ADAPTATION TO ENVIRONMENT

• Climatic Adaptation
  •  The Thermal Environment
  •  Bergmann’s and Allen’s Rules
• Climatic Stress and Man’s Physiological Response and Acclimatization
  •  Hot Climate Adaptation
  •  Cold Climate Adaptation
  •  High Altitude Adaptation

CLIMATIC ADAPTATION

• Every habitat is identified by a specific climatic regime and enforces it on man, his domestic animals, and crops.
• The human body is sensitive to various climatic elements of a region; this sensitivity and responsiveness result from the need to maintain homeostasis.
• Regulatory system acts to maintain the body temperature steady within relatively narrow limits (homeothermy), under conditions of extreme heat or cold.
• At high altitudes the respiratory system keeps the pressure of oxygen and carbon dioxide of the body-fluids adjusted within unwavering limits. Apart from the stresses imposed by thermal factors and low barometric pressure at high altitudes, other environmental hazards are provided by excess of short-wave radiation (ultraviolet light and ionising radiation).

The Thermal Environment

• Climate include the air temperature, humidity, speed and direction of the wind, amount and type of cloud, hours of sunshine and total radiant heat received, rainfall, snow, dust, and other special features.
• The climatic factors of primary physiological significance are those which exert a direct influence on the rate of heat exchange between the body surface (nude or clothed) and the surroundings.
• Heat is lost from the skin when body temperature is higher than environment. On the other hand heat is gained when external objects are warmer than human body.
• The disturbed homeotherm affects the body’s efficiency. The adaptive relations of human body with the thermal environment are of three sorts:
• Physiological adjustments in the heat regulatory, metabolic and circulatory systems make it possible to work and survive in a wide variety of environments. These can be both, short and long-term. The ability to make these adjustments is a highly developed characteristic of the human species as a whole.
•  Specialised physiological and anatomical adaptive responses based on particular genotypes are also observed.
• Cultural and social adjustments engage the provision of shelter, clothing, warmth, and ventilation. These cultural responses to biological requirements are like nutritional requirements, ‘institutionalised’ in Malinowski’s sense. The usefulness of these cultural responses can be judged by biological criteria.

• The efficient adaptation of the human body to climatic change is necessary for: a) Attainment of bodily comfort. b) Performance of habitual physical work without undue fatigue, performance of skilled work calling for alertness and dexterity with minimum errors. c) Attainment of normal growth and development, ability to reproduce successfully.

• An example is given to assess the human capacity to withstand climatic stress in the extreme conditions of human habitation: The village of Verkhoyansk in eastern Siberia almost on the Polar circle, and the oasis of Insalah in the Algerian Sahara. This reinforces that human communities are found surviving efficaciously in the extreme climates .

• The body temperature is highest in the early evening and lowest at about 4 a.m reflecting a diurnal range. This shift takes place regardless of race, e.g., Asian (in America or Europe) have body temperatures of the same values as indigenous peoples.

• The limits to which climatic adjustment can be made must be looked through two biological processes-the maintenance of comfort and the overriding maintenance of heat balance.
• The body is in a state of thermal equilibrium with its environment when it loses heat at exactly the same rate as it gains heat. The basic relation underlying homeothermy is: Heat Gain-Heat Loss = ± Heat Storage,   or, M+C+R-E =+S, where, M = rate of heat produced by metabolism and work, C = rate of heat gain or loss by convection, R = rate of heat gain or loss by radiation, E = rate of heat loss by evaporation of skin moisture, S = net heat exchange. S can be kept relatively constant while the other variables change over a wide range.
• When storage or withdrawal of heat becomes excessive, a number of immediate counter-adjustments are made. Further long-term processes within the life time of an organism reduce the strain, referred as ‘acclimatisation’. These physiological responses have been demonstrated in different races of human.

Bodily characters affecting thermal response: Organism attains equilibrium between heat production and heat loss in order to maintain a constant internal body temperature. The potential avenues of heat exchange include Radiation ( It refers to heat transfer in the form of electromagnetic waves emitted by one object and absorbed by other), Convection ( It refers to transfer of heat by a stream of molecules from a warmer object towards a cooler object.) , Conduction( Heat is lost from the skin as coming in direct contact with an object possessing a colder temperature.) and Evaporation( Heat is lost from the skin by evaporation of sweat from body surface as heat is utilised in conversion of water to vapour).

