4 Adaptation of Growth Rates to Heat Stress

Dr. Monika Bhuker

Contents of this unit

Introduction

1. Adaptation

2. Growth rate

3. Thermoregulation

4. Temperature and growth rate

5. Allen’s rule

6. Bergmann’s rule

7 Why is heat adaptation necessary?

Summary

Self-assessment

Suggested reading

Learning Objectives

• How human body regulate its body temperature
• Why people living in different climates have different body shape and size
• Relationship between heat and growth rate
• To what extent Allen’s rule apply on different populations

INTRODUCTION

The environment can have major influences on human physiology. Environmental effects on human physiology are numerous; one of the most carefully studied effects is the alterations in thermoregulation in the body due to outside stresses. This is necessary because in order for enzymes to function, blood to flow, and for various body organs to operate, temperature must remain at consistent, balanced levels. (http://en.wikipedia.org/wiki/Ecophysiology)

Humans and many other mammals have unusually efficient internal temperature regulating systems that automatically maintain stable core body temperatures in cold winters and warm summers. In addition, people have developed cultural patterns and technologies that help them adjust to extremes of temperature and humidity (http://anthro.palomar.edu).

Two main turning points in evolution are considered as the landmarks of progress in adaption to heat stress. One was the transition from ectothermic to endothermic control of body temperature and the other, morphological changes that occur in response to physical stress placed on bone (Bipedalism) (Drinkwater).

1.1 ADAPTATION

Adaptation is used as a generic term encompassing acclimatization, acclimation and habituation (Folk, 1974). In biology, an adaptation, also called an adaptive trait, is a trait with a current functional role in the life history of an organism that is maintained and evolved by means of natural selection. Adaptation refers to both the current state of being adapted and to the dynamic evolutionary process that leads to the adaptation. Adaptations contribute to the fitness and survival of individuals. Organisms face a succession of environmental challenges as they grow and develop and are equipped with an adaptive plasticity as the phenotype of traits develops in response to the imposed conditions. The developmental norm of reaction for any given trait is essential to the correction of adaptation as it affords a kind of biological insurance or resilience to varying environments. The capacity of any stress to induce adaptation is a function of its application intensity, duration, frequency and variability (Adolf, 1964), and of the genetic, phenotypic and situational status of the individual. In a modification of the training impulse model (Banister, 1980), these variables may be used to quantify the cumulative adaptation impulse (stress volume).

1.2 GROWTH RATE

The changes in height of the developing child can be thought of in two different ways: the height attained at successive ages and the increments in height from one age to the next, expressed as rate of growth per year. If growth is thought of as a form of motion, the height attained at successive ages can be considered the distance travelled, and the rate of growth, the velocity. The velocity or rate of growth reflects the child’s state at any particular time better than does the height attained, which depends largely on how much the child has grown in all preceding years. The blood and tissue concentrations of those substances whose amounts change with age are thus more likely to run parallel to the velocity rather than to the distance curve. In some circumstances, indeed, it is the acceleration rather than the velocity curve that best reflects physiological events. (Britanica.com/human-development)

Figure 2a. Height chart for girls and boys (Encyclopedia Britanica, 2010)

1.3 THERMOREGULATION

Thermoregulation is performed by a physiological control system consisted of central and peripheral thermo receptors, an afferent conduction system, a central control for integration of thermal impulses, and an efferent responses system leading to compensatory responses (Romanovsky, 2007). This system regulates the balance between production (thermo genesis) and dissipation (thermolysis) of heat, in order to maintain the body temperature near a constant level of 36.5°C (Gomes et al, 2013).

Humans lose thermal energy to the environment by convection, radiation and sweating. Dubois (1937) demonstrated that the role of sweating becomes increasingly important in humans as ET rises, as it is impossible to lose heat by other methods when the environment is hotter than the body.

1.3.1 Heat stress

It is defined as an environment that acts to drive body temperature above set-point temperature (Hansen, 2009). Heat stress is composed of two components: 1) heat load which rises from metabolism, heat exchange, radiation, and convection with the environment; and 2) heat dissipation which is release of the heat load through sweat evaporation (Vernacchia, 1998). According to Brotherhood (1987), heat stress is any factor (e.g., high temperatures) or any combination of factors (e.g., high temperature and high humidity) that overloads the thermoregulatory system thereby raising the body temperature.

