The Origin and Evolution of Mammals - Moodle

conducted the experiment of wrapping a lizard in a
fur coat and noting whether it maintained a higher
body temperature. It did not, which suggested to
him that evolving insulation alone does not immediately
confer thermoregulatory ability upon an
ectotherm, insofar as such a crude experiment can
be trusted to show anything significant.
The aerobic capacity hypothesis
A number of arguments have been cited against
any thermoregulation-first hypothesis. Bennett and
Ruben (1979; Bennett 1991) asserted that the level of
thermoregulatory benefit gained from a small initial
increase in BMR would be far outweighed by
the increased cost of food collection. This would be
especially manifest if the organism in question was
a competent ectothermic regulator, for modern reptiles
are able to maintain their diurnal body temperature
to a remarkable degree of accuracy by
entirely behavioural means, and therefore incurring
a very low metabolic cost. It can also be pointed out
that endothermic regulation only works adequately
if the accessory structures and processes such as
insulation and finely variable conductivity have
also evolved, which is unlikely to have been the case
in the initial stages of increase in BMR. These criticisms
are part of the argument in favour of the
hypothesis that the initial selection pressure was for
increased sustainable levels of aerobic activity.
Bennett and Ruben (1979) noted the roughly constant
relationship between an animal’s resting
metabolic rate and the maximum level of sustainable
aerobic metabolism. Without a clear understanding
of why this physiological relationship
should hold, they nevertheless proposed that an
increased level of sustainable aerobic metabolism
was the primary advantage of evolving an increased
metabolic rate, and that any thermoregulatory
advantage was initially insignificant. This was
because even a small increase in BMR would confer
an immediate advantage in terms of an incremental
increase in the level of sustainable activity, and
there would have been no need for the additional
adaptations such as insulation that are necessary
for thermoregulation to have evolved at the same
time. Support for the hypothesis is also claimed
from the fossil record (Ruben 1995). Carrier (1987)
argued that the change in therapsids to a more erect
EVOLUTION OF MAMMALIAN BIOLOGY 125
gait was to permit an increase in ventilation rates
by decoupling the locomotory from the breathing
functions of the axial skeleton. Hillenius (1994)
claimed that maxillo-turbinals, whose function in
living mammals is warming and humidifying
inspired air, and reducing evaporative water loss
from expired air, are first found in therocephalian
therapsids. Both these claims point to the existence
of endothermy in relatively large primitive therapsids,
supposedly with a sufficient degree of inertial
homeothermy for metabolically driven thermoregulation
not to have been necessary. However, neither
of them stands up well to scrutiny, as discussed
shortly in the context of the timing of the appearance
of endothermy.
Testing the aerobic capacity hypothesis experimentally
consists of investigating the relationship
between the resting and the maximum sustainable
metabolic rates. The hypothesis implies that natural
selection acted upon the maximum aerobic rate but
that there is a physiological linkage between that
and the resting rate such that the latter also necessarily
increased as a correlation. Hayes and Garland
(1995) reviewed studies comparing the resting and
maximum aerobic rates between species and found
some supported while others failed to support the
correlation. The ratio between the two individual
species, although typically around 10–15, can be as
low as six and as high as 35. More unexpectedly,
and hard to explain, different tissues are primarily
involved in the two respective rates (Ruben 1995).
Of the total resting heat production in mammals,
around 70% is generated by the visceral organs:
liver, kidneys, intestine, brain, etc., which amount
to only about 8% of body weight. Compared to an
ectotherm, the increase is achieved by a combination
of a twofold increase in the volume of tissue,
and a twofold increase in the density of mitochondria
within it. In contrast, practically all the increased aerobic
metabolism during exercise is due to activity
in the skeletal and cardiac muscle tissue, which
occupies around 45% of the total body mass. Again
compared to an ectotherm, there is a relatively
higher percentage of tissue and of mitochondrial
density, but in this case the individual mitochondria
have a larger membrane surface and therefore
a higher level of metabolic activity as well. Given
this difference in the source of heat related to,