Mammals and birds have evolved complex mechanisms to maintain a constant internal body temperature. While reptiles must rely on exposure to external heat sources to raise body temperature and energy levels, mammals and birds have physiological mechanisms that, within certain limits, increase or reduce the loss of body heat. The optimum body temperature for normal cellular functions is species-specific, and for humans, it’s approximately 98.6°F.
People are found in a wide variety of habitats, with thermal environments ranging from exceedingly hot (in excess of 120°F) to bitter cold (less than −60°F). In such extremes, particularly cold, human life would not be possible without cultural innovations. But even accounting for the artificial environments we live in, such external conditions expose the human body to enormous stress.
Response to Heat All available evidence suggests that the earliest hominins evolved in the warm-to-hot woodlands and savannas of East Africa. The fact that humans cope better with heat, especially dry heat, than they do with cold is testimony to the long-term adaptations to heat that evolved in our ancestors.
In humans as well as some other species such as horses, sweat glands are distributed throughout the skin. This wide distribution of sweat glands makes it possible to lose heat at the body’s surface through evaporative cooling, a mechanism that has evolved to the greatest degree in humans. In fact, perspiration is the most important factor in heat dissipation in humans
The capacity to dissipate heat by sweating is seen in all human populations to an almost equal degree, with the average number of sweat glands per individual (approximately 1.6 million) being fairly constant. However, there is some variation, since people who are not generally exposed to hot conditions do experience a period of acclimatization that initially involves significantly increased perspiration rates (Frisancho, 1993). An additional factor that enhances the cooling effects of sweating is increased exposure of the skin through reduced amounts of body hair. We don’t know when in our evolutionary history the loss of body hair occurred, but it represents a species wide adaptation.
Heat reduction through evaporation can be expensive, and indeed dangerous, in terms of water and sodium loss. For example, a person engaged in heavy work in high heat can lose up to 3 liters of water per hour. To appreciate the importance of this fact, consider that losing 1 liter of water is approximately equivalent to losing 1.5 percent of total body weight, and losing 10 percent of body weight can be life threatening. Thus water must be continuously replaced during exercise in heat.
Basically, there are two types of heat, arid and humid. Arid environments, such as those of the southwestern United States, the Middle East, and parts of Africa, are characterized by high temperatures, wind, and low water vapor. Humid heat, associated with increased water vapor, occurs in regions with a great deal of vegetation and precipitation, conditions found in the eastern and southern United States, parts of Europe, and much of the tropics. Because the increased water vapor in humid climates inhibits the evaporation of sweat on the skin’s surface, humans adjust much more readily to dry heat. In fact, people exercising in dry heat may be unaware that they’re sweating because the perspiration evaporates as soon as it reaches the skin’s surface. While rapid evaporation increases comfort, it can lead to dehydration. Therefore in dry heat, it’s important to keep drinking water, even if you aren’t particularly thirsty.
Another mechanism for radiating body heat is vasodilation, which occurs when capillaries near the skin’s surface widen to increase blood flow to the skin. The visible effect of vasodilation is flushing, or increased redness and warming of the skin, particularly of the face. But the physiological effect is to permit heat, carried by the blood from the interior of the body, to be emitted from the skin’s surface to the surrounding air. (Some drugs, including alcohol, also produce vasodilation; this accounts for the increased redness and warmth of the face in some people after a couple of drinks.)
Body size and proportions are also important in regulating body temperature. In fact, there seems to be a general relationship between climate and body size and shape in birds and mammals. In general, within a species, body size (weight) increases as distance from the equator increases. In humans, this relationship holds up fairly well, but there are many exceptions.
Two rules that pertain to the relationship between body size, body proportions, and climate are Bergmann’s rule and Allen’s rule.
1. Bergmann’s rule concerns the relationship of body mass or volume to surface area. Among mammals, body size tends to be greater in populations that live in colder climates. This is because as mass increases, the relative amount of surface area decreases proportionately. Since heat is lost at the surface, it follows that increased mass allows for greater heat retention and reduced heat loss. (Remem ber our discussion of basal metabolic rate and body size in Chapter 7.)