Heat stress can lead to disruptions in reproductive processes through two general mechanisms. First, the homeokinetic changes to regulate body temperature can compromise reproductive function. One example is redistribution of blood flow from the body core to the periphery to increase sensible heat loss. Another homeokinetic control mechanism for body temperature is reduced feed intake during heat stress. Reducing feed intake reduces metabolic heat production but also can lead to changes in energy balance and nutrient availability that can have large effects on cyclicity, establishment of pregnancy and foetal development. A second mechanism for disruption of reproduction during heat stress is the failure of homeo-kinetic systems to regulate reproduction. (Hansen, 2009)

Children have anatomical and physiological characteristics that differ from adults, such as distinct values of body composition, water, and bone density. Morphologically, children have a higher ratio between surface area and body mass, which leads to a more rapid increase in body temperature when it is thermally stressed by heat (Rowland, 2008 and Inoue et al, 2004). The evaporative heat loss is crucial for the maintenance of thermal homeostasis regardless of age.

Adapted from Little & Hochner, 1974 and Thomas, 1994.

1.3.2 Heat acclimation or Heat acclimatization

The improved tolerance to exercise in heat is known as heat acclimatization. It is specific to the stress imposed on the human body. For example, passive exposure to heat induces some responses, notably an improved ability to dissipate heat. In contrast, physical training in a cool-dry environment results in metabolic, biochemical, hematologic, and cardiovascular adaptations. Heat acclimatization via strenuous exercise induces responses attributed to both passive heat exposure and training in cool environments. (Armstrong, 1998)

1.4 TEMPERATURE AND GROWTH RATE

Climate appears to influence growth and development, helping to determine body size and proportions. According to Bergmann’s and Allen’s rules, body size and proportions of warm-blooded, polytypic animals are related to temperature. Allen’s rule states that longer extremities and appendages relative to body size are found in warmer climates, while the reverse is true in colder climates. Bergmann’s rule states that a larger body size would be expected in colder versus warmer climates. (Cameron, 2002).

A link between adult human body size and environmental temperature, evolved through adaptation to heat stress, was first recognized a century ago and is now well accepted in human biology. Increasing heat stress favours smaller body size and an increased ratio of surface area to mass. Many developing country populations inhabit relatively hot environments compared to industrialized populations, but growth faltering in developing countries is invariably attributed to the combination of poor nutrition and infection. The growth faltering can relieve heat stress in both infancy and childhood. The hypothesis that heat stress plays a role in human growth faltering in hot environments therefore merits empirical investigation (Wells, 2000). According to Newman (1953), the smaller statures observed near the Equator supports Bergmann’s rule, and the shorter legs among the Inuit supports Allen’s rule. Finally, Schreider demonstrated that amount of body surface area tends to increase from cold to hot climates (Bogin, 1988).

A negative correlation between adult body mass and environmental temperature (ET) has been demonstrated by Roberts (1953), Newman (1953), Froment & Hiernaux (1984), Schreider (1964), and Crognier (1981).

As Kleiber (1961) demonstrated in his monograph “The Fire of Life”, heat production is proportional to the body mass, M, but heat loss is proportional to the surface area, SA. The ability of an animal to conserve or lose thermal energy is therefore strongly influenced by the SA(surface area): M (Body mass) ratio.

1.4.1 Adaptation to thermal stress has been invoked to explain the tall lean proportions of the Nilotic populations of east Africa (Hiernaux, 1974), and the small stature of tropical rainforest inhabitants such as the Mbuti (Hiernaux, 1974). Both these adaptations increase the SA: M ratio, facilitating heat loss. In hot environments, cultural factors are less successful at mitigating the thermal stress, and greater variation in body size and shape is seen (Crognier, 1981). Roberts (1978), for example, demonstrated a stronger relationship between size and ET (environmental temperature) in African populations than in populations inhabiting more temperate climates.

Eveleth (1966) studied American middle-class children reared in Brazil and found them to be more linear than expected. Stinson & Frisancho (1978) compared the children Andean migrants to the Amazon jungle with children living in the Andes. Those reared in the jungle were more linear, the length of the extremities clearly greater.

The relationships observed between body proportion and environmental temperature can be explained in terms of the body’s thermoregulatory process. In hot environments, heat dissipation is crucial to avoid hyper thermic stress, or overheating. A body that has greater surface area relative to total body size or volume more efficiently dissipates heat produced by the body’s metabolism and activity. One method through which heat is lost is convection, which is the transfer of heat from the body to the environment through the movement of air over the body; thus, a greater surface area over which wind can pass is beneficial to humans in hotter climates. (Cameron, 2002)

The effect of both growth and maturation as of heat acclimation is the increase of the sweat gland cholinergic stimulus. Moreover, it occurs concomitantly to the acceleration of stimuli toward the peripheral sympathetic nervous system (Lee et al, 2010).