The rate of heat exchange by convection, evaporation and radiation is proportional to the surface area of skin. Larger the surface area the greater will be the loss of heat. When sweating provides the main channel of heat loss the total loss highly correlates with the surface area.

The rate of heat exchange is to some extent influenced by body-size and shape. The amount of heat produced during a physical activity highly correlates with the body-weight of the subject. As smaller subjects have greater surface area per unit of body weight, exposed to environment in contrast to larger individual who have greater body weight to surface area. Thus, smaller individuals dissipate heat at faster rate than larger individuals. This is based on the fact that the bodyweight increases as the cube, while the surface area increases as the square; also for a given body-weight physique which approaches a linear (ectomorphic) rather than a spherical (endomorphic) shape will have a relatively greater surface area. These anatomical properties make it unnecessary for the smaller individual to sweat as much per unit area as the larger individual, and the total water requirement by the smaller individual is both relatively and absolutely less. The heavier subject, putting out more sweat per unit area, will need to have harder-working sweat glands (for it is likely the glands are not present in greater density). Thus, in hot environments individuals of smaller and of linear build would appear to possess some biological advantage.

Body-shape affects heat loss in another way. Both convective and evaporative coefficients (i.e. the heat loss per unit area and per unit temperature or vapour pressure gradient respectively) approach constant values for large surface but increase rapidly as the diameter of the limbs is reduced below a diameter of about 10 cm. The evaporative coefficient of heat loss will be twice for a limb of diameter 7 cm than of 15 cm.

Bergmann’s and Allen’s Rules

Human population has been exposed to a wide variety of different ecological stressors. Adaptive changes to these widely differing selective pressure results into diversity among human population. A strong relationship exists between the physical characters and environment which has led to formulation of several ecological rules.

a) Bergman (1847) rule states that “ within the same species of warm blooded animals, population having less bulky individuals are more often found in warm climates near the equator, while those with greater bulk or mass are found farther from equator, in colder regions”. Basic to Bergman’s rule is the principle that smaller one has larger skin surface in proportion to mass(3) which enhance body heat loss(more rapidly). Therefore, it seems to be a hot climate adaptation. While larger individuals have increased mass to surface area ratio which reduces heat dissipation hence appear to be a cold climate adaptation. Thus indicating that heat production is a function of the total volume of an organism whereas heat loss is a function of total surface area. (Noted observations : more volume of body more heat production + More value of ratio of Surface Area to volume , less rate of heat loss and vice versa)

For example, consider two hypothetical or ganisms whose body shape is that of a cube. Imagine that one cube is 2 cm long and the other is 4 cm long in each dimension . The volume of the 2 cm cube is 8 cm3, and that of 4 cm cube is 64 cm3. As a measure of heat production, the larger cube with greater volume produces more heat. This means, the greater the volume of a mammal, the more is heat produced. Surface area of the 2 cm cube is 24 cm2, and that of the 4 cm cube is 96 cm2. As a measure of heat loss, cube with the greater surface area seems to lose it at a greater rate. This indicates more heat production and loss by the larger cube produce. However, the relevant factor in heat loss in mammals is the ratio of surface area to volume. The surface area/volume ratio is 24/8 = 3 for smaller cube, and 96/64 = 1.5 for the larger cube. Therefore, the larger cube lose heat at a slower rate relative to heat production discerning better adaptation to colder climate while smaller cube lose heat at faster rate favouring adaptation to hot climate.

b) Bergmann’s rule also involves the shape of an object and its relationship to heat loss.

The first object is a 4 cm cube of volume – 64 cm3, surface area – 96 cm2, and surface area/volume ratio – 1.5. The second object is a rectangular block 2 cm wide, 4 cm deep and 8 cm high. The volume of this object is also 64 cm3, but the surface area is 112 cm2 and surface area/volume ratio is 112/64 = 1.75. Even though both objects produce the same amount of heat as measured by their volumes, the rectangular object loses heat more quickly. This means, linear objects such as the rectangular block would be at an advantage in hot climate, whereas less linear objects, such as the cube, would be at an advantage in cold climates.

