KEEPING WARM 
Thermal Necessity 
Fat, feathers, and the Franklin stove provide the warmth needed to sustain life 
by Lisa Heschong 
Life exists within a small range of 
temperatures. It comes to a standstill as 
water freezes, and even the hardiest of 
bacteria are destroyed in boiling water. 
Each species of plant or animal has 
limits within which it can survive and a 
narrow range of temperatures within 
which it can successfully compete with 
other species. Ecologists studying cer- 
tain lakes have discovered that a change 
of as little as two degrees Fahrenheit in 
the average water temperature will shift 
the dominant fish species to either bass 
or catfish, as one species becomes more 
efficient than the other. Thus, not only 
extremes but even subtle variations in 
temperature can be critical to an ani- 
mal’s survival. 
The oceans, where early life evolved, 
provide a particularly stable thermal 
environment in which organisms can 
live without much thermal stress. Fish 
and other coldblooded sea creatures can 
function perfectly well at the tempera- 
ture of their watery environment be- 
cause the temperature of the ocean — 
and thus of their bodies — varies so little, 
and then only very gradually. The land, 
however, experiences great swings of 
temperature. On a daily and a yearly 
cycle, the surface of the earth is con- 
stantly heating up and cooling down. 
The ground, in turn, heats the air 
around it, driving the forces of weather 
that can so dramatically change thermal 
conditions over the course of a few 
hours. The organisms that moved out of 
the oceans and onto the land were those 
equipped to survive both climatic ex- 
tremes and wide daily fluctuations. 
The most direct way to cope with an 
adverse climate is to not be there when it 
gets too hot or too cold. Many of the 
simpler forms of life, such as bacteria, 
fungi, and yeast, suspend metabolism — 
essentially, stop living — when the tem- 
perature becomes inhospitable. Simi- 
larly, many plants and insects live only 
during the most favorable season; when 
that season ends, they die off, leaving 
behind seeds or eggs that will sprout or 
hatch when conditions improve again. 
If an organism is to survive through a 
yearly cycle of seasons, however it must 
possess some thermal “strategy” more 
sophisticated than simply not being 
there. One way of almost not being 
there, short of dying, is to be deciduous, 
an adaptation displayed by plants not 
only in snowy climates but in desert 
climates as well. By dropping all of their 
leaves, deciduous plants reduce their 
exposure to temperature extremes. 
Metabolic activity continues — but at a 
greatly reduced level and only in a pro- 
tected core or in roots sheltered under- 
ground. Coldblooded animals and some 
mammals deal similarly with extreme 
cold: they hibernate, retreating to a 
place sheltered from temperature ex- 
tremes. As body temperatures drop, 
metabolic rates are greatly reduced, en- 
abling the animals to conserve energy 
for survival through the dormant period. 
Animals have a great thermal advan- 
tage over plants because they can move 
about. Good travelers, such as birds and 
large herd mammals, can migrate be- 
tween entire climatic zones so that they 
can avoid the worst thermal conditions 
of any particular region. On a smaller 
scale, animals can also move about to 
take advantage of the variations in cli- 
mate — the “microclimates” — that oc- 
cur within a given landscape. Snakes, 
sluggish from the night’s cold, will crawl 
out of their holes to bask in the morning 
sun. With the sun’s heat they can raise 
their body temperature considerably 
Adapted from Thermal Delight in Architec- 
ture. by Lisa Heschong, by permission of The 
MIT Press, Cambridge. Massachusetts. Copy- 
right © 1979 by The Massachusetts Institute of 
Technology. 
and begin to function effectively even 
though the morning air may still be 
quite cold. 
Indeed, coldblooded animals have 
various techniques to maintain a steady 
body temperature in spite of swings in 
the external temperature. Terrestrial 
coldblooded animals, unlike fish, are not 
completely at the mercy of their envi- 
ronment. In addition to moving to the 
best thermal location, they can also vary 
their muscular activity to generate more 
heat within their bodies. On a cold day, 
butterflies vibrate their wings for sev- 
eral minutes to warm their muscles 
enough so that they can fly. Lizards can 
maintain a body temperature to within 
one degree of their optimum by combin- 
ing two strategies: to warm up, they do 
“push-ups” on a sunny rock; to cool 
down, they retreat to a shady crevice 
and lie still. 
It is the genius of the warmblooded 
animals, the birds and the mammals, 
that they evolved with a system for 
regulating their internal body tempera- 
ture that takes full advantage of the 
heat naturally generated by their metab- 
olism. In mammals, the muscular heat- 
generating technique of the coldblooded 
animals is refined so that a change in 
muscle tone can produce subtle grada- 
tions in the amount of body heat pro- 
duced. Vibrations of the muscles have 
become an automatic shivering re- 
sponse — an emergency heat-generating 
measure. Over longer periods of time, 
many mammals can actually raise their 
basal metabolic rate to acclimate to 
cold. 
Even more significant than the ability 
to change the rate of heat production is 
the capability of warmblooded animals 
to regulate the rate of heat flow away 
from the body. A variety of mechanisms 
allow these animals to release their ex- 
cess heat into the environment in order 
