Human Ecological Adaptations

Human Ecological Adaptations

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The term ecology is derived from the ancient Greek words oikos (house/habitat) and logos (study/science), giving the literal meaning of “science of the habitat.” The German zoologist Ernst Haeckel formally proposed the term in 1866. Ecology is thus defined as the science that deals with the interrelationships between organisms and their surroundings — encompassing all abiotic and biotic components of the environment.

Two landmark works published at the dawn of the twentieth century are considered seminal contributions to the discipline: Oecology of Plants: An Introduction to the Study of Plant Communities by Eugene Warming, and Guide to the Study of Animal Ecology by C.C. Adams. The ecological orientation in natural sciences had been thought to originate when attempts were made to study plants and animals systematically. Social scientists then applied these principles to human populations in the early twentieth century. It was at the Chicago School of Sociology that the term “human ecology” was proposed for the first time in 1921.

Ecology — Ernst Haeckel, 1866

The science that deals with the interrelationships between organisms and their surroundings — including all abiotic and biotic components of the environment.

Ecology as the scientific study

All plants and animals live their life in accordance with the topographic, edaphic, climatic, and hydrospheric conditions of their environment. Ecology is the scientific study of these interrelationships between all the abiotic and biotic components of environments.

Definition and Nature of Human Ecology

• Human ecology has been defined in multiple ways across disciplines. The most comprehensive academic definition is offered by Lawrence (2001):
  • “Human ecology is the study of the dynamic interrelationships between human populations and the physical, biotic, cultural, and social characteristics of their environment and the biosphere.”
  • Human ecology is an interdisciplinary and transdisciplinary study of the relationship between humans and their natural, social, and built environments. In order to survive, all societies must adapt to the opportunities and constraints that their environment presents, given their current culture. Successful adaptation can be said to have occurred when all of a society’s important values are achievable over the long term.
• Because humans live in a greater variety of habitats than any other species on Earth, it is natural to ask how they adapt to these varied environments. Human adaptation involves both biological and behavioural mechanisms. The human behavioural ability to modify the environment is the major factor that has allowed humans to occupy the diverse ecosystems they do. Many of the biological adaptations visible in humans are, in fact, adaptations to environmental conditions that humans themselves have produced.

Anthropological Approach to Human Ecology

• The empirical definition of human ecology in anthropology is that it is concerned with the adaptive ordering of the relations of human groups to the natural environment and with the demographic and sociocultural conditions and consequences of those relations (Helm, 1962).

The ecological approach in anthropology proceeds from the first aspect or level of the adaptive system — man in adjustive and exploitative interaction, through the agency of technology, with his inorganic and biotal milieu. This level has immediate implications for the second aspect: that of the relations between human beings themselves.

— Helm (1962)

• Anthropologists believe that the contribution of environmental factors to human culture is subordinate to ordering principles like kinship, social norms, rituals, and symbolism. The ecological approach is used to explain cultural diversity rather than to reduce culture to environmental determination.
• Surprisingly, humans are not the ideal species to study in order to understand the full range of biological adaptation — despite their obvious success as the dominant species of the Earth. This paradox is attributed to their higher intellectual capacity, which enables them to adjust their behaviour to environmental conditions. Humans therefore tend to exhibit more cultural adjustment than biological adaptation.
• Nevertheless, there are clear instances of physical and biological variation in human populations exposed to different climatic conditions across geographic locations. The best example is skin colour, which likely evolved as a genetic adaptation to ultraviolet radiation. Variation among populations in body size and shape may also relate to climatic adaptation.

Ecological Rules of Human Adaptation

• Most ecological rules were established during the nineteenth century, when it was observed that the form of many warm-blooded animal species varies in a regular way with climate. Three major rules govern morphological variation across latitudes and climates.

Gloger’s Rule

• This rule was proposed by Constantin Wilhelm Lambert Gloger in 1833. It states that races of birds and mammals in warmer regions are more darkly coloured than races in colder or drier regions. This is recognised as a valid generalisation about clines of melanism (Huggett, 2004).
• Gloger’s rule was first observed in birds, but was later seen to apply to mammals such as wolves, foxes, tigers, and hares. Among human populations, it is generally observed that people living in warmer regions closer to the equator are darker-pigmented than people living farther from the equator.
• Example: People living in equatorial Africa, South Asia, and Melanesia have higher melanin concentration than populations of northern Europe. Darker skin acts as a natural UV filter in regions of intense solar radiation.

