KEEPING WARM 
to stay cool or to insulate themselves so 
thoroughly that their body heat is re- 
tained even in the coldest of places. For 
example, by panting or sweating, 
mammals create evaporative cooling, 
the dissipation of body heat through the 
vaporization of moisture. And one of the 
most important ways warmblooded ani- 
mals regulate the flow of heat is through 
the circulation of the blood. In mam- 
mals and birds, the amount of blood 
flowing to the surface of their skin or 
even to entire extremities is controlled. 
Flushing the skin with blood serves to 
pump the heat of the inner body to the 
surface, where it can readily escape. 
Conversely, restricting the flow of blood 
to the surface serves to retain the blood’s 
heat in an animal’s inner core. In many 
species of waterfowl, virtually all blood 
flow to the legs can be cut off, reducing 
the heat lost to cold water. 
Mammals and birds have also evolved 
a marvelous variety of ways to insulate 
their bodies. Fat, which occurs in other 
animals only as lumps that store food 
energy, is deployed in many mammals 
as a special insulating layer under the 
skin. The thickness of this layer varies in 
response to seasonal demands for pro- 
tection against cold. Polar bears and 
huskies have such an effective layer of 
fat that they can comfortably sleep on 
ice. Fur and feathers are other forms of 
insulation, capable of exquisite variation 
in quality, quantity, and even color over 
the course of a year. Smooth and sleek 
and a bit oily in the summer, an animal’s 
coat protects its skin from the sun’s hot 
rays; in winter the downy undercoat is 
thicker and can be fluffed up to provide 
an even more effective insulation. The 
goose bumps that humans get when they 
are suddenly chilled is a remnant of 
response from the time when our ances- 
tors still had fur to fluff. A coat of fur or 
feathers essentially traps a layer of air 
with its own climatic conditions: the 
animal’s skin is not exposed to the pre- 
vailing climate but to the warmer air 
contained within the coat. 
When, in the course of evolution, hu- 
man beings lost the thick mammalian 
fur coat, they also lost its capabilities for 
thermal control. Our naked skin func- 
tions well in the hot, humid tropics but 
needs some assistance in other climates. 
The traditional clothing developed by 
various cultures often has extremely so- 
phisticated thermal functions. The bil- 
lowing white robes of the Arab reflect 
away the sun’s radiation, while helping 
to fan air past the body and increase 
evaporative cooling. At the other ex- 
treme, the Eskimo’s fur parka keeps in 
both body heat and water vapor from 
perspiration so that the Eskimo essen- 
tially lives within a semitropical environ- 
ment. 
Human beings, like other animals, 
also use shelter as a means for thermal 
control. Nest building is, in a way, an 
advanced version of choosing the most 
favorable microclimate. Animals that 
use a rock crevice or a hole in the earth 
to escape the cold (or heat) are indeed 
seeking out a favorable microclimate. 
Digging the hole a little deeper and 
adding a bit of shed fur for insulation 
are simple improvements. All animals 
start their nests by finding the best 
location, but only a few of the most 
talented nest builders — some birds, bea- 
vers, and Homo sapiens — can com- 
pletely transform an environment to 
meet their needs. For example, the 
Anasazi Indians of the southwestern 
United States were remarkably clever in 
choosing sites for their cliff dwellings. 
They invariably chose locations shaded 
in the summer by an overhanging ledge 
of the cliff but exposed to full sun all 
winter long. With their backs to the 
cliff, the dwellings were protected from 
the winter winds and also took advan- 
tage of the thermal mass of the earth to 
moderate the temperature flux. 
Buildings, even in the conventional 
ways they are constructed now, can be 
viewed as a way to modify a landscape 
to create more favorable microclimates. 
As soon as a simple square hut is built, 
at least six new microclimates are cre- 
ated: the south side warmed by a sunny 
wall; the north side in shade most of the 
time; the east side with its morning sun 
and perhaps protected from the prevail- 
ing breeze; the west side warmed in the 
afternoon but buffeted by the wind; the 
interior with its shelter from rain, wind, 
and sun; and the roof, raised above 
ground level and more exposed to the 
elements. A building increases the range 
of available thermal zones so that people 
can select the microclimate most suited 
to their thermal needs. 
Vernacular building traditions all 
over the world display remarkably so- 
phisticated adaptations. Primitive build- 
ers consistently used forms and materi- 
als that effectively moderated prevailing 
climatic conditions. For example, in the 
desert the characteristic problem is ex- 
tremely high daytime temperatures cou- 
pled with uncomfortably low tempera- 
tures at night. The ideal building 
material would have a high heat capac- 
ity — to absorb solar radiation during the 
day and slowly radiate it back at night. 
Thus, buildings of adobe, mud and rub- 
ble masonry, and thick clay mortar on 
twig mesh are found in the American 
Southwest, the Middle East, and across 
Africa. A particularly ingenius response 
to climatic needs is seen in the Eskimo 
igloo. Using material of very low ther- 
mal capacity — dry snow — the Eskimo 
builds a small dome, a shape both resis- 
tant to wind and easily heated by a single 
oil lamp. A glaze of ice, which automati- 
cally forms on the inside surface, seals 
the structure and acts as extra insula- 
tion, along with furs that are draped 
against the wall. Clever use of the insu- 
lating qualities of snow is not unique to 
the human inhabitants of the Arctic. 
Many arctic animals bury their nests 
deep in the snow; protected from winds 
the animals’ body heat can warm the 
enclosure. But the Eskimo has one tran- 
scendent advantage — the use of fire. 
Fire offers enormous potential for af- 
fecting the thermal environment. Plants 
and the simpler animals are totally de- 
pendent upon the sun and weather to 
provide the heat necessary for their sur- 
vival. Coldblooded animals make use of 
their own body heat to help keep them- 
selves warm, and warmblooded animals 
are masters at regulating this internal 
source. Fire allows human beings a third 
mechanism, one independent of the 
fluctuations of climate and their own 
metabolism. With the control of fire, 
people can generate heat at will to warm 
themselves and their environment. 
The most primitive dwelling was typi- 
cally just a small round shelter with a 
fire pit at its center. In Europe, as late as 
the Middle Ages, it was still common 
practice to have an open fire in the 
center of a room with the smoke escap- 
ing through a hole in the roof gable. 
Such fireplaces were extremely ineffi- 
cient, causing as many heating problems 
as they solved. Flames were more val- 
ued than embers, but large, open fires 
consumed great quantities of oxygen, 
pulling the cold outside air in through 
the cracks of the building as quickly as 
the heated air escaped through the roof 
hole. Since the air inside the dwelling 
would not stay heated, the only way to 
benefit from the fire’s warmth was to sit 
directly in front of the flames. 
About the end of the Middle Ages 
chimneys began to be used to channel 
off the smoke of a fire. Early chimneys 
were made of wood, but people soon 
34 
