The Origin and Evolution of Mammals - Moodle

cold stress, the part that usually fails first is the central
nervous system, and this is the structure most
obviously associated with a highly complex network
of interacting elements.
The second function of endothermy is the very
high rate of sustainable aerobic activity that is possible.
For reasons not readily explained, there is a
roughly constant ratio between the basal or resting
metabolic rate and the maximum sustainable aerobic
metabolic rate of all vertebrates. The latter is
typically 10–15 times the former, although there
are exceptions. If the BMR of a typical ectotherm
such as a lizard is taken as one unit of energy per
unit time, then its maximum sustainable aerobic
metabolic rate is about 13 units. For a typical mammal
of the same body weight, the figures are a BMR
of about 7 units and an expected maximum sustainable
aerobic rate of 91 units, a huge increase in the
latter property over the ectotherm. The mechanism
behind the increase primarily involves a far larger
number of mitochondria in the skeletal muscle, coupled
with a greatly enhanced oxygen delivery system
to them. The enhanced aerobic capacity does
not affect the total maximum power output, or the
top running speed attainable, because ectotherms
can achieve similar values by anaerobic metabolism.
However, ectotherms can maintain this level
of exercise for no more than a very few minutes,
after which time activity has to cease as the oxygen
debt is repaid and lactic acid removed, a process
that can take some hours to complete. In contrast,
the maximum power output, and therefore maximum
speed that can be sustained indefinitely, or at
least until the body’s food reserves are exhausted, is
far greater in mammals than reptiles. The biological
functions of this enhanced endurance are fairly
obvious: food capture, predator avoidance, size of
territory, vagility, and energy available for courtship
all spring immediately to mind.
There are several hypotheses about the evolutionary
origin of endothermy in mammals (Bennett 1991;
Hayes and Garland 1995; Ruben 1995). They are all
predicated on the assumption that endothermy in
living mammals is too complex a character to have
evolved in a single step. Therefore, one of the two
major functions must have evolved first, whilst the
other evolved secondarily later on. Consequently,
there are two categories of hypothesis currently on
EVOLUTION OF MAMMALIAN BIOLOGY 123
offer. One regards thermoregulation as the initial
function, the other that high aerobic capacity was
the first to evolve. Even within this dichotomy,
there is more than one version of each, differing in
what they take to be the initial selection pressure.
Miniaturisation: the physiological
thermoregulation hypothesis
McNab (1978) pointed out that there was a general
evolution in body size through the mammal-like
reptiles to the first mammals. From the ancestral
synapsids, which are assumed to have been small,
there was a tendency to increase in size through the
pelycosaurs and primitive therapsids. There then
followed a trend of decreasing body size through
the cynodonts, which culminated in the extremely
small first mammals. His proposal was that temperature
physiology tracked these changes in body
size. The ancestral starting point was a small tetrapod
with lizard-like ectothermic temperature physiology.
The large-bodied therapsids had acquired
the ability to maintain a relatively constant body
temperature, not by increasing the metabolic rate,
but by virtue of their large mass. The low surface
area to volume ratio of a large organism reduces the
relative rate of heat exchange with the environment
and therefore of temperature change—a process
termed inertial homeothermy. He suggested that
some degree of insulation of the skin might also
have existed, since this would enhance the effect. In
the course of size reduction in the lineage that led
ultimately to the ancestral mammal, the process
would be expected to have reversed as the surface
area to volume ratio increased, and the animals
gradually to have returned to ectothermy. However,
McNab argued, the benefits of the relatively constant
body temperature having once been gained,
they could not be secondarily lost. Instead, as body
size reduced selection for continuance of the constant
body temperature ensured the evolution of
increasing relative metabolic rate and effective insulation
in the form of fur. Thus metabolically driven
thermoregulation gradually took over from heat
inertial endothermy as the means of maintaining
the constancy of the body temperature.
Attractive as it is, direct evidence in favour of
McNab’s miniaturisation hypothesis is limited. The
hypothesis predicts that the origin of morphological