Accordingly, Bergmann’s rule predicts that mammals in hot climates will have linear body shapes and mammals in cold climates will have less linear body shapes.

Another zoologist, J. Allen, applied these principles to body limbs and other appendages. Allen’s rule (1877) states “among warm blooded organism, individual in population of same species living in warmer climates near the equator tend to have longer limbs than do population living away from the equator in colder environments”.

CLIMATIC STRESS AND MAN’S PHYSIOLOGICAL RESPONSE AND ACCLIMATIZATION

Hot Climate Adaptation

Responses to heat: The instant physiological response to overheating is a compensatory increase in heat dissipation from the body accomplished mainly through adjustment of cardiovascular system followed by sweating. There is increased flow of blood through skin due to vasodilatation to start with. Due to additional strain there is an elevated cardiac output accompanied by increased pulse rate. The heat brought to the surface thus elevates the skin temperature facilitating dissipation of heat to the surroundings by convection and radiation. This heat loss per unit surface area is proportional to the temperature gradient between skin and external environment; and square-root of the wind velocity.

When environmental temperature increases above the skin temperature circulatory adjustment are not adequate for heat dissipation by convection and radiation because of negative gradient, in fact it gains heat. This results in sweating and heat loss by evaporation of evenly distributed sweat is effective. The rate of heat loss by evaporation (latent heat of vaporisation) influences the variance between the vapor pressure at the skin surface and that of the air, extent of the wetted surface area and air movement. The rise in skin temperature reflects an increased vapor pressure at the skin surface. The sweating rate is raised by increasing the number of active sweat glands and rate of fluid output of each gland.

The flexibility of physiological response in a variety of habitats is clearly demonstrated by the way homeothermy is maintained against heat load. The hot environment can be classified as:

• Hot, humid, and still-air condition,
• Hot, dry conditions with moderate air movement and high solar radiation,
• Cool, dry condition with solar radiation, and
• Hot and moderate temperatures.

Acclimatisation to heat: Repeated exposure to heat results in acclimatisation enhancing tolerance to work under heat stress. The heat regulatory system became more efficient. The body and skin temperature, which rise rapidly to high levels on the early exposures, rises slowly or attains a ‘plateau’ on continued exposure to the heat. The bodily changes which are elicited under artificial conditions (acclimation) can also be revealed in natural environment (hot climates-equatorial or desert). The complex physiological changes which lead to acclimatisation has been demonstrated in population of different races living in hot climates e.g. Nigerians, Chinese, Indians, and Malayans living in Malaya, Kalahari Bushmen and South African Bantu, as well as in Europeans habituating tropics or hot deserts.

Since the emergence of genus ‘Homo’ in tropical region Homo sapiens have occupied diverse tropical and equatorial habitats. Acclimatisation as a consequence of physiological changes enable individual to inhabit wide variety of hot environments. Genetic selection of various bodily characters for life in diverse climates has been superimposed on physiological plasticity.

Cold Climate Adaptation

Responses to cooling: Within the thermo-neutral temperature range of 25ºC to 27ºC the individual is in thermal equilibrium, but below this range nude individual responds immediately through mechanism which permits both increased heat production and conservation. The major mechanism concerned with heat conservation is vasoconstriction (constriction of subcutaneous blood vessels). The lowered blood flow decreases skin temperature, reduces temperature gradient between skin surface and environment, consequently reducing the rate of heat loss or thermal conductance. Other important factor affecting heat loss is the degree of artificial or natural insulation. Subcutaneous fat layer has lower thermal conductance. The level to which skin temperatures fall on exposure to cold is highly correlated with skin-fold thickness. Differences in thickness of subcutaneous fat-layer are significant among populations. Fatter subjects show longer endurance on immersion in cold water. To survive in the extreme cold condition of Arctic man must be supplemented with extra insulation which can be obtained by the use of animal skins or other clothing material. Greater the insulatory layer the lower is the heat loss. Eskimo clothing of caribou fur 3.75-7.5 cm thick. Eskimo dwellings are kept at 21°C during the day and 10°C during the night. Eskimos spend roughly 1-4 hours per day outdoors in winter, 5-9 hours in summer. Only the face and occasionally the hands are exposed to chilling.

