Plants has been divided into the following four categories on the basis of their heat tolerating capacity : (i) Megatherms: plants growing in regions where high temperatures prevail throughout the year, e.g., desert vegetation and tropical rain forests, (ii) Mesoderms: plants of the regions where high temperature alternating with low temperature, e.g., tropical deciduous forests and aquatic plants, (iii) Microtherms: plants of the regions where low temperature prevail throughout the year, e.g., mixed coniferous forests, (iv) Hekistotherms: plants growing in regions with very low temperature, e.g., alpine vegetation.
Except for the time they sprout from seeds or vegetative cuttings, plants receive all of their energy from radiation in the environment and from convection. To balance the input, plants lose heat by radiation, convection and evapotranspiration. They have some control over the latter by opening and closing the stomata and by changing the shape and position of the leaf.
Further, the ability to endure low temperature extremes varies among plants. They are not equally resistant at all stages of the life cycle. Flowers are more sensitive to low temperatures than are fruits and leaves, and young leaves are more resistant than old ones. Trees may be more severly injured than herbaceous plants. In general, adaptations among plants to endure low temperatures are primarily protoplasmic (Smith, 1977).
On the basis of temperature tolerance, fungi have also been classified into the following three kinds: thermotolerant, thermophilic and mesophilic fungi (R. Emerson, 1968), Thermophilic fungi require optimum temperature 45°C for growth and they occur in man-made self-heated habitats and also in natural habitats where geothermal heat, body heat of warm blooded animals, microbial metabolism and perhaps sun heating provide elevated temperature required for their growth. Thermophilic fungi has been reported in nesting material of American alligator (Tansey, 1973) and of birds. T. Satyanarayana, B. N. Johri and S. B. Saksena (1977) have reported the occurrence and seasonal variation of mycoflora of nesting material of the following Indian birds: crow (Corvus splendens), house sparrow (Passer domesticus), pipit (Anthus rufulus), bee-eater (Merops superviliesus) and crow-pheasant (Centropus sinensis).
In the nest of these birds, they reported few thermotolerant fungi (that have optimum growth temperature 45°C and are constantly found throughout the year), e.g., Aspergillus fumigatus, and A. flavus ; many thermophilic fungi such as Chaetomium thermophile, Humicola grisea, Mucor pusillus, Rhizopus microsporus, R. rhizopodiformis, Sporotrichum thermophile, Thermoascus aurantiacus, Thermomyces lanuginosus and Thielavia minor ; and many mesophilic fungi (having optimum growth temperature 28°C) such as Penicillium citrinum, Trichoderma viride, four species of Aspergillus and one species each for Mucor and Rhizopus.
In contrast to plants and fungi, animal’s response to energy environment is more complex. Not only do animals produce a considerable amount of heat by their own metabolism, but they are also able to move about to seek a favourable temperature regime and in other ways, both behaviourally and physiologically, to maintain some control over their body temperature.
As far as temperature control is concerned, animals can be divided into the following three groups:
Poikilothermic or Ectothermic animals:
Poikilcthermic means ‘having a variable temperature’. Poikilothermic animals are described as “cold-blooded”, their body temperature changing with fluctuations in the environmental temperature. If the environment is cold, so is their blood.
Poikilothermic animals such as most invertebrates and most chordates excluding birds and mammals exhibit high rates of thermal conductance and low rates of heat production (usually less than 2°C). Consequently these animals exploit sources of heat energy other than metabolism (solar radiation, reradiation, etc.). Thus, poikilothermic animals are ectothermic which means that they cannot regulate their body temperature by physiological means but have gain heat from the environment, i.et, from outside the body.
Within the range of temperatures that poikilotherms can tolerate, the rate of metabolism and thus, oxygen consumption rises with temperature. Lacking any homeostatic devices, poikilothermic animals of terrestrial environments in particular must depend on some behavioural control over body temperatures. Most aquatic poikilotherms generally encounter temperature fluctuations of lesser magnitude and usually have more poorly developed behavioural and physiological capabilities than do terrestrial forms.
Homeothermic or Endothermic Animals:
Homeothermic is a Greek word meaning having the same temperature. Homeothermic animals are popularly described as ‘warm blooded’; their body temperature is independent of environmental temperature so that in cold conditions their blood is at a temperature higher than that of their surroundings.
The homeothermic animals are birds and mammals which possess a sufficiently high rate of oxidative metabolism and a sufficiently low rate of thermal conductance, that is, the body temperature is the product of the animal’s own oxidative metabolism (thermogenesis). Due to this fact, homeotherms are also called endothermic animals, since they generate heat from within the body, and keep it there.
