Adapting to Climate Extremes

 


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.

In very cold climates, there is a constant danger of developing hypothermia click this icon to hear the preceding term pronounced, which is a life threatening drop in core body temperature to subnormal levels.  The normal temperature for humans is about 98.6° F. (37.0° C.).  However, individual differences in metabolism click this icon to hear the preceding term pronounced, hormone levels, physical activity, and even the time of day can cause it to be as much as 1° F. (.6° C.) higher or lower in healthy individuals.  It is also normal for core body temperature to be  lower in elderly people.  Hypothermia begins to occur when the core body temperature drops to 94° F. (34.4° C.).  Below 85° F. (29.4°C.), the body cools more rapidly because its natural temperature regulating system (in the hypothalamus click this icon to hear the preceding term pronounced) usually fails.  The now rapid decline in core body temperature is likely to result in death.  However, there have been rare cases in which people have been revived after their temperatures had dropped to 57-60° F. (13.9-15.6° C.).  This happened in 1999 to a Swedish woman who was trapped under an ice sheet in freezing water for 80 minutes.  She was found unconscious, not breathing, and her heart had stopped beating, yet she was eventually revived despite the fact that her temperature had dropped to 56.7° F. (13.7° C.).

In extremely hot climates or as a result of uncontrollable infections, core body temperatures can rise to equally fatal levels.  This is hyperthermia click this icon to hear the preceding term pronounced.  Life threatening hyperthermia typically starts in humans when their temperatures rise to 105-107° F. (40.6-41.7° C.).  Only a few days at this extraordinarily high temperature level is likely to result in the deterioration of internal organs and death.

Body size and shape are significant factors in how efficiently an individual responds physiologically to cold and hot climates.  Two 19th century naturalists, Carl Bergmann and Joel Allen, formulated rules concerning these factors.


Bergmann's Rule

In 1847, the German biologist Carl Bergmann click this icon to hear the preceding term pronounced observed that within the same species of warm-blooded animals, populations having less massive individuals are more often found in warm climates near the equator, while those with greater bulk, or mass, are found further from the equator in colder regions.  This is due to the fact that big animals generally have larger body masses which result in more heat being produced.  The greater amount of heat results from there being more cells.  A normal byproduct of metabolism in cells is heat production.  Subsequently, the more cells an animal has, the more internal heat it will produce.

In addition, larger animals usually have a smaller surface area relative to their body mass and, therefore, are comparatively inefficient at radiating their body heat off into the surrounding environment.  The relationship between surface area and volume of objects was described in the 1630's by Galileo.  It can be demonstrated with the cube shaped boxes shown below.  Note that the volume increases twice as fast as the surface area.  This is the reason that relatively less surface area results in relatively less heat being lost from animals.

Comparison of cube surface areas and volumes illustrating Bergmann's rule
 
drawing showing a comparison of cube surface areas and volumes illustrating Bergmann's rule--a 2 by 2 cube has a surface area of 24 and a volume of 8, while a 4 by 4 cube has a surface area of 96 and a volume of 64; in other words, the larger cube has a 4 times larger surface area and an 8 times larger volume
  Massive polar
bear bodies are
predicted by
Bergmann's rule		   photo of a large wild polar bear walking by a pickup truck with two people inside watching the bear cautiously

Polar bears are a good example of this phenomenon.  They have large, compact bodies with relatively small surface areas from which they can lose their internally produced heat.  This is an important asset in cold climates.  In addition, they have heavy fur and fat insulation that help retain body heat.

Bergmann's rule generally holds for people as well.  A study of 100 human populations during the early 1950's showed a strong negative correlation between body mass and mean annual temperature of the region.  In other words, when the air temperature is consistently high, people usually have low body mass.  Similarly, when the temperature is low, they have high mass.  However, there are exceptions.  Our clothing and technologies that allow us to keep buildings warm in the winter and cool in the summer tend to offset the effects of natural selection now in shaping our bodies.  In addition, culturally guided mate selection criteria also somewhat counter Bergmann's rule for humans. 

Negative correlation between
environmental temperature
and body mass in humans
and other warm blooded
animals

   illustration of the negative correlation between environmental temperature and body mass in humans and other warm blooded animals--if the environmental temperature is high, body masses tend to be low and if the temperature is low, body masses tend to be high

A corollary of Bergmann's rule stated that a linear shaped mammal will lose heat to the environment faster than a more compact one of similar size.  The boxes below illustrate this fact.  Note that the long, narrow box has the same volume but greater surface area.  It is comparable to a tall, slender animal.

