The Origin and Evolution of Mammals - Moodle

122 THE ORIGIN AND EVOLUTION OF MAMMALS
a considerable array of structures and processes,
including a regulating system for the high metabolic
rate, variable conductivity of the skin by use of hair
and cutaneous capillaries, neurological mechanisms
for bringing about panting and shivering, and so on.
Before addressing the question of the origin of
endothermy, it will be helpful to review the mechanisms
and functions associated with it in living
mammals. There are four physiological features
associated with, and therefore effectively defining,
endothermic temperature physiology:
● High basal or resting metabolic rate: normally
between 6 and 10 times the basal metabolic rate
(BMR) of an ectotherm of the same body mass.
● Elevated body temperature: somewhere between
28 and 40 ºC.
● Constant body temperature: regulated to within
about 2 ºC of the thermostat setting.
● High aerobic scope: the maximum aerobically
sustainable metabolic rate of tetrapods is generally
around 10–15 times the BMR, so it is up to 10 times
higher than an ectotherm of the same body mass.
The high BMR has no biological function per se, as
is clear from several observations. To begin with,
the BMR is related to body mass by the well-known
Kleiber or ‘mouse to elephant curve’, which shows
that the total metabolic rate varies as body mass to
the power of 0.75. Thus the metabolic rate per unit
mass of the elephant is only a few percent of the
mouse, yet all the other aspects of the temperature
physiology of these two mammals are similar.
There are also differences in BMR between closely
related species that are attributable to such factors
as habitat or food type. For example in temperate,
semi-arid, and desert species of hedgehog, the BMR
varies in the ratio, respectively, of about 1 : 0.75 : 0.5.
The relatively high body temperature of endotherms
does not have any direct function either, as indicated
for example by the many ectothermic reptiles
that are active during the daytime at a body temperature
equal to, or often higher than the endotherms.
Therefore, the first two features of endothermy
listed above must be interpreted as the mechanisms
causing the second two features: the constant body
temperature and the high level of maximum sustainable
aerobic activity are the two respective
biologically significant functions.
The constancy of the body temperature is maintained
to the very high level of accuracy by controlling
the rate of heat loss from the body surface on
a moment-by-moment basis. To achieve this, it is
first necessary to maintain a temperature gradient
between the body and the environment, and the
role of the high metabolic rate is to raise the body
temperature to a value that will normally be higher
than the ambient temperature. It is also essential, or
at least vastly less wasteful, to have insulation of
the body surface. The third necessity is for the conductance
of the body surface to be instantly variable,
so that the rate of heat loss can increase or decrease
rapidly and at low metabolic cost, and this is
achieved by the familiar thermoregulatory devices
of vasodilation–vasoconstriction of the skin capillaries,
piloerection–pilodepression of the hairs, and
postural changes exposing different areas of the
skin to the environment. A fourth requirement is to
have what might be called emergency devices in
place. If the maximum possible conductance of the
skin is still too low to allow a high enough rate of
heat loss under conditions of high ambient temperature,
or high levels of activity, evaporative cooling
by panting or sweating can temporarily relieve the
problem. Conversely, if under cold conditions the
minimum level of conductance is still too high to
prevent excessive heat loss, the temporary measure
available is an increase of metabolic heat production
by shivering, or by switching on non-shivering
thermogenesis in certain specialised tissues.
The adaptive significance of a constant body
temperature is hard to describe succinctly because
it so permeates the total biological organisation of a
mammal. In one respect, this mode of thermoregulation
expands the niche so that activity can continue
under a wider range of ambient temperatures, both
cooler and hotter. In a second respect, accurate thermoregulation
increases the possible complexity of the
organism because the various processes of enzymatic
reaction, diffusion, muscle contraction, and so on
take place at a constant, and therefore predictable
rate. Therefore, a greater number of elemental biochemical,
physiological, and physical processes can
be reliably linked together into the more complex
functional networks that underlie more complex
levels of biological activity. It is significant that
when a mammal is subjected to excessive heat or