30 Adaptation to ecological stress-temperature, latitude and altitude

Imkongtenla Pongen and Meenal Dhall

Contents:

Introduction

1. Ecological rules

2. Standard temperature trials

3. Acclimatization to temperature

4. Adaptation to hypoxia

5. Organ-system adaptations

6. Individual adaptation

6.1 Physical performance

6.2 Nervous system: Sensory and psychological performance

6.3 Growth and development

6.4 Nutrition

6.5 Reproduction

6.6 Morbidity and mortality

6.7 Cross tolerance and resistance

7. Population adaptation Summary

Learning Objectives:

1. To summarize the ecological rules

2. To describe standard temperature trials and acclimatization to temperature

3. To describe adaptation to hypoxia and organ-system adaptations

4. To describe the various facets of individual adaptation to hypoxia

5. To explain population adaptation to hypoxia

Introduction

In this module, we will focus only on those aspect of adaptation that is of interest to anthropologists. This aspect comprises genetic adaptation that are characteristic of groups of people some of whose phenotypes better suited to environment were produced as a matter of natural selection. Restriction of this module only to heat and altitude stress would be arbitrary since most of the population undergoes cold stress on a seasonal or diurnal basis.

The primary problem for man in hot environment is heat dissipation and there are no major racial differences in the physiological mechanisms involved in dissipating heat. Man relies on convection and radiation forms of heat loss upto the point when the surrounding air and objects become hotter than the skin. Adjustment of heat loss is accomplished by altering the flow of blood through the capillary bed of the skin by vasolidation (widening of the blood vessels to increase the blood flow and result in the loss of heat) and vasoconstriction (narrowing of the blood vessels and reduction in the flow of blood which results in decrease in the loss of heat). While exercising, heat is loss by thermal sweating (evaporative heat loss) and this evaporation is the only way to lose heat when air temperature exceeds the temperature of the skin.

The assessment of adaptation of man to high altitudes presents an interesting paradigm of dual problem-delineating the actual pattern of structural, functional or behavioral response and evaluation of the adaptive value to a response based on the benefit of that response to the individual or population. The most apparent stresses for man at high altitude include hypoxia, high solar radiation, cold, aridity, high winds, limited nutritional base and rough terrain. Of these, hypoxia is of predominant interest because the other stresses are present equally or in greater degrees in other geographical zones, but hypoxia resulting from reduced barometric pressure at altitude stands alone. Barometric pressure and the degree of hypoxia varies in different geographical locations at elevations that are comparable (Moore, 1968). Hypoxia is interesting to physiological anthropologist even though it affects only a small segment of the people. It is due to the fact that hypoxia is an ever-present stress which can be ameliorated by altering the behavioral patterns. It also affects all organ systems and physiological functions of the human body. Even though hypoxia is present only at high altitude, it is encountered commonly at sea level. Evaluation of genetic basis for population adaptation to environmental stress is another reason for altitude effect studies on human populations.

1. Ecological rules

Bergmann’s rule: Bergmann (1847) observed that the body size increases with decrease in temperature of the habitat within a single, polytypic, warm-blooded species.

Allen’s rule (1877): Allen observed that mammals living in cold climates have smaller extremities and appendages than those who stay in warmer climates.

The aforesaid rules are applicable only when the species are subjected to wide range of climatic extremes and the species have not differentiated to the point where apparent thermoregulatory response is seen.

2. Standard temperature trials

To explore differences in group of people to heat adaptation,they are subjected to heat stress and their responses are compared which is termed as the “experimental approach” as coined by Henschel in 1967. Skin temperature and some measures of internal temperature, rate of sweating, heart rate and oxygen consumption are used as measure of metabolic response in such studies. A combination of work and heat is utilized in most of these studies because this shortens the time period to achieve high level of protective response (production of sweat) since it imposes both an external and internal heat load. There are many factors that influence the performance of an individual in combination of heat and exercise, some of which are body size, state of heat acclimatization, state of physical fitness, nutrition, state of health, etc.

Temperature of skin is not a vital measure in these experiments although it is recorded. A subsequent rise of skin temperature above 36 ̊ C indicates sweating and is interpreted as a sign of danger . Measurement and interpretation of evaporation is essential as it is the only way of heat loss.