Comparison of different shaped
box surface areas and volumes
illustrating a corollary of
Bergmann's rule relating to
body shape
 

  

drawing showing a comparison of different shaped box surface areas and volumes illustrating Allen's rule--a 4 by 4 cube has a surface area of 96 and a volume of 64, while an elongated box 2 by 4 by 8 has a surface area of 112 and a volume of 64; in other words, both boxes have the same volume, but the elongated one has 1.75 times greater surface area


Allen's Rule

In 1877, the American biologist Joel Allen went further than Bergmann in observing that the length of arms, legs, and other appendages also has an effect on the amount of heat lost to the surrounding environment.  He noted that among warm-blooded animals, individuals in populations of the same species living in warm climates near the equator tend to have longer limbs than do populations living further away from the equator in colder environments.  This is due to the fact that a body with relatively long appendages is less compact and subsequently has more surface area.  The greater the surface area, the faster body heat will be lost to the environment. 

This same phenomenon can be observed among humans.  Members of the Masai click this icon to hear the preceding term pronounced tribe of East Africa are normally tall and have slender bodies with long limbs that assist in the loss of body heat.  This is an optimal body shape in the hot tropical parts of the world but it would be a disadvantage in subarctic regions.  In such extremely cold environments, a stocky body with short appendages would be more efficient at maintaining body heat because it would have relatively less surface area compared to body mass.

Slender
East Africans
with long arms
and legs
predicted by
Allen's rule

   photo of a group of tall, slender, young East African men in traditional tribal clothing dancing together with their spears--they are jumping vertically more than 3 feet   photo of a young woman illustrating her loss of body heat by radiation, conduction, evaporation, and convection in a moderate climate--60% is lost through radiation, 3% through conduction into the bench she is sitting on, 22% through evaporation of sweat, and 15% through convection into the air surrounding her
   

 

  
Normal processes of body heat loss in a moderate climate

We lose heat to the surrounding environment in several ways, as shown in the illustration above on the right. However, simple radiation is the process that is responsible for most of the loss, except in hot dry climates where evaporative cooling, or sweating, can be more significant.


Cold Climate Responses

Many people living in freezing climates drink alcohol to warm themselves.  This increases blood flow to the body extremities, thereby providing a feeling of warmth.  However, it results only in a temporary warming and can speed up the loss of heat from the vital internal organs, resulting in more rapid death from hypothermia.  A much more effective cultural response to extremely cold temperatures is the use of insulating clothing, houses, and fires.  People all over the world also adapt by limiting outdoor activities to warmer times of the day.  In some societies, sleeping in family groups with bodies pushed up against each other is also done in order to minimize heat loss during the cold months of the year.

When the environment is very cold, life can depend on the ability of our bodies to reduce heat loss and to increase internal heat production.  As Bergmann and Allen observed, the human physiological response to cold commonly includes the evolution of more massive, compact bodies with relatively less surface area.  However, short term acclimatization to the cold also occurs.  A normal initial physiological response is the narrowing of blood vessels near skin surface (vasoconstriction).  This preserves core body heat by reducing peripheral blood flow.  As a consequence, the skin cools and less heat is lost from the body by radiation.  However, if the environmental temperature is below the freezing point, prolonged vasoconstriction can result in dangerous frostbite.  As a consequence, the body's internal temperature regulating mechanism responds by dilating the peripheral blood vessels (vasodilation), thereby increasing the flow of warm blood near the skin surface.  The body normally alternates back and forth between vasoconstriction and vasodilation to compensate for the risks of both conditions.  This cycling is known as the Lewis hunting phenomenon).  Shivering can also cause a short-term warming effect. The increased muscle activity in shivering results in some heat production. 

There are three additional important types of biological responses to cold conditions found among some human populations around the world:

1.   increased basal metabolic rate
2.   fat insulation of vital organs
3.   long term change in blood flow patterns

People living in harsh subarctic regions, such as the Inuit click this icon to hear the preceding term pronounced (Eskimo) of the far northern regions of the western hemisphere and the Indians of Tierra del Fuego at the southern end, traditionally consumed large quantities of high calorie fatty foods.  This significantly increases the basal metabolic rate, which, in turn, results in the production of extra body heat.  These peoples also wore heavy clothing, often slept in a huddle with their bodies next to each other, and remained active when outdoors.

map of the Americas showing the Inuit territory at the far north of North America and Tierra del Fuego at the far south of South America

   photo of an Inuit man in the winter--he is wearing a traditional parka; his eye brows and mustache are caked with ice
Inuit man

The Ju/'hoansi click this icon to hear the preceding term pronounced of Southwestern Africa and the Aborigines click this icon to hear the preceding term pronounced of Australia usually respond physiologically to the cold in a different way.  Thick fat insulation develops around the vital organs of the chest and abdomen.   In addition, their skin cools due to vasoconstriction at night.  As a result, heat loss is reduced and the core body temperature remains at normal levels.  However, the skin feels very cold throughout the night.

map of Africa, South Asia, and Australia showing the !Kung and Australian Aborigine Territories--the !Kung are in Southwest Africa, while the Aboringines once occuppied all of Australia

This response would not be adaptive if the Ju/'hoansi and the Aborigines lived in consistently freezing environments because the concentration of body heat in their torsos would allow the loss of fingers, toes, and other appendages from frostbite.  The loss of fingers in particular would make it difficult for these hunters and gatherers to acquire food.  Their physiological adaptation is to environments that rarely stay below freezing long and that do not have abundant high calorie fatty foods.


