The Origin and Evolution of Mammals - Moodle

124 THE ORIGIN AND EVOLUTION OF MAMMALS
characters associated with increased metabolic
rates should be correlated with decreasing body
size. The earliest cynodonts such as Procynosuchus,
Dvinia, and Thrinaxodon confirm this since, along
with their relatively small size, they possess a secondary
palate and possibly a diaphragm. However,
looked at in more detail the trend in size through
the mammal-like reptiles is not at all clear-cut, and
there are large, medium, and small members at
every stage. Many pelycosaurs are large-bodied,
but even amongst later pelycosaurs, varanopseids,
and some caseids have body lengths less than 1 m.
There are small dicynodonts and therocephalians
that could not possibly have been inertial homeotherms,
existing alongside large forms. While the
earliest cynodonts are indeed relatively small, there
are plenty of large-bodied genera throughout
the Triassic. Even the highly advanced, extremely
mammal-like tritylodontids include species with
skull lengths as much as 25 cm, which are far from
miniature synapsids. In fact, the only part of the
record that clearly supports the hypothesis is that
of the earliest mammals, which were very small,
shrew-sized animals.
Arguments against the miniaturisation hypothesis
are mainly those against any version of the
thermoregulation-first hypothesis. Bennett et al.
(2000) attempted to test it experimentally. They
increased the metabolic rate of a resting lizard by
introducing a large meal directly into its stomach,
and then measuring whether the body temperature
increased and the rate of cooling declined, both of
which would be predicted by the hypothesis that
endothermy is caused by a simple increase in
metabolic rate. In fact they found that although the
metabolic rate was elevated by three to four times,
there was only a very small (0.5 ºC), increase in
temperature and no significant decrease in rate of
cooling. Of course, like all such experiments, the
results must be treated with considerable reservation
since so many of the possible differences between
the experimental circumstances and the real evolutionary
event cannot be controlled for.
Nocturnalisation: the ecological
thermoregulation hypothesis
Crompton et al. (1978) offered a different version of
the thermoregulation-first view, which stressed the
significance of maintaining a constant body temperature
for nocturnal activity, rather than for general
biological organisation. Having observed that
certain primitive nocturnal mammals such as
monotremes, tenrecs, and hedgehogs have significantly
lower metabolic rates and body temperatures
than other similar-sized mammals, they
proposed that these forms represent the primitive
condition for mammals. From this perspective,
endothermy arose initially as an adaptation for
entering a nocturnal, insectivorous niche, as the
first mammal is believed to have done. All that was
necessary was to evolve an insulating layer to
reduce the rate of heat loss. At the low ambient temperatures
of night-time, a relatively low constant
body temperature of around 30 ºC would have created
an adequate temperature gradient for thermoregulation,
which in turn would have allowed
the animal to remain active. This relatively low
level of body temperature could be maintained by
means of the still relatively low metabolic rate.
Only with a subsequent shift to diurnal activity in
various later lineages of mammals was a higher
body temperature needed in order to maintain a
large enough temperature gradient with the environment.
Therefore, only at this later stage did the
metabolic rate have to evolve the typical modern
mammalian level.
It has since become appreciated that the various
living mammals with low BMR are unrelated and
therefore that the condition probably evolved convergently
as a specialisation, although this does not
actually refute the hypothesis. Tenrecs, for example,
could nevertheless be regarded as suitable analogues
for the ancestral mammalian mode of temperature
physiology, although even they are now
known to have metabolic rates some four times
those of comparable-sized ectotherms and not the
very low, reptilian-type energetics originally attributed
to them (Ruben 1995). The main argument
against the ecological thermoregulation hypothesis
is that, contrary to McNab’s miniaturisation hypothesis,
it requires no increase in metabolic rate prior to
the origin of the mammals themselves. This leaves
unexplained the evidence pointing towards elevated
metabolic rates in the cynodonts, namely the
secondary palate, possible diaphragm, and increased
masticatory ability. Many years ago Cowles (1958)