Created by: CK-12/Adapted by Christine Miller
Figure 6.7.1 Men from Maasai Mara, Kenya.
Built for Heat
These tall, slender men live near the equator in Kenya, East Africa — one of the hottest regions of the world. These and the other people of his tribal group, called the Maasai, are among the tallest, most linear people on the planet. Their body build is thought to be an adaptation to their climate, which is hot year-round.
refers to the average weather conditions in a region over a long period of time. One of the main determinants of climate is temperature. Both hot and cold temperatures are serious environmental stresses on the human body.
In the cold, there is risk of , which is a dangerous decrease in core body temperature. The normal temperature of the human body is 37 degrees C (98.6 degrees F). Hypothermia sets in when body temperature drops to 34.4 degrees C (94 degrees F). If body temperature falls below 29.4 degrees C (85 degrees F), it starts to cool very rapidly because the body’s temperature regulation mechanism starts to fail.
The opposite problem occurs in the heat, where the risk is , which is a dangerous increase in core body temperature. If human body temperature rises above about 40.6 degrees C (105 degrees F), hyperthermia may become life threatening. If a temperature this high persists more than a few days, it generally damages the brain and other internal organs, leading to death.
Human Adaptation to Heat and Cold
Humans are the most widespread species on the planet, and they have lived in extreme climates for tens of thousands of years. As a result, many human populations have had to cope with extreme temperatures for hundreds of generations, which has forced them to develop genetic adaptations to these climate extremes.
The size and proportions of the human body may play an important role in how well an individual is able to handle hot or cold temperatures. In general, people with a tall, slender build, like the Maasai man pictured in Figure 6.7.1, are well adapted to heat, whereas people with a short, stocky build (like the Indigenous North American Inuit pictured in Figure 6.7.2) are well adapted to cold. These relationships between body build and climate were first noticed in other animal species in the 1800s by biologists Carl Bergmann and Joel Allen. These scientists formulated what are now known as Bergmann’s and Allen’s rules.
Figure 6.7.2 Indigenous North American Inuit.
states that within a broadly distributed taxonomic group, populations or species of larger size are found in colder environments, whereas populations or species of smaller size are found in warmer environments. Bergmann’s rule has been shown to generally apply to widespread species of mammals and birds, although there are also many exceptions to the rule.
What explains Bergmann’s rule? Larger animals have a lower surface area to volume ratio than smaller animals, which is illustrated in Table 6.7.1 for a simple shape, a cube. From the table, you can see how the surface area to volume ratio of a cube decreases dramatically as the size of the cube increases. Because heat is lost through the surface of the body, an animal with a smaller surface area to volume ratio radiates less body heat per unit of mass. The larger body mass also allows the animal to generate more heat. A larger animal has more cells, so it can produce more body heat as a byproduct of cellular metabolism. Both of these factors allow a larger animal to stay warmer in a cold climate.
Relationship of Surface Area to Volume in Cubes of Different Sizes
|Relationship of Surface Area to Volume in Cubes of Different Sizes
|Side of Cube (cm)
|Surface Area of Cube (cm2)
|Volume of Cube (cm3)
|Surface Area:Volume Ratio
Warmer climates impose the opposite problem: body heat generated by metabolism needs to be dissipated quickly rather than stored within the body. Smaller animals have a higher surface area to volume ratio that maximizes heat loss through the surface of the body and helps cool the body. With less mass and fewer cells, smaller animals also generate less heat due to cellular metabolism.
Anthropologists have found that many human populations tend to follow Bergmann’s rule. For example, a study of 100 human populations in the 1950s found a strong negative correlation between mean body mass and average yearly temperature. In other words, higher body mass was generally found in colder places, and lower body mass was generally found in hotter places.
