Evolution And History

Human-Made Stressors of a Changing World 305

Adaptation to Heat

The human body’s primary physiological mechanism for

coping with extreme heat is sweating or perspiring. Sweat-

ing is a process through which water released from sweat

glands gives up body heat as the sweat evaporates. There-

fore, water availability is a crucial aspect of adaptation to

heat. Without drinking enough water to replace that which

is lost through sweating, exposure to heat can be fatal.

Though there is some individual and population vari-

ation, each human has roughly 2 million sweat glands.

These glands are spread out over a greater surface area

on tall, thin bodies, facilitating water evaporation and

heat loss. Thus Bergmann’s and Allen’s rules also apply

to heat adaptation. The more surface area a body has,

the more surface for the sweat glands. In addition,

because heat is produced by unit of volume, having a

high surface area-to-volume ratio is beneficial for heat

loss. A long, slender body is best for dissipating heat. In

a hot and humid environment such as a rainforest, water

evaporation is a major challenge. In this environment,

human populations have adapted to minimize heat

production through a reduction in overall size while

keeping a slender, lean build.

Human-Made Stressors

of a Changing World

Traditionally, culture has played an important role in mod-

ifying natural stressors such as heat and cold. Housing,

diet, and clothing traditions can alter such stressors con-

siderably. But in today’s globalizing world, the effects of

culture are much more complex. Rather than alleviating

physical stressors through simple cultural adaptations

such as housing, diet, and clothing, cultural processes are

adding stressors such as pollution, global warming, and

exhaustion of the world’s natural resources.

People cannot biologically adapt to these human-

made stressors because biological change simply cannot

keep pace with the rapid rate at which humans are chang-

ing the earth. Until human cultures cooperate to collec-

tively address these global challenges, unnatural stressors

will inevitably lead to sickness and suffering. An inte-

grated, holistic anthropological perspective has much to

contribute to alleviating if not eliminating these human-

made stressors.

In extreme cold, the limbs need enough heat to pre-

vent frostbite, but giving up heat to the periphery takes it

away from the body core. Humans balance this through

a cyclic expansion and contraction of the blood vessels

of their limbs called the hunting response. Blood vessels

oscillate between closing down to prevent heat loss and

opening up to warm the hands and feet. When first ex-

posed to cold as gloves are taken off, blood vessels im-

mediately constrict. Initial alternations between open

(warm) and shut (cold) and the corresponding tempera-

ture of the skin range dramatically. But the oscillations

become smaller and more rapid, allowing a hunter to

maintain the warmth-derived manual dexterity required

for tying knots or positioning arrows.

Eskimos (including the Inuit) also deal with cold

through a high metabolic rate: the rate at which their

bodies burn energy. This may result from a diet high in

protein and fat (whale blubber is the common food). In

addition, genetic factors likely also contribute to Eskimos’

high metabolic rate.

One short-term physiological response to cold is shiv-

ering. Shivering generates heat for the body quickly but

cannot be maintained for long periods of time. Instead, as

an individual acclimatizes to the cold, adjustments to diet,

activity pattern, metabolic rate, and the circulatory system

must occur.

4

4

(44 of each side)  6 sides  96 [(24)  2 sides] 

[(28)  2 sides] 

[(48)  2 sides]  112

4

Surface area  96

Volume  64

8

2

4

Surface area  112

1.75 times greater

surface area

Volume  64

Same volume

Allen’s Rule

hunting response A cyclic expansion and contraction of the

blood vessels of the limbs that balances releasing enough heat to

prevent frostbite while maintaining heat in the body core.

Figure 13.4 Allen’s rule refers to the observation that in

two bodies that have the same volume, the one that is long

and lean rather than short and squat will have a greater

surface area. This accounts for the tendency for mammals

living in cold climates to have shorter appendages (arms and

legs) than the same species living in warmer climates. Heat

can be dissipated through long limbs or conserved through

short ones.

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