Sweat production ranges from almost none to 2 litres per hour and it may decrease from early peak levels if the exposure is prolonged. Dripping sweat refers to the marked capacity of man to produce sweat more than can be evaporated on the skin. It is a maladaptive phenomenon unique to humans.

Exposure to heat causes a slight increase in heart rate which compensates for the increased vascular bed resulting from peripheral vasodilation. However exercise with exposure to heat also increases heart rate to fulfill the greater metabolic demand and the skeletal musculature.

When exercise and heat are combined, there is a dramatic increase in heart rate at the initial stage followed by a slow increase subsequently. Any rise or fall in heart rate indicates collapse. Accomplishing a given workload with least increase in heart rate is an efficient response when there is adequate transport of heat due to the fact that the volume per beat i.e., stroke volume is relatively fixed. Physical training increases stroke volume through repeated exercise and allows the workload to be performed with small increase in heart rate. Extensive exposure to heat without exercise (passive heat acclimatization) has no effect on heart rate hence this kind of measurement is only indicative of physical fitness.

3. Acclimatization to temperature

Artificial heat acclimatization is a set of changes in response to controlled exposures of heat in extent of sweat production, changes in heart rate, and changes in endocrine function brought about by loss of sodium contained in the sweat. These changes can be observed in healthy subjects exposed to a heat work situation after 5-10 days. Natural heat acclimatization is a thermoregulatory adaptation which requires frequent stimulation for full maintenance.

Internal temperature does not show as much initial increase and level off or rise very slowly with exposure. Sweat is produced sooner and reaches a high point earlier and is maintained at a constant rate for an extended period of time. Heart rate is not much increased as indicated earlier that it is primarily a function of exercise. Temperatures of skin were reported to be lower in acclimatization involving dry temperature/heat (Lee, 1964) and higher in humid heat (Ladel, 1964). An additional change that occurs in heat acclimatization is the reduced concentration of salts, particularly sodium chloride in each unit of sweat .But this is not of importance for assessing the effects of short term exposure to heat because the higher production of sweat balance the decreased concentration of the salts. In 1966, Wyndham observed acclimatized Negros having lower hourly sweat rates than acclimatized Europeans but when the group averages were corrected for body weight or surface area, the differences diminished.

Sweating is an efficient mechanism to maintain internal temperature only when the sweat can be totally evaporated as fast as it is produced. But this is quite impossible in humid heat, thus man’s tolerance is reduced in such environmental conditions. It would be beneficial if man regulates sweat production to fit the evaporation rate but he does not do so. Regardless of how much is evaporated and how much drips to the ground, greater sweating is promoted by an increase in internal temperature because sweating is regulated mainly by internal temperature than by skin temperature except for special circumstances as high input or application of hot water to the skin.

This module has emphasized the uncertainties and the complexities involved in comparing different groups of people with respect to heat adaptation. A given population may have ignored or buffered the heat stress for generations or may have exploited maximum biological potential. One way in which anthropologist can contribute to the understanding of temperature adaptation is by trying to identify the interface between what people can do and what they actually do starting with the assumption that these parameters are not identical.

4. Adaptation to hypoxia

4.1 Assessing adaptation to hypoxia

The basis for assessing human adaptation to hypoxia comprises of evaluation of relative benefit or degree of necessity to environmental stress (Mazess, 1975). Adaptive implies selective advantage or fitness.

The adaptive abilities of highlanders to lowlanders are enhanced ventilator efficiency or an increased work capacity. Adaptation at the individual level is assesses at adaptive domains which includes physical performance (exercise and motor abilities/skils), functioning of nervous system (sensory, motor and neural functions), growth and development (progression in rate and attainment), nutrition (requirements, utilization, efficiency), reproduction (survival, reproductive advantage), health (morbidity, mortality, resistance to diseases), cross-tolerance and resistance (stress resistance), affective functioning (happiness, tolerance, sexuality) and intellectual ability (learning, expression).

Adaptation at the infra-individual level implies adaptation that is of benefit or necessity to the individual. This benefit or necessity are translated as homeostasis and resistance/tolerance. At high altitude, hypoxia produces stress at the beginning of oxygen transport chain. The responses at infra-individual level are by increasing oxygen transport at any level between the inspired air and tissue or by allowing function to continue despite lower oxygen tension. A cellular change that maintains homeostasis is an infra-individual adaptation but if it benefitted the individual or population then it is termed as individual or population adaptation.

