Environmental Engineering Reference
In-Depth Information
mous variation of expenditures within each animal class.
Finally, it is not surprising that an attempt to express the
effect of temperature on metabolic rate by a single uni-
versal equation has not met with uncritical acceptance.
Gillooly et al. (2001) used a century-old Arrhenius
equation relating temperature, reaction rate, and the
equilibrium constant to formulate their expression for
the variation of metabolic rate (B) of all organisms:
some interesting transitory adaptations) to cope with
nonoptimal temperatures: ectothermy and endothermy
(or poikilothermy and homeothermy).
All higher ectotherms—arthropods, fish, amphibians,
and reptiles—have very low specific BMRs and poor
body insulation. Their thermoregulation is behavioral;
their goal is to raise body temperatures into the preferred
range (@10
C for salamanders, 35
C-40
C for helio-
thermic lizards) by seeking optimal microenvironments.
In terrestrial ectotherms, body size is a critical determi-
nant of sustainable body temperatures and hence of their
behavior and niches (Stevenson 1985). Large ectotherms
with high heat capacity warm up slowly, but their maxi-
mum body temperature ranges are narrow (3.5
C-5.5
C
in giant tortoises, and 2
C would be expected for an
ectothermic 3 t dinosaur). They are able to maintain a
larger gradient between body and ambient temperature,
and their considerable thermal inertia allows for longer
periods of activity.
In contrast, small insects unable to raise their tempera-
ture above the ambient level control it by moving
around; they can heat up quickly or retreat rapidly to
protected microenvironments. Basking is very important
and uses both direct absorption and reflected radiation.
Butterflies, the most attractive basking ectotherms, either
use spread wings to capture the radiation and transfer it to
the body or hold their wings in a wide variety of angles
above the body (Kingsolver 1985). Thin wings are poor
conductors, so species living at higher altitudes have
darker wing bases to maximize absorption. Baskers with
very narrow angles can convey radiation to the body
from a much larger area than more open baskers.
Remarkably, many flying insects can be endothermic
during the short periods preceding takeoff, when they
warm up the flight muscles by shivering. Winter moths,
BAM
3
=
4
e
E
=
kT
,
where E is the activation energy, k is Boltzmann's con-
stant, and T temperature in K. But while temperature
governs metabolism because of its effect on the rate of
biochemical reactions, such a simple mechanistic explana-
tion ignores the fact that heterotrophic metabolism is
influenced by a large number of interacting physiological
processes (Clarke 2004). As a result, evolutionary adapta-
tions have resulted in reaction rates that are relatively
independent of the temperature at which an organism
habitually metabolizes, and responses to temperature
cannot be reliably predicted from first principles
(Hochachka and Somero 2002; Clarke and Fraser 2004).
4.2 Ectotherms and Endotherms
The most obvious consequence of high specific BMRs in
tiny organisms is the limit on the size of the smallest
warm-blooded animals. Creatures lighter than shrews
and hummingbirds would have to feed incessantly to
compensate for rapid heat losses. In contrast, low specific
BMRs make it considerably easier for larger creatures to
cope with prolonged environmental stresses because they
can draw on accumulated fat reserves for relatively long
periods of time. Metabolic imperatives thus dictate two
very different grand strategies used by heterotophs (with
