There are many variations in temperature and these affect plants distribution in various ways. It also affects animals. All animals are broadly classified into two group:

1. Homoiothermic or endothermic animals (warm-blooded)

These animals like birds and mammals are able to maintain their body temperature at a constant level irrespective of the environmental temperature.

2. Poikilothermic or ectothermic animals (cold-blooded)

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These are animals as reptiles, fishes, amphibians in which the body temperature fluctuates with changes in the environmental temperature.

Thermoregulation in Homoiotherms

Though several factors may operate, the most important role is played by their skin. Almost each of the various structures presents in the skin play some part in the temperature regulation. A homoiotherm has different responses to cold or heat as follows.

(a) Response to cold

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When a typical homoiotherm as a mammal is subjected to severe cold, following adaptations help the animal from excessive cooling.

(i) The subcutaneous fat serves as insulator and reduces heat loss from body.

(ii) The hair is raised and brought up into a more-or-less vertical position by the contraction of the erector-pili muscles. Thus air gets trapped in the spaces between hairs. This air is warmed by the body and being a poor conductor of heat it serves as an insulatory layer around the animals. In birds, the same function is done by feathers.

(iii) The superficial blood vessels in the skin constrict so that blood is directed from the surface of the deeper layers. This reduces loss of heat from the blood to surrounding atmosphere.

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(iv) Extra heat is produced by increase in the metabolic rate particularly of the liver and muscles.

Homoiotherm besides above said structural adaptations also has physiological capability of increasing their heat production.

(b) Response of heat

Heat production is cut down and heat loss encouraged as follows:

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(i) Animals of hot climates have relatively little subcutaneous fat. The fat deposits are localised. Thus camel’s fat is stored only in hump and in buffalo and bison on top of the neck.

(ii) The hair is lowered by relaxation of the erector-pili muscles on that it lies flat against the body surface. There is no space between the hair and no air can be trapped against the skin.

(iii) The superficial blood vessels are dilated so that blood is brought up near the surface from which it can lose heat to atmosphere.

(iv) Sweating or panting occurs. Sweating by sweat glands of skin and evaporation of sweat from the body surface cools the skin and the blood flowing through it.

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Thus in mammals the hypothalamic centre functions as a thermostat. It is sensitive to temperature charges of blood flowing through it and responds by sending nerve impulses through efferent nerves to the appropriate effectors.

If the temperature of the blood is slightly higher than it should be, the thermo-regulatory centre of hypothalamus of brain detects this and sets into motion process that collectively encourage heat loss. On the other hand, if the temperature falls below the normal the centre initials processes that produce and conserve heat.

(v) Metabolic rate fails in hot conditions so that less heat is generated by the body.

Thermoregulation in Poikilotherms

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Some poikilohterms avoid both heat and cold by underlying dormant in period of environmental stress. Many insects, some crustaceans, mites and snails, enter diapause, a state of dormancy and arrested growth. Eggs, embryonic larvae and pupae are involved. Some poikilotherms become dormant in temperature extreme conditions. Amphibians and turtles bury them in the pond mud, snakes and lizards seek burrows.

A few exceptional poikilotherms, especially insects, certain amphibians and reptiles, exercise some degree of thermoregulation by physiological or behavioural mechanism. For example, hawk-moths can raise the temperature of their flight muscle to 32°-36°C by vibrating the wings before takeoff and gregarious butter fly larvae may raise their temperature 1.5-2°C by clustering together. Locust and grasshoppers may increase temperature 10°C by basking sideways in the sun. Ants move their larvae to warm or cool places.

When temperature drops, lizards bask in the sun to achieve more temperature. Once this temperature is attained, they will divide their time between sun and shade to maintain it. The frogs and reptiles lower their body temperature slightly by evaporating cooling through the skin or via the respiratory tract by panting.

Temperature affects the distribution of animals also, as different species have different ranges of their temperature requirements. For instance, coral reefs require a minimum temperature of 21°C for their suitable growth, hence are completely absent in colder regions, where prevailing temperatures are below 21°C. Speckled trout, Salvelinus fontinalis, perfers 14°C to 19°C, and thus cannot tolerate a temperature above 25°C, being thus completely absent from streams where the temperature exceeds 24°C, for some time.

Distribution of intertidal barnacles is also affected by temperature. For example, an intertidal barnacle, Pectin groenlandicus, is hardly known to occur in areas with 0°C temperature, whereas another one, Balanus balanoides, occurs at a temperature below 7.2°C.