4.2 Problems in assessing adaptation to altitude

Assessment needs to examine relative benefit or necessity in an adaptive domain.It is not only sufficient to show that a response occurred or that individuals and populations differed in their responses but rather some benefit has given. Large chest or high hematocrits in native highlanders are viewed as distinctive characteristics which are beneficial (Monge, 1948). For instance, hyperventilation i.e., increased oxygen transport might be beneficial to individual or populations without demonstrating advantage in any of the adaptive domains.

Secondly, all phenotypic stable adaptive (aptitudes) characteristics are not genetic adaptations. Acclimatization responses are far more vital than aptitudes in altitude hypoxia. The only common example of a hypoxic aptitude mistakenly considered as genetic adaptation is large size of chest and large lungs . However, large chest and large lungs are mere ventilator acclimatization as hyperventilation.

It is also essential to note that the adaptive benefit maybe temporal, spatial and population specific. For instance, those characteristics that occur with long term exposure may not be beneficial in short term exposure and may be also harmful. In the first few days to months at altitude exposure, increased hypoxic sensitivity and hyperventilation is acclimatizational. A diminished hypoxic ventilator defect occurs during developmental period in long term exposure. Hyperventilation would be advantageous to a population with a rightward shift on the hemoglobin-dissociation curve where arterial saturation is maximized by high alveolar oxygen tension than to a population with leftward shift.

5. Organ-system adaptations

A variety of responses enhance the oxygen transport or increase the resistance to hypoxia at organ system level. The only major aptitude considered important at organ system level in altitude hypoxia is a large chest and a large lung.Hurtado (1964) was of the view that it is not certain if this larger chest is directly associated with larger lungs or whether larger lung volume would be functionally important because the Andean highlanders differ from lowlanders only in their larger lung residual volume. Therefore, a large chest cannot be assumed to be an adaptive aptitude because it does not have a direct relation to increased transport of oxygen.

Following are the infra-individual adaptations to hypoxia which are acclimatizational:

i) Red blood cells: Increase in blood viscosity may affect the flow of blood and circulation.

ii) Hemoglobin-Oxygen dissociation curve: This curve shifts towards decreased affinity leading to enhanced release of oxygen to the tissue. This shift is more advantageous to the newcomers than the residents.

iii) Circulation: There is increase in cardiac output at rest during first days of exposure and then return to normal levels. Such circulatory alterations such as reduced flow to kidneys and heart occur with greater flow to muscle and brain.

iv) Capillarity: Increase in capillarity aids the process of diffusion in the tissue.

v) Ventilation: Among native residents and long sojourns there is diminished hypoxic sensitivity and hyperventilation.

vi) Cellular metabolism: There is higher concentration of muscle myoglobin and phosphates, increase activity of glycolytic and oxidative enzymes and enhanced aerobic glycolysis in the tissues of highlanders. These changes lead to increase in resistance to tissues to low oxygen tensions.

6. Individual adaptation

Here the effects of individual adaptive domain are described.

6.1 Physical performance

Exposure to high altitude impairs physical abilities. The maximal work capacity of newcomers shows a decreasing trend and submaximal work becomes more stressful leading to increase in heart rates and rapid breathing decreasing the endurance time. Decrease in aerobic capacity is directly proportional to the training extent: athletes show greatest decrease, sedentary individuals the least and active individuals the intermediate. Submaximal work at high altitude use a greater percentage of reduced capacity of total work load which is known as relative work load.

Highlanders have a slightly increased aerobic capacity at sea level indicating that they have slight reduction in maximal performance at high altitude. The high endurance of submaximal work in them reveals their maximal capacity and the lower relative work at a fixed oxygen intake. This maybe due to their small body build, and their consequent usual activities involving moving of body weight and their endurance ability to work due to their vigorous activity level.

6.2 Nervous system: Sensory and psychological performance

Nervous tissue is susceptible to oxygen deprivation and functioning of sensory and motor system is not possible at altitude extremes. Development of central nervous system is delayed at high altitude. Brief anoxia in fetal stage causes damage and retardation of brain with late appearance, eg: Andean Quechua infants. Even at moderate altitude, visual and colour sensitivity decreases.