Bergmann’s Rule (Size Rule)

• Also known as the size rule, it was established by Carl Bergmann in 1847. It states that species of birds and mammals living in cold climates are larger than their congeners in warm climates. It applies to a wide range of birds and mammals.
• The physiological basis: big animals have a thermal advantage over small ones in cold climates. As an object increases in size, its surface area becomes relatively smaller (increasing by the square) than its volume (increasing by the cube) (Huggett, 2004). A larger body retains heat more efficiently because its surface-area-to-volume ratio is lower.
• Examples: Pygmies are predominantly found in tropical areas and are smaller in body size. Inhabitants of Northern Europe such as Swedes tend to be taller and heavier than inhabitants of Southern Europe such as Italians.

Allen’s Rule (Proportional Rule)

• The proportional rule was propounded by Joel A. Allenin 1877. Allen’s rule extends Bergmann’s rule to include protruding parts of the body — necks, legs, tails, ears, and bills. Allen found that protruding parts in wolves, foxes, hares, and wild cats are shorter in cooler regions. Like large body size, short protruding parts help reduce surface area and conserve heat in cold climates (Huggett, 2004).
• The principle: shorter appendages = reduced heat loss in cold environments; longer appendages = enhanced heat dissipationin warm environments.
• Example: People of East Africa and other tropical regions generally have tall, slender bodies with long limbs that assist in the loss of body heat. Arctic peoples such as Inuit have shorter limbs and compact body shapes.

Types of Adaptation: Key Definitions

• Human beings differ from other organisms in their usage of regulators to define, modify and control their living conditions — using mechanisms such as thermoregulation and circadian rhythms to ensure and maintain vital needs. Adaptation is the result of a complex set of biological, ecological, cultural, societal, and individual human mechanisms (Lawrence, 2001).

• The key distinction: adaptation is genetically fixed and permanent; acclimatization occurs during an individual’s lifetime in response to natural environmental stress; acclimation is a laboratory-induced variant of acclimatization.
• A strategy alternative to physiological adaptation is the creation of a microenvironment — a protective layer around the organism that minimises or eliminates the environmental stress itself. This is usually achieved by changing natural behavior and is referred to as cultural adaptation. The Eskimo suit is a classic communal example of microenvironment creation.

Stress → Strain → Adaptation Pathway

Environmental stress (e.g., low oxygen at altitude) → Physical strain in organism (respiratory/circulatory strain) → Body adapts through multiple mechanisms → Adaptation is brought about by strain, which is caused by environmental stress.

Major Environmental Stresses

Abiotic Stresses

• Temperature extremes — heat and cold
• Altitude — reduced oxygen partial pressure
• Water availability — aridity or flooding
• Light — photoperiod, UV radiation
• Soil conditions — edaphic factors

Biotic Stresses

• Pathogens — disease agents
• Parasites — resource extraction organisms
• Predators — direct predation
• Competition — intra/inter-specific resource competition

Levels of Adaptation

Adaptations can occur at a variety of levels ranging from the genetic to the cultural. Each level has a different degree of permanence, rate of occurrence, and scope of effect.

Adaptation to Heat

Heat Stress & Thermoregulation

Core temperature maintained between 35°C and 41°C

• Body heat is produced as a result of oxidation of carbohydrates and fat. Humans produce heat to regulate body temperature by balancing heat production, absorption, and loss (Lim et al., 2008). Heat stress results from the interaction of environmental conditions (temperature, humidity, sun), physical work rate, and wearing of heavy clothing or equipment that impedes heat loss.
• Humans regulate core temperature within a narrow range (35°C to 41°C) through two parallel processes:
  • Physiological Thermoregulation
    • Operates independently of conscious voluntary behavior. Controls: (a) rate of metabolic heat production, (b) body heat distribution via blood from core to skin, (c) sweating. Most important during heat stress.
  • Behavioral Thermoregulation
    • Operates through conscious behavior. Includes: modifying activity levels, changing clothes, seeking shelter. Most important during cold stress (Sawka et al., 2002).

Physiological Mechanisms in Heat Adaptation

• Sweating: When skin temperature rises above 35°C, sweat glands start functioning. Perspiration begins from the trunk region and proceeds towards extremities. Helps in heat dissipation through evaporation.
• Vasodilation: Dilation of blood vessels. During body heating, skin blood flow increases to facilitate heat transfer from core to skin surface. Resultant heat dissipation essential to maintain normal body temperature.
• Cardiovascular response: Helps manage baroreflex (maintaining blood pressure) — control of systematic vascular resistance, causing elevated heart rate and blood pressure to decrease.