Hot Climate Responses

Adapting to hot environments is as complex as adapting to cold ones.  However, cold adaptation is usually more difficult physiologically for humans since we are not subarctic animals by nature.  We do not grow dense fur coats nor do we usually have thick layers of fat insulation like polar bears.  Despite this reality, more people die from heat than cold in the United States every year.  Those who succumb are usually babies left in locked cars on hot days and the elderly poor who cannot afford air conditioning.

The effect of heat on our bodies varies with the relative humidity of the air.  High temperatures with high humidity make it harder to lose excess body heat.  This is due to the fact that when the moisture content of air goes up, it becomes increasingly more difficult for sweat to evaporate.  The sweat stays on our skin and we feel clammy.  As a result, we do not get the cooling effect of rapid evaporation.

  Evaporative cooling experiment.  Put a little rubbing alcohol on the back of one of your
       hands and water on the back of the other.  Wave them in the air.  Feel the difference as
       they both evaporate.  What did you learn about evaporative cooling?  (HINT: alcohol is more
       volatile than water--it evaporates more rapidly.  Therefore, the hand with alcohol replicates
       sweating in a very low humidity environment.)

In dry, hot weather, humidity is low and sweat evaporates readily.  As a result, we usually feel reasonably comfortable in deserts at temperatures that are unbearable in tropical rain forests.  The higher the desert temperatures, the more significant of a cooling effect we get from evaporation.  This relationship between relative humidity and air temperature is quantified in the table below.  When the apparent temperature is in the light yellow range, heat exhaustion and cramps are likely for humans.  In the bright yellow range, life threatening heat stroke is likely.

table of apparent temperatures at different air temperatures and relative humidities--these data show that fatal heat stroke can occur in humans if the humidity of the atmosphere is close to 100% and the air temperature is 85 degrees Fahrenheit

   ( Source: U.C. Berkeley Wellness Letter, Aug.1996)

While evaporative cooling is very effective in dry climates, there is a major drawback.  That is the rapid loss of water and salts from the body through sweat.  This can be fatal in less than a day if they are not replaced.  It is common to lose a quart or more of water through sweating each hour in harsh summer desert conditions.  Commercial "sport drinks" are designed to help people in these situations rehydrate and replenish lost mineral salts.  It is easy and inexpensive to create your own equivalent drink without the unnecessary food coloring and sugar that the commercial drinks often include to make them more appealing to customers.  Diluted lemonade with added salt can satisfactorily serve the same purpose.

Most people have the ability to physiologically acclimatize to hot conditions over a period of days to weeks.  The salt concentration of sweat progressively decreases while the volume of sweat increases.  Urine volume also reduces.  In addition, vasodilation of peripheral blood vessels causes flushing, or reddening, of the skin because more blood is close to the surface.  That blood brings heat from the core body areas to the surface where it can be dissipated easily into the environment by radiation.

Evaporative cooling also plays an important protective role when vigorous physical activity results in overheating of the body.  Researchers at Osaka International University and Kobe University in Japan have discovered recently that men generally are more effective at sweating than women when they do strenuous work or exercise.  Men usually begin sweating earlier and sweat more than women in response to the same amount of exertion.  This would imply that men and women are not equal in heat tolerance.  However, further research with different populations around the world are necessary to verify this conclusion.

  Function of fever--could it be an advantage in surviving infections?
           This link takes you to an external website.  To return here, you must click
           the "back" button on your browser program.    (length = 7 mins 32 secs)

 

NOTE:  Sweating is not only a mechanism for getting rid of excess body heat.  Our sweat contains a number of different substances, including pheromones click this icon to hear the preceding term pronounced that can have powerful effects on the hormone systems of others who are physically close to us.  Researchers at the Monell Chemical Senses Center in Philadelphia have shown that pheromones in the sweat of men can cause an increase in the amount of luteinizing hormones released from a woman's pituitary gland at the base of her brain.  This in turn can shorten the time until the next ovulation.  Subsequently, human male pheromones are now being considered as potential future fertility drugs for women.  Pheromones released by sisters and other women living together can cause a synchronization of their menstrual cycles.  It is likely that human males also respond subconsciously to female pheromones in a way that affects their reproductive systems.

NEWS:  In the March 2, 2012 issue of the Journal of Biological Chemistry, Wei Cheng et.al reported that some human nerve cells have proteins on their surfaces that enable them to differentiate between several different temperatures in the mildly warm to hot range.  This sensing ability may play an important role in the way we respond physiologically to hot temperatures.  A summary version of this information is available in ScienceDaily.

 

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