The Origin and Evolution of Mammals - Moodle

132 THE ORIGIN AND EVOLUTION OF MAMMALS
their internal environment, which is to say, that
have the highest degree of homeostatic ability. The
most profound challenge to the potential of homeostasis
was the shift in habitat from water on to
land. If the internal environment is to be maintained
constant in dry air, then large water and
chemical gradients between the animal’s tissues and
the external environment have to be maintained by
suitable regulatory mechanisms of differential intake
and excretion. On top of this, the daily temperature
fluctuation in air is huge in the absence of the high
heat capacity and consequent buffering effect of
water, which challenges the temperature regulation
mechanism to maintain the internal body temperature
within the much narrower limits of viability. The
very physical patency of an organism on land, and its
ability to control its own movement is challenged by
the absence of the upthrust of water. Aside from these
three major problems of life on land there are several
lesser ones such as the absence of suction as a means
of food intake, and of external water as a medium for
gamete transfer and embryo development.
Different grades of tetrapods have adapted to
terrestrial existence to different extents. Modern
amphibians, with their permeable skin and small
size, are unable to regulate their internal environments
physiologically to any great extent against
water, chemical, or temperature gradients, and are
therefore restricted to humid, nocturnal habitats.
Thus they may be described as avoiders in that they
avoid places where such gradients would be high.
Living reptiles by and large have solved the water
gradient problem by a strategy of reducing loss.
They have also solved the temperature problem by
a regulatory ability that depends on differential
uptake of environmental heat during the daytime.
However at night, this process of ectothermic thermoregulation
is not possible and therefore inactivity
is imposed upon the animal.
The mammals (and equally the birds) have
achieved the highest levels of regulation of the
internal environment of all, and are therefore
the tetrapods most highly adapted to the habitat of
dry land. They are able to regulate the chemical composition
and the temperature of the body in the face
of higher gradients. Chemoregulation is achieved
mainly by the ability of the kidney tubule to create
hyperosmotic urine by means of the mechanism of
the loop of Henle. By concentrating the urine, the
mammal can afford to utilise the soluble urea as its
prime nitrogen-excreting molecule without incurring
an unacceptably high rate of water loss. The liquid
ultrafiltrate entering the proximal end of the kidney
tubule passes down the full length of the tubule and
as it does so the concentration within it of water, the
various ions, urea, pH, etc. is adjusted by a balance
between reabsorbtion and secretion. Under fine
hormonal control, the level of each of these
constituents is adjusted to that which is necessary on
a moment-by-moment basis to maintain the plasma
concentration at the optimal composition.
Mammalian endothermic temperature regulation
works by an analogous mechanism. An excess of
heat is generated and the rate at which it flows out
of the body is finely, and almost instantaneously,
adjusted by varying the conductivity of the skin so
as to keep the internal temperature constant. For
the mechanism to operate, there has to be a gradient
from a higher body temperature to a lower
ambient temperature, so that the heat flow is continuous
and therefore continuously adjustable.
Thus, there has to be a high enough permanent
metabolic rate to raise the body temperature to a
thermostat setting above that of the environment.
In case, on occasion, the gradient is temporarily lost
or reversed because of hot conditions, the emergency
expedient of evaporation by panting or sweating
has to be available which is effective for a while but
inconvenient and short term due to the need to
replace the lost water. Conversely, if, under cold
conditions, the temperature gradient becomes too
large for the basic level of heat production to maintain,
there are several ways in which extra heat can
be generated by elevating the metabolic rate, again
on a temporary and expensive basis, such as by shivering,
exercise, and non-shivering thermogenesis.
But regulation is metabolically expensive.
Maintaining chemical gradients requires the energetic
process of active transport of molecules at the
cellular level. Maintaining the temperature gradient
requires a high level of aerobic respiratory activity by
the mitochondria. Together these dictate the need for
the 6–10 times greater BMR of endotherms over
ectotherms. In order to achieve this, the rate of gas
exchange and the rate of food assimilation need to
increase proportionately. Efficiency of food detection,