6.3 Growth and development

Organs such as lung, heart and spleen gets reduced in weight although not proportional to body weight while thymus and kidney appear to be so much reduced. The reduction in hypoxia is due to reduction in number of cells with normal or large size of cells. Growth of genetically similar people is advanced in mountainous regions than the disease prone low -landers. There is relative increase in rate of growth of the chest in highlanders without functional importance to ventilator function. The reduced size of the body of the highlanders may be beneficial during chronic food shortage, famine and/or energetics of work.

The overall diminution of growth and development that occurs in hypoxia indicates that adjustment even with chronic exposure is not complete. Results of alterations in growth that occur in hypoxia may be beneficial (increased diffusion capacity of lungs), harmful (reduced ventilator drive) or uncertain (alteration of pulmonary circulation).

6.4 Nutrition

People living in high altitudes are subjected to seasonally deficient calorie and nutrient intake than the lowlanders. Acute exposure to hypoxia is associated with loss in body weight, including loss of both water and tissues. Hypohydration and anorexia occurs at varying degrees. We see negative balances of nitrogen and calories and other nutrients in response to extreme exposures leading to reduced efficiency of food utilization. There is exposure of fat mobilization from the body leading to increased serum free fatty acids. There is alteration of carbohydrate and protein metabolism as well.

6.5 Reproduction

There is effect of moderate altitude on reproduction during the first days or weeks of exposure. There is marked effects on litter size, birth weights, lactation, gonadal function and success of pregnancy due to exposure to extreme altitudes. Adaptation to high altitude among neonates include increased placental size and vascularity and increased oxygen capacity of the fetal blood due to increase in mass of hemoglobin. Better care of neonates had lead to lower mortality in the US.

6.6 Morbidity and mortality

Lack of medical facilities in high altitude regions has caused two problems: lack of adequate data on incidence of disease and cause of death and the statistics that exists are biased by this lack of medical care in the region. Infectious disease seem be higher among the highlanders than the lowlanders. Altitude also has debilitating effects on the lungs thereby exacerbating pulmonary disease. There is low incidence of chronic diseases and hypertension of some altitude residents which could be primarily due to genetic and dietary factors rather than of hypoxia. Varicose veins occurs at high altitudes in relation with higher hematocrits and greater blood viscosity. Acute mountain sickness that includes symptoms such as dizziness, shortness of breath , stomach upset is most widely prevalent. Pulmonary edema is the most severe of altitude sickness which is characterized with impaired exchange of pulmonary oxygen and pulmonary hypertension. Congenital abnormalities of different kinds appears to be more common at high altitudes since these are marked by fetal anoxia.

6.7 Cross tolerance and resistance

Humans exposed to hypoxia continuously or for a short period of time show enhanced responses and survive in more severe stress of hypoxia. Cross tolerance and resistance that occur in hypoxia are results of specific hypoxic responses rather than from “general adaptation syndrome”. In chronically exposed individuals to severe stresses such as mountain climbing, adrenal activity and catecholamine appear normal. Increased vascularity affecting both capillary and larger vessels occur in hypoxia enhancing diffusion of oxygen to the cells.

7. Population adaptation

Population adaptation is assessed in association to demographic optimality, spatial-temporal spread, ecological efficiency and reproductive success. Infra-individual adaptations demonstrated among natives have made investigators view altitude native as an adapted individual and consider the population as also adapted. Humans have mildly impaired fertility in hypoxia which is associated with increase in perinatal mortality but there is no such mortality at moderate elevations. Census data show reduced reproductive success of native highlanders but enhanced reproductive success of the former with sojourners indicate selective advantage of the former group.

Altitude population show high proportion of males except where there is selective out migration of males and relative deficit of older people which may be beneficial in a nutritionally marginalized society. There is evidence of temporal permanence of native highlanders. Density of altitude populations are less as compared to lowlanders which reflects the technological and cultural patterns.

Summary

The primary problem for man in hot environment is heat dissipation and there are no major racial differences in the physiological mechanisms involved in dissipating heat. Man relies on convection and radiation forms of heat loss upto the point when the surrounding air and objects become hotter than the skin. Adjustment of heat loss is accomplished by altering the flow of blood through the capillary bed of the skin by vasolidation and vasoconstriction. While exercising, heat is loss by thermal sweating and this evaporation is the only way to lose heat when air temperature exceeds the temperature of the skin.

Bergmann (1847) observed that the body size increases with decrease in temperature of the habitat within a single, polytypic, warm-blooded species. Allen observed that mammals living in cold climates have smaller extremities and appendages than those who stay in warmer climates.

