The Origin and Evolution of Mammals - Moodle

of feasibility that a taxon of endothermic, primarily
nocturnal animals like the mammals had a significant
enough ecological overlap with large to very
large, probably diurnal animals like the dinosaurs.
The presumed differences between the two groups
in metabolic physiology and mode of food preparation
were surely quite as great as between many
co-existing groups of organisms sharing comparable
body size: mammals and reptiles; carrion birds
and mammals; different groups of ungulate mammals.
Even accepting the possibility of competitive
exclusion to the extent that it could have prevented
mammals and dinosaurs from evolving extensively
overlapping size ranges, it leaves unanswered
the question of why there were no mammals of
medium-size. The ancestral size of dinosaurs was
large, and though a few modest sized taxa evolved,
the vast majority were relatively large animals. The
presence of dinosaurs might perhaps account
for the absence of Mesozoic mammals above, say,
100 kg body weight, but it certainly does not offer a
plausible explanation for the complete absence of the
equivalents of foxes, leopards, beavers, mediumsized
antelopes, goats, and such like, animals of the
order of 1–100 kg body weight. Perhaps the main
competitive excluders of medium-sized mammals
were juvenile dinosaurs, and that they were active at
night as well as day, and that they had the same order
of efficiency of feeding mechanisms as mammals.
This is not a convincing scenario.
The physiological constraints hypothesis. The alternative
hypothesis for the universally small body size
of Mesozoic mammals is that their biological design
was such that larger body size would not have been
viable. The idea that the particular body plan of an
organism can constrain its size is commonplace (e.g.
Calder 1984). The surface area to volume considerations
that limit the size of a unicellular organism
dependent on diffusion, the muscular power output
to body mass relationship that prevents flying birds
from exceeding 10–20 kg, and many more examples
are well appreciated.
In the case of size in Mesozoic mammals, there is
one obvious possible source of constraint on large
body size. Bakker (1971) suggested that the problem
lay in the absence of well-developed evaporative
ability for cooling. Therefore, the surface area to
volume ratio had to remain high to allow adequate
THE MESOZOIC MAMMALS 185
heat loss during high levels of muscle activity. It is
also perhaps implicit in Crompton et al.’s (1978)
hypothesis that the origin of endothermy in mammals
was an adaptation to nocturnal conditions.
At the relatively low body temperature of around
30 °C assumed by this hypothesis, a tendency to
overheat during high levels of activity may have
been the main thermoregulatory problem in the
early forms. It is, however, difficult to accept such a
simple thermoregulatory explanation (Lillegraven
1979a). For one thing, many modern mammals do
not rely to any significant extent on sweat glands
for evaporative cooling, but on the much simpler
device of panting. For another, it does not answer
the question of why, post-Mesozoic, larger size suddenly
became possible, unless by the coincidental
evolution of enhanced cooling ability in several
separate mammalian lineages.
Comparable objections can be made to any other
suggestion that a simple, single biological attribute
caused the size restriction. For example, larger
mammals must have a relatively more massive
skeleton and muscles to support and move the body,
a longer lifespan, a lower reproductive rate, and so
on. However, in every such case, the invoked physical
or biological relationship is neither a bar to large
body size in modern mammals, nor offers an explanation
for the sudden post-Mesozoic release from
the constraint.
It may be more fruitful to consider the Mesozoic
mammals in the wider context of their terrestrial
ecosystem as a whole. Compared to an equal sized
ectothermic tetrapod, an endothermic mammal
requires around 10 times as much energy intake per
unit time, and therefore requires a sufficiently nutritious
and reliable source of food to provide this
amount. Consider four possible basic diets. Insects
and other small invertebrates offer an adequate diet,
but only for mammals of small body size. The discrete
occurrence and diminutive size of each individual
food item set a limit on the absolute rate of food
intake for any individual organism. Within this limit,
however, little in the way of extreme specialisation is
required. The teeth are not subjected to excessive
wear, digestion is simple, and at small body size a
secretive, nocturnal, or crepuscular way of life is possible.
This is the ancestral mode of life of mammals.
A second possible diet consists of high-energy
plant material, seeds, fruits, and storage organs.