like projection on the head, which attains its maximum size during summer, and during next winter the head becomes round again. Thus degree of warmth of water and size of the helmet are in one way or the other correlated.