Artificial heat acclimatization is a set of changes in response to controlled exposures of heat in extent of sweat production, changes in heart rate, and changes in endocrine function brought about by loss of sodium contained in the sweat. Natural heat acclimatization is a thermoregulatory adaptation which requires frequent stimulation for full maintenance. Sweating is an efficient mechanism to maintain internal temperature only when the sweat can be totally evaporated as fast as it is produced. Dripping sweat refers to the marked capacity of man to produce sweat more than can be evaporated on the skin. It is a maladaptive phenomenon unique to humans.

A given population may have ignored or buffered the heat stress for generations or may have exploited maximum biological potential. One way in which anthropologist can contribute to the understanding of temperature adaptation is by trying to identify the interface between what people can do and what they actually do starting with the assumption that these parameters are not identical.

The most apparent stresses for man at high altitude include hypoxia, high solar radiation, cold, aridity, high winds, limited nutritional base and rough terrain. Of these, hypoxia is of predominant interest because the other stresses are present equally or in greater degrees in other geographical zones, but hypoxia resulting from reduced barometric pressure at altitude stands alone.

The basis for assessing human adaptation to hypoxia comprises of evaluation of relative benefit or degree of necessity to environmental stress (Mazess, 1975). Adaptation is examined at different levels of hierarchy, namely, organ system, individual and population. In infra-individual level, many adjustments maintain homeostasis or increase hypoxia tolerance. Some abnormalities in reproduction occurs and physical performance, growth and development and nervous system deteriorates in extreme altitudes. The only advantage of hypoxia happens to be enhancement of resistance and cross-tolerance produced by increased cellular metabolism and oxygen transport. The adjustments that occur at the infra-individual level doesn’t provide full adaptation to the individual.

There is little or no benefit at altitude for any population. Some population may in fact be in disadvantage. High altitude populations have different demographic structures but their significance is unknown. Highlanders have survived of their own but have shown no relative benefits/advantage. Hypoxia is one of the environmental variables and both the pattern and adaptive value of responses is modified by interaction of many environmental and cultural variables of the population.

you can view video on Adaptation to ecological stress-temperature, latitude and altitude

Glossary

Allen’s rule Mammals living in cold climates have smaller extremities and appendages than those who stay in warmer climates.

Bergmann Rules Body size increases with decrease in temperature of the habitat within a single,polytypic,warm-blooded species

Pulmonary edema Most severe of altitude sickness which is characterized with impaired exchange of pulmonary oxygen and pulmonary hypertension.

Vasoconstriction The process of reducing the flow of blood to decrease the loss of heat Vasolidation The process of expanding the blood flow to increase the loss of heat

Self-Assessment

References

Baker, P.T. (1958). Racial differences in heat tolerance. Amer.J.Phys.Anthropol.n.s.16:287-305

Baker, P.T. (1969).Human adaptation to high altitude.Science,163:1149-56.

Consolazio, C.F. and Shapiro R. (1964). Energy requirements of men in extreme heat. Environmental Physiology and Psychology in Arid Regions Proceedings of the Lucknow Symposium. Paris: UNESCO.

Dalmont A (Ed.). (1975). Environment and Human Adaptation in the Sahara. In Physiological Anthropology. New York, NY: Oxford University Press.

Dalmont A (Ed.). (1975). Human Adaptation to High Altitude.. In Physiological Anthropology. New York, NY: Oxford University Press.

Haldane J.B.S.(1932).The Causes of Evolution. London:Longmans Green.

http://www.sciencedirect.com/science/article/pii/B9780128026526000189

Suggested Readings:

Hock, R. (1970). The physiology of high altitude. Sci Amer.(Feb):53-62.

Wyndham, C.H. (1964). Heat reactions of different ethnic groups. Environmental Physiology and Psychology in Arid Regions. Proceedings of theof ospital Interest Lucknow Symposium. Paris: UNESCO.

http://onlinelibrary.wiley.com/doi/10.1525/aa.1953.55.3.02a00020/full

http://www.jstor.org/stable/41455380?seq=1#page_scan_tab_contents

http://www.sciencedirect.com/science/article/pii/S0169534707002819

https://academic.oup.com/icb/article/46/1/18/661204/Andean-Tibetan-and-Ethiopian-patterns-of