{"id":4668,"date":"2019-06-24T14:04:57","date_gmt":"2019-06-24T14:04:57","guid":{"rendered":"https:\/\/pressbooks.ccconline.org\/acchumanbio\/chapter\/8-9-human-responses-to-extreme-climates-3\/"},"modified":"2023-11-30T18:47:19","modified_gmt":"2023-11-30T18:47:19","slug":"8-9-human-responses-to-extreme-climates-3","status":"publish","type":"chapter","link":"https:\/\/pressbooks.ccconline.org\/acchumanbio\/chapter\/8-9-human-responses-to-extreme-climates-3\/","title":{"raw":"6.7\u00a0Human Responses to Extreme Climates","rendered":"6.7\u00a0Human Responses to Extreme Climates"},"content":{"raw":" \r\n\r\n[h5p id=\"525\"]\r\n\r\nFigure 6.7.1 Men from Maasai Mara, Kenya.<\/span><\/em>\r\n

Built for Heat<\/h1>\r\nThese tall, slender men live near the equator in Kenya, East Africa \u2014 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.\r\n
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Climate Extremes<\/h1>\r\n<\/div>\r\n[pb_glossary id=\"2741\"]Climate[\/pb_glossary]<\/strong>\u00a0refers 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.\r\n\r\nIn the cold, there is risk of\u00a0[pb_glossary id=\"2742\"]hypothermia[\/pb_glossary],<\/strong>\u00a0which 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\u2019s temperature regulation mechanism starts to fail.\r\n\r\nThe opposite problem occurs in the heat, where the risk is\u00a0[pb_glossary id=\"2743\"]hyperthermia[\/pb_glossary],<\/strong>\u00a0which 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.\r\n
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Human Adaptation to Heat and Cold<\/h2>\r\n<\/div>\r\nHumans are the most widespread species on the planet, and they have lived in extreme climates for tens of thousands of years.\u00a0As a result, many human populations have had to cope with extreme temperatures for hundreds of generations, which has\u00a0forced\u00a0them to develop genetic adaptations to these climate extremes.\r\n\r\nThe 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\u2019s and Allen\u2019s rules.\r\n\r\n[h5p id=\"526\"]\r\n\r\nFigure 6.7.2 Indigenous North American Inuit.<\/em>\r\n

Bergmann\u2019s Rule<\/h3>\r\n[pb_glossary id=\"2748\"]Bergmann's rule[\/pb_glossary]<\/strong>\u00a0states 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\u2019s rule has been shown to generally apply to widespread species of mammals and birds, although there are also many exceptions to the rule.\r\n\r\nWhat explains Bergmann\u2019s 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.\r\n\r\nTable 6.7.1<\/strong>\r\n\r\nRelationship of Surface Area to Volume in Cubes of Different Sizes<\/em>\r\n\r\n\r\n\r\n\r\n\r\n\r\n\r\n\r\n\r\n
Relationship of Surface Area to Volume in Cubes of Different Sizes<\/strong><\/td>\r\n<\/tr>\r\n
Side of Cube (cm)<\/strong><\/td>\r\nSurface Area of Cube (cm<\/strong>2<\/strong>)<\/strong><\/td>\r\nVolume of Cube (cm<\/strong>3<\/strong>)<\/strong><\/td>\r\nSurface Area:Volume Ratio<\/strong><\/td>\r\n<\/tr>\r\n
2<\/td>\r\n24<\/td>\r\n8<\/td>\r\n3:1<\/td>\r\n<\/tr>\r\n
4<\/td>\r\n96<\/td>\r\n64<\/td>\r\n3:2<\/td>\r\n<\/tr>\r\n
6<\/td>\r\n216<\/td>\r\n216<\/td>\r\n3:3<\/td>\r\n<\/tr>\r\n
12<\/td>\r\n864<\/td>\r\n1728<\/td>\r\n3:6<\/td>\r\n<\/tr>\r\n
20<\/td>\r\n2400<\/td>\r\n8000<\/td>\r\n3:10<\/td>\r\n<\/tr>\r\n<\/tbody>\r\n<\/table>\r\nWarmer 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.\r\n\r\n[caption id=\"attachment_2752\" align=\"alignleft\" width=\"432\"]\"Bergmann's Figure 6.7.3 According to Bergmann's Rule, species of larger size are found in colder environments and species of smaller size are found in warmer regions.<\/em>[\/caption]\r\n\r\nAnthropologists have found that many human populations tend to follow Bergmann\u2019s 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.\r\n\r\nThere 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.\u00a0Humans, 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.\r\n

Allen\u2019s Rule<\/h2>\r\n[pb_glossary id=\"5625\"]Allen\u2019s rule[\/pb_glossary]<\/strong>\u00a0is a corollary of Bergmann\u2019s 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\u2019s rule is similar to the rationale behind Bergmann\u2019s rule. Longer extremities maximize an animal\u2019s 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.\r\n\r\nAnthropologists 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 \u2014 like those of other members of his population \u2014 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 \u2014 consistent with Allen's rule \u2014people tend to have shorter limbs at higher altitudes.\r\n
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Other Human Responses to Heat<\/h1>\r\n<\/div>\r\nHumans exhibit several other responses to high temperatures that are generally considered either short-term physiological responses or examples of longer-term acclimatization.\r\n

Sweating and Humidity<\/h2>\r\n[caption id=\"attachment_2753\" align=\"alignright\" width=\"346\"]\"\" Figure 6.7.4 Sweating is a normal response to heat stress.<\/em>[\/caption]\r\n\r\nBecause 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.\r\n
\r\n\r\nHow 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, \u201cIt\u2019s not the heat, but the humidity.\u201d<\/span>\r\n\r\n<\/div>\r\nThe\u00a0heat index (HI)<\/strong> 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.\u201d 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.\r\n\r\n \r\n\r\n[caption id=\"attachment_2754\" align=\"aligncenter\" width=\"638\"]\"NOAA's Figure 6.7.5 NOAA's National Weather Service Heat Index <\/em>[\/caption]\r\n\r\n
\r\n\r\nAcclimatization to Heat<\/span>\r\n\r\n<\/div>\r\n\r\n[caption id=\"attachment_2755\" align=\"alignright\" width=\"269\"]\"\" Figure 6.7.6 Thirsty! The loss of water in hot temperatures may cause severe dehydration if the water is not replaced by drinking much more than usual.\u00a0<\/em>[\/caption]\r\n\r\nIf 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 \u2014 particularly while exercising or doing physical work \u2014 can reduce the risk of these effects.\r\n\r\nIt 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\u00a0[pb_glossary id=\"2756\"]vasodilation[\/pb_glossary].<\/strong>\u00a0This brings more heat from the body core to the skin, and from there it may be radiated out into the environment.\r\n\r\nBecoming acclimatized to heat allows one to safely perform more exercise or work in the heat. It also helps prevent heat-related illnesses\u00a0by reducing\u00a0strain on the body. Heat-related illnesses \u2014 from least to most serious \u2014 include heat cramps, heat exhaustion, and heat stroke.\r\n