The oxygen consumption of homeotherms decreases linearly with increasing temperature to some critical point where it is independent of environmental temperature. In other words, the lower the temperature, the more metabolic work the animal does to maintain a uniform temperature. But within a certain range of temperatures, the amount of energy needed to maintain body temperatures is minimal.
At all ambient temperatures, homeotherms maintain their metabolic rates and body temperatures by means of a closely regulated feedback system mediated by the hypothalmus. The metabolic costs of such a system, however, are high. Homeotherms use some 80 to 90 per cent of their oxidative energy to maintain thermal homeostasis.
Some animals such as monotremes and some marsupials have a limited power of temperature regulation. They respond to temperature extremes by aestivating and hibernating.
Both poikilotherms and homeotherms have following methods of thermo-regulation:
Thermo-regulation in poikilotherms:
Certain poikilothermic animals avoid both heat and cold by undergoing into a slate of dormancy during a period of environmental stress. Many species of insect as well as certain crustaceans, mites, and snails, enter diapause, a state of dormancy and arrested growth. Eggs, embryonic larvae, or pupal stages may be involved.
In addition to tiding the animal over unfavourable periods, diapause also synchronizes the life cycle of a species with the weather and ensures that active stages will coincide with the climatic conditions and food supply that favour rapid development and high survival.
Fig. 11.13. Heat gain and losses by a poikilothermic (ectothermic) animal (i.e., lizard). By absorbing heat from, or losing it to, its surroundings the animal maintain its body temperature at approximately 36°C (after Roberts. 1971).
Some poikilothermal animals become dormant during periods of temperature extremes. Amphibians and turtles bury themselves in the mud of pond bottoms; snakes and lizards seek burrows and dens in rocky hillsides. There they remain in a suspended animation until the temperature warms or cools again. Winter dormancy is called hibernation; summer dormancy is aestivation. Most poikilotherms become inactive when the temperature of their surroundings goes below 8°C or rises to 42°C.
Like the poikilothermic animals, certain homeothermic animals, such as bats, ground squirrels, woodchucks and jumping mice also undergo hibernation .and aestivation. During hibernation, they too have a state of cold torpor (i.e., slow metabolic rate, low oxygen consumption, slow body movement and letharginess), like the poikilotherms, but there is a difference.
When reptiles and amphibians are exposed to cold, the animal cools because it has no way to stay warm. As a poikilotherm’s system becomes cooler, the heart rate and metabolic rate decline and animal become torpid. But when a mammal enters hibernation, first the heart rate and metabolic rate decline and then the body temperature drops (Smith, 1977).
A few exceptional poikilothrems, especially insects, certain amphibians and reptiles, exercise a degree of thermo-regulation by either physiological or behavioural mechanisms. For example, Hawk-moths can rise the temperature of their flight muscle to 32°—36°C by vibrating the wings before take-off and gregarious butterfly larvae may raise their temperature 11/2 — 2°C when clustered together.
Locust, and grasshoppers may increase their temperature 10°C by basking sideways in the sun. Ants move their larvae to warm or cool places within the nest and bees maintain temperatures within their hives between 13° and 25°C by fanning with their wings to evaporate water droplets when it is too hot, or releasing body heat through increased metabolic activity, when too cold.
When temperature drops, lizards bask in the sun to achieve the desired body temperature (Fig. 1M3); once this thermal level is attained, they will divide their time between sun and shade to maintain it. Desert lizards tend to be most active in the early morning and evening when it is neither too cold nor too hot.
These are the times when one sees them scurrying about for food. In the heat of day they retire to a shady place; and at night, when it can be extremely cold, they may burrow or seekout a crevice in which to build up a warm atmosphere from the heat generated by their metabolism.
Some poikilotherms such as frogs and reptiles lower their body temperature slightly by evaporative cooling through the skin or via the respiratory tract by panting.
Fig. 11.14. V. S. of mammalian skin stereogram showing structures involved in temperature regulation (after Roberts, 1971).
Thermo-regulation in homeotherms:
Although many different systems co-operate in the process, the key structure involved in the temperature regulation of any endotherm is the skin. Literally all the structures included in the skin (Fig. 11.14) play some part in temperature regulation. A homeothermic or endothermic animal will have different responses to cold or heat as follows:
A. Response to cold by an homeothermic animal:
When a mammal, a typical homeothermic animal is subjected to severe cold, following physiological processes and thermal adaptations may save it from excessive cooling and ultimate death:
1. The subcutaneous fat in the dermis of the skin serves as an insulator and reduces heat loss from the body. In fact, animals living in very cold habitats, the polar bear and seal for example, have a particularly thick layer of sub-cutaneous fats.