There are also exceptions to the rule, in part because we use cultural responses to temper environmental stresses so we do not need to change genetically or physiologically in order to cope. Humans, for example, use clothing and heated buildings to stay warm in cold climates, which tends to counter the effects of natural selection changing human body shape in cold climates.
is a corollary of Bergmann’s rule. It states that animals living in hotter climates generally have longer extremities (such as limbs, tails, snouts, and ears) than closely related animals living in colder climates. The explanation for Allen’s rule is similar to the rationale behind Bergmann’s rule. Longer extremities maximize an animal’s surface area, allowing greater heat loss through the surface of the body. Therefore, having long extremities is adaptive in hot climates where the main challenge is dissipating body heat.
Anthropologists have noted that, in populations that have lived in tropical regions for long periods of time, the limbs of people tend to be longer in proportion to overall body height. The Maasai man pictured in Figure 6.7.1 is a clear example. His exceptionally long limbs — like those of other members of his population — are optimally proportioned for the hot climate in Kenya. The shorter-limbed body proportions of the Inuit people (Figure 6.7.2) suit them well for their cold climate. Marked differences in limb length have also been observed in related populations that have lived for long periods of time at different altitudes. High altitudes have colder climates than lower altitudes and — consistent with Allen’s rule —people tend to have shorter limbs at higher altitudes.
Other Human Responses to Heat
Humans exhibit several other responses to high temperatures that are generally considered either short-term physiological responses or examples of longer-term acclimatization.
Sweating and Humidity
Because humans are basically tropical animals, we generally have an easier time dealing with excessive heat than excessive cold. Evaporation of sweat is the main way we cool the body. The dancer in Figure 6.7.4 is sweating copiously while working out in a hot environment. Why does sweating cool us? When sweat evaporates from the skin, it requires heat. The heat comes from the surface of the body, resulting in evaporative cooling.
How well we can deal with high air temperatures depends in large part on the humidity of the air. We have a harder time losing excess body heat when the humidity is high because our sweat does not evaporate as well as it does when the humidity is low. Instead, the sweat stays on the skin, making us feel clammy and warmer than we would feel if the humidity were lower. If the air is dry, on the other hand, sweat evaporates readily, and we feel more comfortable. For this reason, we are able to tolerate higher temperatures when the humidity is low. This is the basis of the common aphorism, “It’s not the heat, but the humidity.”
The heat index (HI) is a number that combines air temperature and relative humidity to indicate how hot the air feels due to the humidity. The heat index is also called “apparent temperature.” Figure 6.7.5 shows the heat index at different combinations of air temperature and relative humidity. As you can see, when the humidity is very high, even a 90-degree F (32 degrees C) temperature can be very dangerous.
Acclimatization to Heat
If humidity is low, evaporation of sweat can be an effective way to keep the body from overheating. However, the loss of water and salts in sweat can also be dangerous. In very hot conditions, an adult may lose up to four litres of sweat per hour and up to 14 litres per day. Such water losses may cause severe dehydration if the water is not replaced by drinking much more than usual. The loss of salts may also upset the normal salt balance in the body, which can be dangerous. Becoming acclimatized to heat by gradually increasing the exposure time to high temperatures — particularly while exercising or doing physical work — can reduce the risk of these effects.
It may take up to 14 days to attain maximum heat acclimatization. As the body becomes acclimatized, sweat output increases, and sweating begins sooner. The salt content of the sweat also declines, as does the output of urine. These and other physiological changes help the body lose heat through the evaporation of sweat, while maintaining the proper balance of salts and fluids in the body. There may also be increased blood flow to the body surface through the widening of blood vessels near the skin. This is called . This brings more heat from the body core to the skin, and from there it may be radiated out into the environment.
Becoming acclimatized to heat allows one to safely perform more exercise or work in the heat. It also helps prevent heat-related illnesses by reducing strain on the body. Heat-related illnesses — from least to most serious — include heat cramps, heat exhaustion, and heat stroke.
- Heat cramps are muscle spasms caused by loss of water and salts. They often follow prolonged sweating brought on by over-exertion in hot weather.
- Heat exhaustion is a condition in which over-heating of the body causes dizziness, headache, profuse sweating, rapid heartbeat. and other symptoms. Without prompt treatment, heat exhaustion can lead to heat stroke.
