Environmental Engineering Reference
In-Depth Information
nearly 40
C across their blubber; their BMRs are two to
three times those of similarly sized terrestrial mammals
(Kanwisher and Ridgway 1983), a high price for their
ability to move and to feed in frigid waters.
Small endotherms are especially at risk during long
winters. Because the capacity to store fat is directly
proportional to body mass, large mammals can maintain
constant temperatures for months without eating. Con-
sequently, no hibernating animal heavier than about 5
kg needs to reduce its winter body temperature by more
than a few degrees, but in small mammals it drops some-
times by more than 30
C until spring, when the animal's
brown fat tissue is activated to rewarm it and energize it
for mating (Lyman 1982; French 1988). And some ani-
mals can become repeatedly hypothermic in order sustain
amazing physical feats. Antarctic king penguins dive so
deeply (
>
500 m) and for so long (maxima
>
15 min) be-
cause they reduce their abdominal temperature to as low
as 11
C during such deep, prolonged diving (Handrich
et al. 1997). Similarly, Arctic mammals habitually resort
to deep hypothermy in their feet.
Given these challenges, it is logical to ask why the
homeotherms are thermoregulating at such high levels
(approaching the level of protein decay), why not at
lower levels, perhaps just around 20
C? Regulation
at such levels would call for lower metabolic rates but
for high rates of evaporative cooling; with surface body
temperatures below the ambient level in warm climates
and during summers there would be no conduction or
convection heat loss. These high evaporative heat losses
would pose excessive risks of desiccation and restrict the
radiation of homeotherms in arid climates. Maximization
of evaporative cooling would also require sparse insula-
tion, which would restrict diffusion in cold environments.
And lower temperatures would reduce the efficiency of
diation and the necessity of higher metabolic outputs,
whereas raising the furry species in temperatures colder
than their normal habitat results in the growth of thicker
insulation. The plumage of birds is about one-third more
insulative than the pelage of similarly sized mammals.
Light-colored species reflect over 50% of visible wave-
lengths, dark-colored ones may reject just 15%, but
actual effects on radiation balance are not that simple.
White coats scatter incoming radiation both away from
the animal and toward its skin. Individual hairs are actu-
ally colorless, appearing white only as their central core
scatters incoming radiation; the hair shaft may also con-
duct scattered radiation to the skin. The white pelts of
Arctic mammals absorb UV radiation, a desirable prop-
erty in cold environments. In contrast, black covers ab-
sorb incoming radiation, and the amount of solar energy
reaching the skin may be higher in a lighter-colored
animal.
This apparent paradox is most pronounced in birds in
windy environments, and it explains why dark colors of
such desert species as corvids or vultures and light colors
of arctic ptarmigans are not maladaptations: with high
winds heat load on erected black plumage is below that
on white feathers (Wolf and Walsberg 2000). Insulation
quality (measured as conductance) of furs is a linear func-
tion of their thickness. Naturally, density of hair also mat-
ters, as does the degradation of the insulative layer by
sweat or rain. At normal temperatures water conducts
24 times more than the air, and the fur of a polar bear
swimming in ice water will lose virtually all of its out-
standing insulation capacity. Consequently, all entirely
marine mammals will have an especially difficult task of
thermoregulating in cold waters. The smallest cetaceans
(harbor and porpoise dolphins) are at the greatest disad-
vantage as they try to maintain temperature differences of