With physical growth, especially of body surface area, a decrease in the density of sweat glands activated by heat is verified. This phenomenon does not favour increased perspiration, but, on the other hand, glands increase in size (hypertrophy), increasing the amount of sweating rate (Inoue et al, 2004). Growth and maturation provide increased ability to sweat glands, however, unevenly. This means that some areas may have a higher cholinergic sensitivity compared to other ones (Inoue et al, 2004). Crognier (1981) studied the relationship between climate and anthropometric measurements in 85 East African, European, and Middle Eastern populations. He found that mean annual low temperature was strongly correlated with cranial measurements, but postcranial measurements were strongly correlated to heat and dryness.

Malina and Bouchard (1991) suggested that the typical body shapes associated with extremes in temperature, growth in hot environments is prolonged, since there is an association between delayed maturation and a linear body type. Studies of the mean age at menarche, however, contradict this conclusion, because a negative correlation to annual mean temperature has been observed indicating that maturation occurs earlier in hotter climates.

1.5 ALLEN’S RULE

Allen’s rule is a biological rule posited by Joel Asaph Allen in 1877.The rule says that the body shapes and proportions of endotherms vary by climatic temperature by either minimizing exposed surface area to minimize heat loss in cold climates or maximizing exposed surface area to maximize heat loss in hot climates. The rule predicts that endotherms from hot climates usually have ears, tails, limbs, snouts, etc. that are long and thin while equivalent animals from cold climates usually have shorter and thicker versions of those body parts (Asaph, 1877 and Holstun, 1986).

Katzmarzyk and Leonard (1998) said that there is a negative association between body mass index and mean annual temperature for indigenous human populations, meaning that people who originate from colder regions have a heavier build for their height and people who originate from hotter regions have a lighter build for their height.

1.6 BERGMANN’S RULE

Bergmann’s rule is an ecological principle which states that body mass increases with latitude. Human populations, who live near the poles, including the Inuit, Aleut, and Sami people, are on average heavier than populations from mid-latitudes, consistent with Bergmann’s rule (Holiday, 2009). They also tend to have shorter limbs and broader trunks, consistent with Allen’s rule (Holliday, 2009) According to Marshall T. Newman in a 1953 article for the Journal of the American Anthropologist, Native American populations are generally consistent with Bergmann’s rule although the cold climate and small body size combination of the Eastern Eskimo, Canoe Indians, Yuki, Andes natives and Harrison Lake Lillouet runs contrary to the expectations of Bergmann’s rule. Newman (1953) contends that Bergmann’s rule holds for the populations of Eurasia, but it does not hold for those of sub-Saharan Africa.

1.7 WHY IS HEAT ADAPTATION NECESSARY

The ability to respond to heat is seen in all extant human populations, regardless of the environment in which they now live or how many generations they have been removed from the heat (Edholm & Weiner, 1981). It is very clear that physiological adaptations remain of paramount importance in survival. Heat adaptation are very much necessary not just for human beings but for every single organism for their better survival in hot environments.

Humans acclimate to heat in a rapid and effective manner. Acclimatization involves increased sweating which reduces body temperature and circulatory strain. The stimulus is elevated body temperature for short periods over several, consecutive days. In addition to a higher rate of perspiration, acclimatization also involves a greater sensitivity to environmental heat, as evidenced by the lower temperature at which sweating begins, as well as a redistribution of recruitment patterns and a greater sodium and chloride economy (Hanna & Brown, 1983). Well adapted human beings more easily tolerate heat and enhance the level of heat tolerance (Gisolfi et al, 1969). Acclimatization to heat has a positive effect on work performance and improves maximum work capacity (Shvartz et al, 1977). A linear body build is of advantage in hot climates if it promotes heat loss.

SUMMARY

• Heat Acclimatization is necessary to prevent or reduce the severity of heat illness.
• Humans are remarkably well adapted to tolerate heat whether derived from environmental or from metabolic sources.
• Higher surface area to mass ratio in homeotherms is beneficial in terms of heat loss.
• Stature, weight and limb length are phenotypically plastic and have a significant environmental input.
• With physical growth, especially of body surface area, a decrease in the density of sweat glands activated by heat is verified.
• Allen’s rule states that longer extremities and appendages relative to body size are found in warmer climates, while the reverse is true in colder climates.
• Bergmann’s rule states that a larger body size would be expected in colder versus warmer climates.
• Heat adaptation are very much necessary not just for human beings but for every single organism for their better survival in hot environments.