- Heat stroke is potentially life threatening and a medical emergency. Heat stroke results from prolonged exposure to high temperatures, usually in combination with dehydration. It leads to failure of the body’s temperature control system and is diagnosed when the core body temperature exceeds 105 degrees F (40 degrees C). Symptoms may include nausea, seizures, confusion, disorientation, and coma.
Acclimatization to heat, like other types of acclimatization, is a reversible process. Just as quickly as heat acclimatization occurs, the physiological changes fade away in the absence of heat exposure. The body returns to its baseline state within a week or two of no longer exercising or working at high temperatures.
Other Human Responses to Cold
Besides genetic difference in body build, there are two major ways the human body can respond to the cold. One way is by producing more body heat, and the other way is by conserving more body heat. An immediate response to cooling of the body is shivering. This is an involuntary and simultaneous contraction of many tiny muscles in the body. These muscle contractions generate a small amount of heat. Another early response to cold temperature is a narrowing of blood vessels near the skin. This is called . This helps to shunt blood away from the body surface so more heat is held at the body core. The skin cools down and radiates less heat into the environment.
At temperatures below freezing, vasoconstriction can be dangerous if it lasts too long. The extremities become too cold because of lack of blood flow, and cold injury (such as frostbite) may occur. is tissue destruction that occurs when tissue freezes. You can see a mild-to-moderate case of frostbite of the fingers in Figure 6.7.7. If frostbite is severe, it may lead to gangrene and amputation of the affected extremities.
The body counters the possibility of cold injury with a reaction called the . This is a process of alternating vasoconstriction and vasodilation in extremities exposed to cold. About five to ten minutes after the start of cold exposure, the blood vessels in the extremities suddenly dilate, which increases blood flow and subsequently the temperature of the extremities. This is soon followed by another phase of vasoconstriction, and then the process repeats.
The hunting response occurs in most people, but several factors may influence the strength of the response. People who live or work regularly in cold environments show an increased hunting response. Through acclimatization, however, tropical residents can develop an increased response, which is indistinguishable from that of arctic residents. Genetic factors may play a role in the hunting response, but this is uncertain because it is difficult to differentiate between adaptation and acclimatization.
Where temperatures rarely fall below freezing but are repeatedly very chilly, the hunting response may not occur. Instead, vasoconstriction may persist to keep heat within the body at the expense of cooling the skin. As long as the temperature stays above freezing, cold injury (such as frostbite) will not occur. This type of response has been shown to occur in indigenous desert dwellers in southern Africa and Australia, where the temperature is hot during the day and very cold at night. People in these populations also tend to deposit fat around the organs in their chest and abdomen. The fat serves as insulation, protecting vital structures from the cold.
Besides shivering, another way to increase body heat is to raise the basal metabolic rate. The (BMR) is the amount of energy that a person needs to keep the body functioning at rest. The higher the BMR, the more heat the body generates, even without exercise or physical labor. The BMR can be increased by consuming large quantities of high-calorie fatty foods. People living in very cold subarctic regions, including the Inuit, traditionally ate whale and seal blubber and other high-fat foods, which helped them maintain a high BMR and stay warm.
Figure 6.7.8 Whale and seal blubber (mainly on abundant ring seals) is an important part of the traditional Inuit diet.
Feature: Human Biology in the News
Too many news stories report young children being seriously injured or dying from heat stroke in hot vehicles. On average, 38 children die in hot vehicles each year from heat-related deaths after being trapped inside. Most often, this happens by accident, when a parent or caregiver unknowingly leaves a sleeping child in a car. In other cases, children get into cars on their own, and then cannot get out again.
A child’s thermoregulatory system is not as efficient as that of an adult, and a child’s body temperature may increase as much as five times faster. This makes children prime candidates for heat stroke. A motor vehicle is also easily heated by direct sun. The windows of the vehicle allow solar radiation to pass through and heat up objects inside. A dark-coloured dashboard or seat may quickly reach a temperature of more than 180 degrees F (82 degrees C)! These hot surfaces can just as quickly heat the adjacent air, rapidly increasing the temperature of the air trapped inside the vehicle.