Heat production is increased involuntarily by shivering and voluntarily by muscular activity. Shivering can provide about 3 times the resting heat production. It is induced by reflex stimulation of hypothalamic centre by cold receptors in response to fall in skin temperature. Large amounts of heat can be produced by voluntary exercise but is limited by physical fitness and availability of food. Eskimos have learned to run for long periods behind their sledges at a rate sufficient to keep them warm but not to exhaust them, and their fitness measured by standard tests is higher as compared to Canadian Whites (Shepherds).

When a body is exposed to extremely low temperature the skin surface of exposed area such as fingers of hand and feet freeze (frost bite).Thus there is intense vasoconstriction followed by vasodilatation. A cyclic continuation of these vasomotor changes prevents the tissue temperature dropping to frostbite level.

Acclimatisation to cold: On exposure to cold acclimatisation, that is, increased tolerance to cold develops gradually. Increased heat production and conservation reduces the extent of fall in the rectal temperature. On severe exposure to cold, physiological and metabolic process improve to maintain thermal equilibrium accounting for body comfort. For example, newcomers to Arctic regions will wear all their available clothing at the onset; but as air temperature falls further no protection is sought. Later they often wear less at work, or while asleep. It has been observed that members on expedition who spent the greater part of their time indoors suffered frostbite at low temperatures in 1 minutes, whereas an outdoor group resisted this for nearly 10 minutes. At the same time well-adapted individuals have a keen admiration of their lower limb and face and will take the necessary action to prevent frostbite (Irving). Evidences points to increased basal metabolic rate in concern to maintenance of thermal equilibrium. Eskimos show a larger increase than Europeans living in comparable cool conditions. However, it remains uncertain whether raised metabolism depends on cold exposure, altered endocrine function, or diet. The total food energy intake in the Arctic is high relative to that in the tropics but fluctuates widely, depending on indoor conditions as well as on work. Some believe that a higher protein intake account for the increased metabolic heat production. It seems, however, that fat is equally or perhaps more important than protein in exerting a protective effect against cold. Many Arctic explorers and others living in the north report that fat becomes positively desired. United States members of an Antarctic expedition who had an abhorrence of fat at home would eat fat in great quantity. Thus, the two processes common in the animal kingdom for enhancing adaptation to cold are also operative in man-(a) increase in heat production and (b) increase in fat insulation layers, though man depends, of course, on artificial insulation as well.

Acclimatisation has been demonstrated in indigenous peoples also. Central Australian Aborigines do not wear clothing except for genital covering. The degree of cold exposure in sleeping microenvironment is below the thermoneutral temperature. Despite this cold stress they sleep naked comfortably without shivering, whereas the European controls studied under the same conditions shivered continuously and were unable to sleep. The Aborigines were able to endure greater fall of skin temperature than the Europeans.

The Bushmen of the Kalahari and the Australian Aborigines sleep in extremely cold conditions with single covering and a small fire as protection. A high degree of cold tolerance can to a large extent be acquired by Europeans. Norwegian students who lived in the open for 6 weeks with minimal protection were initially unable to sleep but subsequently were able to do so in spite of shivering.

Eskimos have greater tolerance to cold in the hands than white men. Evidence from the laboratory from a comparative study of finger temperatures following immersion in water at freezing-point among Eskimo, Indian, Negro and European shows greater tendency of Negroes to frostbite. In tests of sensory-motor function Eskimos and Indian young men in the North-West Territory of Canada were unquestionably superior to a similar group of European workers with only 6 weeks’ residence.

The capacity of acclimatised men, and especially the Eskimo, to use the hands well at low temperatures is linked with an increased blood-flow.

High Altitude Adaptation

The effects of altitude: Approximately 10 million people live permanently at heights 3600m-4000m. Life at high altitudes imposes a complex ecological stress of low barometric pressure (which acts by lowering the oxygen and carbondioxide pressure in the inspired air), cold , low moisture content of the air and intense radiation. In addition, the rough terrain imposes higher muscular activity.