Factors Influencing Individual Response to Heat Tolerance

• Age
  • Middle-aged men and women (45–64 years) are more work-heat-intolerant and suffer more physiological strain during heat acclimation than younger individuals.
• Physical Activity
  • About 10–20 fold higher metabolic heat is produced during physical exercise than resting. The dissipation of 80% heat occurs through evaporation during exercise.
• Body Shape & Proportion
  • Per Allen’s rule, people in hot environments have longer extremities and lesser body mass. Robert (1972) proposed a higher surface-area-to-mass ratio is useful for heat loss.
• Obesity
  • Obese people show high body heat content due to higher resting metabolic heat production and proportionality of heat loss to skin surface area (Savastano et al., 2009).
• Ethnicity
  • Adaptation to heat stress is influenced by ethnic background. According to Shonkoff et al. (2009), immigrant people of Australia show potential vulnerability to heat.

Adaptation to Cold

Cold Stress & Thermoregulatory Response

Three distinct patterns of cold adaptation

• Human thermoregulatory adaptations to chronic cold exposure are more modest and less understood than adaptations to chronic heat. Where chronic heat exposure induces a fairly uniform pattern of thermoregulatory adjustments, chronic cold exposure induces three different patterns of adaptation:
  • Habituation
    • Characterised by blunted physiological responses during cold exposure. The most commonly occurring cold adaptation — characterised by shivering and vasoconstriction response, or both.
  • Metabolic Adaptation
    • Characterised by enhanced thermogenic responses to cold. Includes non-shivering thermogenesis by brown adipose tissue.
  • Insulative Adaptation
    • Characterised by enhanced body heat conservation during cold exposure through subcutaneous fat and peripheral vasoconstriction (Sawka et al., 2002).

Key Physiological Mechanisms

• Shivering:Various grades of muscle contraction — from increased muscle tone through barely perceptible tremor to vigorous shivering (Heminway, 1963). One of the mechanisms for delaying decrease in body temperature along with peripheral vasoconstriction.
• Vasoconstriction:Cold exposure stimulates sympathetic noradrenergic vasoconstrictor nervous system, reducing skin and extremity blood flow due to narrowing of blood vessels (Charkoudian, 2010; Rintamaki et al., 2005).
• Non-shivering thermogenesis:Mammals including humans contain brown adipose tissue for producing heat from food. Cold exposure causes sympathetic nerves to release catecholamine, stimulating proliferation of brown fat cells and releasing heat (Cannon and Nedergaard, 2004).
• High metabolic rate:Higher metabolic rate in cold environments increases energy expenditure — dietary thermogenesis associated with animal protein consumption, acclimatization to climatic stress, and genetic adaptations (Leonard et al., 2005).
• Cardiovascular response:Immersion of hand in cold water causes increase in heart rate and blood pressure, whereas face cooling increases systolic and diastolic blood pressure but decreases heart rate.

Factors Affecting Cold Adaptations

• Surface Area
  • Individual’s response to cold is heavily affected by body size and shape. Individuals with large body size require less heat to maintain constant body temperature than those with small body size.
• Insulation of Fat
  • Subcutaneous fat represents the most important form of natural insulation. The greater the fat layer, the lower the skin temperature, and consequently the smaller the gradient between body surface and environment (Frisancho, 1993).
• Physical Fitness
  • Sensitivity of thermoregulatory system increases with physical fitness. Physically fit individuals have more effective thermoregulatory abilities against cold than less active individuals.
• Age
  • Younger individuals have better thermoregulatory responses than older when exposed to cold. Young responses are caused by increased vascularization, peripheral blood flow and higher metabolic rate.
• Malnutrition
  • Nutrition-deprived conditions cause reduced response to cold stress. Lack of essential fats leads to increased intensity of shivering.

Adaptation to High Altitude

High Altitude Stresses

• An elevation above 2500 m is defined as high altitude (Beall et al., 2010). A high altitude environment presents several stresses to humans:
  • (a) Hypoxia
  • (b) High solar radiation
  • (c) Cold
  • (d) Low humidity
  • (e) High winds
  • (f) Limited nutritional base
  • (g) Rough terrain
• From the physiological point of view, hypoxia is the most importantstress, since the other stresses are present in an equal or greater degree in other geographical zones (Frisancho, 1993).

Hypoxia and Its Effects

• The decrease in oxygen availability due to the falling partial pressure of oxygen at high altitude. All organ systems and physiological functions are affected by hypoxic conditions.
• Common symptoms observed in hypoxia include: headache, loss of appetite, nausea, vomiting, fatigue, weakness, dizziness, light-headedness, and sleep disturbance.

Factors Affecting Individual Response to High Altitude

Age

• Individual response to high altitude is affected by age. Acclimatization studies indicate that young individuals show increased hemoglobin concentration than older individuals during the first few days at high altitude.