2. Allen’s rule concerns the shape of the body, especially appendages. In colder climates, shorter appendages, with increased mass-to-surface ratios, are adaptive because they’re more effective at preventing heat loss. Conversely, longer appendages, with increased surface area relative to mass, are more adaptive in warmer climates because they promote heat loss.
According to these rules, the most suitable body shape in hot climates is linear, with long arms and legs. In cold climates, a stockier body with shorter limbs is more adaptive. Considerable data gathered from several human populations demonstrate that, in general, humans conform to these principles. In colder climates, body mass tends, on average, to be greater and to be characterized by a larger trunk relative to arms and legs. People living in the Arctic tend to be short and stocky, while many sub-Saharan Africans, especially East African pastoralists, are tall and linear (Fig. 15-6). But there’s a great deal of variation in human body proportions, and not all populations conform so obviously to Bergmann’s and Allen’s rules.
Response to Cold There are two basic types of physiological responses to cold: those that retain heat and thosethat increase heat production. Of the two, heat retention is more efficient because it requires less energy. This is an important point, because energy is obtained from dietary sources. Unless food is abundant, and in winter it frequently is not, any factor that conserves energy can be beneficial.
Increases in metabolic rate and shivering are short-term responses that generate body heat. Increases in metabolic rate (the rate at which cells break up nutrients into their components) release energy in the form of heat. Shivering also generates muscle heat, as does voluntary exercise. But both these methods are costly because they require an increased intake of nutrients to provide needed energy. (Perhaps this explains why we tend to have a heartier appetite during the winter and why, during that season, we also tend to eat more fats and carbohydrates, the very sources of the energy we require.) In general, people exposed to chronic cold (meaning much or most of they ear) maintain higher metabolic rates than people who live in warmer climates. The Inuit (Eskimo) people living in the Arctic maintain metabolic rates between 13 and 45 percent higher than that observed in non-Inuit control subjects (Frisancho, 1993). What’s more, the highest metabolic rates are seen in inland Inuit, who are exposed to even greater cold stress than coastal populations. Traditionally the Inuit had the highest animal protein and fat diet of any population in the world. Their diet was dictated by the available resource base, and it served to maintain the high metabolic rates required by exposure to chronic cold.
Vasoconstriction is another shortterm response, but instead of producing heat, it minimizes heat loss and therefore is more energy efficient. Vasoconstriction restricts capillary blood flow to the surface of the skin, thus reducing heat loss at the body surface. Because retaining body heat is more economical than creating it, vasoconstriction is very efficient provided that temperatures don’t drop below freezing. However, if temperatures do fall below freezing, continued vasoconstriction can lower skin temperature to the point of frostbite or worse.
Long-term responses to cold vary among human groups. For example, in the past, desert-dwelling native Australian populations experienced wide temperature fluctuations from day to night. Because they wore no clothing and didn’t build shelters, they built sleeping fires to protect themselves from night time temperatures hovering only a few degrees above freezing. Also, they experienced continuous vasoconstriction throughout the night, which permitted a degree of skin cooling most people would find extremely uncomfortable. But since there was no threat of frostbite, continued vasoconstriction helped prevent excessive internal heat loss.
By contrast, the Inuit experience intermittent periods of vasoconstriction and vasodilation. This compromise provides periodic warmth to the skin, which helps prevent frostbite in below-freezing temperatures. At the same time, because the vasodilation is intermittent, energy loss is restricted to retain more heat at the body’s core.
Humans and some other animals also have a subcutaneous (beneath the skin) fat layer that provides insulation throughout the body. In many overfed populations today, this fat layer is an annoyance to many and a major health issue for others. But in the not too distant past, our hunting and gathering ancestors relied on it not only for some protection against the cold but also as a source of nutrients when food was scarce.
These examples illustrate two of the ways in which adaptations to cold vary among human populations. Obviously winter conditions exceed our ability to adapt physiologically in many parts of the world. Consequently, if our ancestors hadn’t developed cultural innovations, they would have remained in the tropics.