Here are several simple tips that parents and caregivers can follow to prevent heat stroke tragedies:
- Never leave children alone in or around cars — not even for a minute.
- Always open the back door and check the back seat before leaving your vehicle to be sure no child has been left behind.
- Put something you will need, such as your cell phone or handbag, in the back seat so you will have to open the back door to retrieve it whenever you park the car.
- Keep a large stuffed animal in the child’s car seat, and when the child is placed in the car seat, put the stuffed animal in the front passenger seat as a visual reminder that the child is in the back.
- Make sure you have a strict policy in place with everyone involved in the care of your child that you should always be called whenever your child does not show up at daycare or school as scheduled.
- Keep vehicles locked at all times, even in driveways and garages. Ask home visitors, child care providers, and neighbors to do the same.
- Keep car keys and remote vehicle openers out of reach of children.
- If a child is missing, immediately check the inside passenger compartments and trunks of all vehicles in the area. Check vehicles even if they are locked, because a child may lock a vehicle after entering and not be able to unlock it again to get out.
- If you see a child alone in a vehicle, call 911 immediately. If the child seems hot or sick, get them out of the vehicle as quickly as possible.
- Pay for gas at the pump and use drive-throughs at the bank, pharmacy, or wherever else they are available.
- Both hot and cold temperatures are serious environmental stresses on the human body. In the cold, there is risk of , which is a dangerous decrease in core body temperature. In the heat, there is risk of, which is a dangerous increase in core body temperature.
- According to , body size tends to be negatively correlated with temperature, because larger body size increases heat production and decreases heat loss. The opposite holds true for small body size. Bergmann’s rule applies to many human populations that are hot- or cold-adapted.
- According to , the length of body extremities is positively correlated with temperature, because longer extremities are better at dissipating excess body heat. The opposite applies to shorter extremities. Allen’s rule applies to relative limb lengths in many human populations that have adapted to heat or cold.
- Sweating is the primary way that humans lose body heat. The evaporation of sweat from the skin cools the body. This only works well when the relative humidity is fairly low. At high relative humidity, sweat does not readily evaporate to cool us down. The heat index (HI) indicates how hot it feels due to the humidity.
- Gradually working longer and harder in the heat can bring about heat acclimatization, in which the body has improved responses to heat stress. For example, sweating starts earlier, sweat contains less salt, and vasodilation brings heat to the surface to help cool the body. Full acclimatization takes up to 14 days and reverses just as quickly when the heat stress is removed.
- The human body can respond to cold by producing more heat (by shivering or increasing the basal metabolic rate) or by conserving heat (by vasoconstriction at the body surface or a layer of fat-insulating internal organs).
- At temperatures below freezing, the hunting response occurs to prevent cold injury, such as frostbite. This is a process of alternating vasoconstriction and vasodilation in extremities that are exposed to dangerous cold. Where temperatures are repeatedly cold but rarely below freezing, the hunting response may not occur, and the skin may remain cold due to vasoconstriction alone.
- Compare and contrast hypothermia and hyperthermia.
- State Bergmann’s and Allen’s rules.
- How do the Maasai and Inuit match the predictions based on Bergmann’s and Allen’s rules?
- Explain how sweating cools the body.
- What is the heat index?
- Relate the heat index to evaporative cooling of the body.
- Identify three heat-related illnesses, from least to most serious.
- How does heat acclimatization occur?
- State two major ways the human body can respond to the cold, and give an example of each.
- Explain how and why the hunting response occurs.
- Define basal metabolic rate.
- How does a high-fat diet help prevent hypothermia?
- Explain why frostbite most commonly occurs in the extremities, such as the fingers and toes.
What happens when you get heat stroke? – Douglas J. Casa, TED-Ed, 2014.
Hailstones’ Inupiaq Traditions | Life Below Zero, National Geographic, 2014.
How An Igloo Keeps You Warm, It’s Okay To Be Smart, 2017.