2. The hair is raised and brought up into a more-or-less vertical position by contraction of the erector-pili muscles. The advantage of this is that air gets trapped in the spaces between the hairs. This air is warmed by the body and being a poor conductor of heat it serves as an insulatory layer around the animal.
In birds the same function is performed by the feathers. Mallard and black ducks (Anas platyrhynchos and Anas rubripes) with oil on their feathers show higher metabolic rates as the concentrations are increased (Hartung, 1967). The oil, thus increases the insulating capability of the feathers.
3. The superficial blood vessels in the skin constrict so that blood is diverted from the surface to the deeper layers. This reduces loss of heat from the blood to the surrounding atmosphere. In exposed structures such as the ears there are special shunt vessels interconnecting the arterioles and venules that take blood to and from the superficial capillaries of the skin.
In cold conditions these shunt vessels dilate so that blood by-passes the surface of skin. This is aided by a general reduction in the total volume of circulating blood, achieved by some of the blood being taken up into reservoirs such as spleen. In prolonged conditions of extreme cold the blood may be diverted from the surface to such an extent and for so long that the cells die, resulting in frost-bite.
4. Extra heat is produced by an increase in the metabolic rate, particularly of the liver and muscles. This starts as a general increase in muscular tone but may be followed by rhythmical involuntary contractions of the skeletal muscles (shivering) and contraction of the smooth muscles in the skin (goose flesh).
Heat production increases when an animal is active. For example, the heat produced while standing compared with lying to be 15 per cent greater for sheep and 17 per cent greater for cattle (Ritzman and Benedict, 1931).
Homeotherms also have the physiological capability of increasing their heat production without overt activity. Summit metabolism is one example of this; newborn lambs can increase their heat production up to five times the basal rate without muscular activity.
Postnatal tissue (i.e. brown fat) which is typically located between the shoulder blades and contains numerous mitochondria, produces large quantities of heat. This initial source of thermo- genesis is followed by the lamb’s increasing dependence on milk and forage for energy (Moen, 1973).
Out of these four techniques of endothermic animals to cope with environmental cold fall, first three are physical mechanisms designed to conserve heat by increasing the body’s powers of insulation; the fourth is a chemical mechanism in which body actively generates heat.
B. Response to heat by an endothermic animal:
The response to excessive environmental heat involves the reverse of the above processes. Heat production is cut down and heat loss encouraged by following methods:
1. Animals living in hot climates have comparatively little subcutaneous fat. The fat deposits rend to be localized so as not to impede the loss of heat. Thus the camel’s fat is stored in hump, and in the buffalo and bison it is located on top of the neck.
2. The hair is lowered by relaxation of the erector-pili muscles so that it lies flat against the surface of the body. With no space between the hairs no air can be trapped against the skin. Insulation is reduced and heat can be more readily lost by radiation and convection.
However, when the environmental temperature becomes higher than the body temperature, heat cannot be lost in this way. Under such circumstances the hair becomes important in insulating the body against excessive heat uptake. This is important in large animals like the camel which finds it difficult to escape the heat of the sun.
3. The superficial blood vessels are dilated so that blood is brought up near the surface from which it can lose heat to the surrounding atmosphere. The shunt vessels are constricted and the total blood volume is raised, thereby further increasing the flow of blood to the surface.
4. Sweating or panting occur. Sweating by sweat glands of mammalian skin and evaporation of the sweat from the surface of the body cools the skin and the blood flowing through it. The evaporating power of the atmosphere is greatly enhanced by air movements.
In the dog and cat families there are no sweat glands (except in the pads of the paws) and heat is lost by panting. This greatly speeds up evaporation from the lungs with concomitant cooling of the blood. It also facilitates loss of heat from the blood as it flows through the pulmonary capillaries.
5. The metabolic rate falls in hot conditions so that less heat is generated by the body. That is why animals are generally less active in hot weather than in colder conditions.
Thus, when the temperature of the environment changes, the body takes the necessary steps to maintain its own temperature at a constant level. Figure 11.15 depicts the summary of the structures involved in the reflex control of body temperature in a mammal.
Thus in mammals the hypothalmic centre function as a thermostat. It is sensitive to temperature changes of the 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 (Fig. 11.16).
On the other hand, if the temperature falls below the optimum level, the centre initiates processes that produce and conserve heat (Fig. 11.16). The skin receptors detect temperature at the surface and enable the animal to feel whether the external environment is hot or cold. This information, acting via the thermo-regulatory centre, initiates voluntary activities such as taking muscular exercise in severe cold or moving into the shade if it is very hot.