Lowered barometric pressure: Oxygen constitutes about 21 per cent of the atmosphere by volume; hence the pressure of oxygen is roughly one-fifth of the total barometric pressure. When the pressure is high as occurs at the sea-level (150mmHg) the oxygen diffuses across the lung membranes and proportion of hemoglobin saturated with oxygen is sufficient for all needs. While at altitude decreased barometric pressure reduces the rate of oxygen diffusion into lungs, proportionately decreasing the amount of oxygen available to the working tissues. At an altitude of 2400-3000 m and above oxygen demand during physical work is not met and beyond 4000m the rate of oxygen transfer is insufficient to satisfy efficiently even the needs of sedentary man. Immediate exposures of body to such conditions displays limited compensatory response and accompany breakdown. However, secondary response of physiological acclimatisation ensures physical work and permanent residence at high levels. The cultivated land in Tibet is at altitudes of 2700-4500 m, and permanent settlements in the Andes at 5200m, and daily visits to 5800m for mining and pastoralism with agriculture above 4000m substantiate it.

Responses: The immediate response to lack of oxygen (hypoxia) is an increase in the volume of air respired per minute. This is brought about by rapid and deeper respirations. The increased ventilation leads to the ‘washing out’ of carbon dioxide from the air passages and consequently from the blood. This loss of carbon dioxide alters the homeostatically controlled acid base balance of the body to a more alkaline level termed ‘alkalosis’. This inhibit the stimulus for increased ventilation, which is counteracted by excretion of alkaline urine (bicarbonate ions) by kidney thereby shifts pH of blood to normal level. The augmented heart rate and cardiac output attained in response to immediate exposure decline with acclimatisation. Heart rate reduces to normal sea level followed by reduced cardiac output.

Oxygen transport is aided by the pressure gradient between lung and blood at pulmonary level and between blood and tissue at organ level. At high pressure more oxygen and hemoglobin combine while hemoglobin dissociate from oxygen at low pressure. Exposure to hypoxia favors increase in red blood cell and consequently hemoglobin concentration, enhancing oxygen carrying capacity of blood. There is linear relation between hemoglobin (Hb) and barometric pressure. Upto 3500m it rises steeply. Augmented viscosity accompanying polycythemia contributes to increased pulmonary arterial pressure. This enhances effective blood gas interfacial area of alveoli and diffusing capacity of lung which permit effective arterial blood oxygenation. Long term morphological changes ensuethe enlarged thorax with the increased respiration and the vital capacity. Increased Hb concentration along with respiratory adjustments enables native residents to achieve arterial oxygen content above that of pressure at sea-level and increased ability of tissues to work at low oxygen tensions. Increased pulmonary arterial pressure is associated with right ventricular hypertrophy indicating increased workload characteristic of native population. Natives rely on moderate increase in pulmonary ventilation and polycythemic response while more on diffusion of oxygen from blood to tissue. At 4500m acclimatisation takes place in approximately10 days. Clothing, shelter, and heating arrangements are generally effective in protecting against extreme cold climate of high altitude (Tibet, 5000m, -33°C and summer temperature rise only to 13°C), though some degree of physiological acclimatisation has been observed.

Populations at high altitude: Slight adaptive physiological changes have been found in population even at 2000m like Caucasus, Pyrenees, and even the High Veld of South Africa. A variety of racial groups inhabiting higher altitude includes the Tibetans living up to 4500 m, the inhabitants of Kashmir at 3000 m, those of the high uplands at 2500m, and the Andes at 5500m. They provide ample evidences of acclimatisation. For example the native inhabitants of Peru may have red-cell counts 30 per cent above the sea-level value. Though the responses described here are secondary acquisitions, it is quite conceivable that some characteristics are fixed genetically due to selection pressure. The llama, the vicuna, and Bolivian goose, all native to high altitudes, have high values for red-cell count and increased affinity of the hemoglobin to oxygen and these characters are retained by first-generation llamas born at sea level. Monde argues that selection among the Spanish invaders of the Andes, eliminated the infertile individual and pure Spaniards established themselves after several generations. But convincing evidence of genotypes adaptive to altitude stress in man is yet to be established.

Pathological response : High altitude hypoxia elicit direct and indirect responses, some of them can cause mild to severe malformation, eventually becoming deleterious to organism. Monge disease or Mountain sickness is a complex pathophysiological condition that occur when normally acclimatised individual lose their ability to adapt to altitude as a consequence of anoxia and alkalosis. Symptoms include nausea, vomiting, headache, insomnia, acceleration of heart rate, deterioration of neuromuscular co-ordination, diminished auditory perception, diminution of visual activity and fatigue.