Physical Fitness

• Individuals with high physical fitness are better able to tolerate the stress of hypoxia than unfit individuals — mainly due to positive effects of physical activity: increased vascularization, maximum aerobic capacity, and increased size of striated and cardiac muscles.

Application of High Altitude Studies in Understanding Human Diseases

• Hypoxia is commonly observed in human diseases like ischemic heart disease, stroke, anemia, chronic obstructive pulmonary disease (COPD) and pulmonary hypertension. Hypoxia can be detected in any tissue of pathological conditions. Understanding the mechanisms of hypoxia is useful for optimal organism and tissue response management (Bigham and Lee, 2014).

Nutritional Adaptations

• Nutrition plays an important role in adaptation. Metabolic adaptations to heat, cold, and high altitude are all associated with nutritional requirements. Inadequate nutrition may impair metabolic response (Murthy and Singh, 2003).

Key insight — Summary from Murthy & Singh, 2003

Oxidation of carbohydrate is higher than fat, therefore carbohydrate provides maximum energy in cold climate. High intake of carbohydrate and dietary protein is considered advantageous in high altitude environments. In hot climate, adequate hydration and salt replacement are critical to prevent heat illness.

Cultural Adaptations: Detailed Case Studies

• Human adaptive techniques include material cultural adjustments and behavioural adaptations.
• According to Hanna (1983), material culture provides habitations and clothing which establish a favorable microclimate and counter the high potential of radiation, convection, and conductive heat gain. Behavioral techniques center largely upon avoidance.

Cultural Adaptations to Heat — Desert Environments

• Common material cultural techniques to adapt to hot-dry environments (Hanna, 1983):
  • High heat-capacity construction:Houses built using adobe and stone — materials that absorb large amounts of heat before passing it into the interior. Stored heat is lost at night by radiation and convection.
  • Subterranean excavation:Excavating into the earth as an alternative — mean temperature of subsoil is more comfortable than the surface, and much cooler during the day.
  • Compact building design:Houses built close together — increasing internal volume with disproportionate increase in surface area, providing mutual shading of exterior walls and wind protection.
  • Window management:Limiting size and number of windows with shutters reduces convective and radiative heat entry.
  • Reflective surfaces:Painting in light colours or white-washing reflects radiation and reduces radiative heat gain.
  • Separated cooking spaces:Kitchens and cooking areas built away from residential structures to prevent heat entry.
  • Clothing as protection:Clothing reduces abrasions and prevents sunburn. An equally important effect is reducing solar heat gain, which reduces the level of perspiration required to maintain thermal equilibrium.

Cultural Adaptations to Cold — The Eskimo/Inuit Model

• Much of what is known about cultural adaptations to cold comes from Eskimo practices. Eskimo traditional clothing has wide openings to allow airflow in and out by releasing or closing drawstrings.
  • Multilayered clothing:Contains numerous layers in which trapped air acts as an insulator. The outside layer is windproof and impermeable, holding heat in and keeping cold and wetness out.
  • Specialised footwear:Soles of boots made from seal skin and sewn with sinew without holes to ensure waterproofing. Stocking made from fur with a grass pad placed between soles of boot and stocking to absorb moisture from external environment or foot perspiration.
  • Sealskin mittens:Kept with grasspad to protect hands.
  • Snow shelters (Igloo):Constructed during nomadic activities. Small air cells in ice act as insulators. Sea oil lamp provides heat, light and melting of snow surfaces.
  • Permanent settlements:Houses constructed using semi-subterranean stone or driftwood covered with turf and snow for insulation at hillside spots. Stone igloos have ventilation holes for air circulation and to prevent overheating.

Cultural Adaptations to High Altitude — Sherpas and Aymaras

• Sherpas of Tibet/Nepal
  • The Sherpas occupy the high valleys around the base of Mount Everest, Nepal. They construct houses as a cluster at the center of village’s agricultural land. Houses are two stories, made of stone. Roofs are flat and made of wood, weighed down by heavy stones. Both sexes wear a long inner shirt over a pant-like garment made of wool. Both males and females wear high, woolen boots with hide soles.
• Aymaras of Bolivia/Ethiopia
  • The Aymaras are residents of Andean mountains of Bolivia. Houses are oblong buildings using adobe; at lakes, houses are built using reeds. Roofs are thatched with reeds and grasses. In the altiplano (high altitude), men wear cotton trousers and woolen caps with ear flaps and ponchos. Women wear home-spun skirts and sweaters. Both sexes use sandals or shoes.