Why do we sweat? – John Murnan, TED-Ed, 2018.
Wim Hof Method, Wim Hof, 2011.
- Maasai warrior by Ninaras on Wikimedia Commons is used under a CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/deed.en) license.
- Smiling man from Maasai Mara, Kenya by Sneha on Unsplash is used under the Unsplash License (https://unsplash.com/license).
- Inuit-Kleidung women by Ansgar Walk on Wikimedia Commons is used under a CC BY-SA 3.0 (https://creativecommons.org/licenses/by-sa/3.0/deed.lv) license.
- Kulusuk, Tunumiit Inuit couple by Arian Zwegers on Wikimedia Commons is used under a CC BY 2.0 (https://creativecommons.org/licenses/by/2.0/deed.en) license.
Heat_Index by U.S. National Oceanic and Atmospheric Administration (NOAA) on Wikimedia Commons is in the public domain (https://en.wikipedia.org/wiki/Public_domain).
- Ringed seal preparation by Ansgar Walk on Wikimedia Commons is used under a CC BY-SA 2.5 (https://creativecommons.org/licenses/by-sa/2.5/deed.en) license.
- Butchering a narwhal by Spencer & Carole on Flickr is used under a CC BY-NC-SA 2.0 (https://creativecommons.org/licenses/by-nc-sa/2.0/) license.
- Inuit children playing while the family is on seal hunt, by GRID-Arendal on Flickr is used under a CC BY-NC-SA 2.0 (https://creativecommons.org/licenses/by-nc-sa/2.0/) license.
It’s Okay To Be Smart. (2017, January 9). How an igloo keeps you warm. YouTube. https://www.youtube.com/watch?v=1L7EI0vKVuU&feature=youtu.be
National Geographic. (2014, April 7). Hailstones’ Inupiaq traditions | Life below zero. YouTube. https://www.youtube.com/watch?v=_Ifq73REJiM&feature=youtu.be
TED-Ed. (2014, July 21). What happens when you get heat stroke? – Douglas J. Casa. YouTube. https://www.youtube.com/watch?v=PpHM4DfPZQU&feature=youtu.be
TED-Ed. (2018, May 15). Why do we sweat? – John Murnan. YouTube. https://www.youtube.com/watch?v=fctH_1NuqCQ&feature=youtu.be
Wim Hof. (2011, June 19). Wim Hof Method. YouTube. https://www.youtube.com/watch?v=j3sY67aGFXY&feature=youtu.be
The long-term average of weather, typically averaged over a period of 30 years. Some of the meteorological variables that are commonly measured are temperature, humidity, atmospheric pressure, wind, and precipitation.
A medical emergency that occurs when your body loses heat faster than it can produce heat, causing a dangerously low body temperature. Normal body temperature is around 98.6 F (37 C). Hypothermia occurs as your body temperature falls below 95 F (35 C).
A group of heat-related conditions characterized by an abnormally high body temperature — in other words, the opposite of hypothermia. The condition occurs when the body's heat-regulation system becomes overwhelmed by outside factors, causing a person's internal temperature to rise.
An ecogeographical rule that states that within a broadly distributed taxonomic clade, populations and species of larger size are found in colder environments, and species of smaller size are found in warmer regions.
The principle holding that in a warm-blooded animal species having distinct geographic populations, the limbs, ears, and other appendages of the animals living in cold climates tend to be shorter than in animals of the same species living in warm climates.
The widening of blood vessels. It results from relaxation of smooth muscle cells within the vessel walls, in particular in the large veins, large arteries, and smaller arterioles. The process is the opposite of vasoconstriction, which is the narrowing of blood vessels.
A narrowing of blood vessels so less blood can flow through them.
An injury to body tissues caused by exposure to extreme cold, typically affecting the nose, fingers, or toes and sometimes resulting in gangrene.
A process of alternating vasoconstriction and vasodilation in extremities exposed to cold. The term Lewis reaction is used too, named after Thomas Lewis, who first described the effect in 1930.
The number of calories required to keep your body functioning at rest.