my working hypothesis for body size

["Colette H. Adams" <cadams@hh.gpz.org>]

The tendency to invoke competition as an explanation for trends in
evolution is very strong, and I feel very tempted myself.  The problems I
have with it are: (1) It is difficult to see how large taxonomic groups are
in some sort of macro-competition with each other.  Presumably this implies
a competition between traits, such as furriness vs. scaliness, etc.  I see
nothing in population or community ecology that would indicate that an
insectivorous mammal is inherently more or less likely to survive than an
insectivorous dinosaur of similar size and habitat preferences.  (2) The
supposed competition between two groups does not take into account the
presence of other taxa.  Why would mammals in particular be competing with
dinosaurs in particular?  Why wouldn't mammals be more in competition with
lizards, birds, or insects for that matter?  I have seen numerous
references to such "lifeways" as "large terrestrial predator."  I assume
these refer to niches.  There has been a great deal of controversy in
community ecology about niches, which I won't go into, but the basic
problem is that it is not at all clear why a large animal should
necessarily compete with another large animal.  In a given ecosystem you
may have two species of herbivores that consume the lion's share of the
vegetation.  One may be an insect, the other a large mammal.  But even here
it is by no means clear that one will affect the population of the other.

So competition is a difficult problem.  Even where it is obvious that two
species are feeding on the same things, it is often very difficult to
demonstrate that one affects the population of the other.  Yet we have
these trends in the fossil record that seem to suggest that large taxonomic
groups do affect the trends in one another.  Here is my working hypothesis
for the body size patterns in dinosaurs and mammals during the Mesozoic.
Mammals at this time were by and large insectivores and herbivores.  Today
almost all insectivorus mammals are small (actually most mammals are
smaller than a rabbit).  Insects are small, so insect predators are small.
The exceptions are species that feed on highly social insects, which were
absent during most of the Mesozoic.  There is nothing to particularly
select for large body size.  The herbivores might get more efficient by
increasing their body size, but probably, as with rodents today, their
small size gave them the advantage of being able to scoot into holes and
escape predators.  Dinosaurs, on the other hand, begin as fairly small
predators in the Triassic, but almost as soon as herbivorous species
evolve, dinosaur body size begins to increase.  I believe this was the
beginning of an evolutionary "arms race", with dinosaur herbivores trying
to achieve an escape from their predators.  This continued through the
Jurassic, reaching a crescendo with some of the largest animals ever to
walk on land, some of which may well have been pushing the limits of how
large a terrestrial animal can possibly get.  Dinosaur predator size
tracked the herbivores.  The Cretaceous saw this battle ease off, as
grassland-type habitats increased and herbivorous dinosaurs adapted to
them.  But a large herbivore is an efficient herbivore, and herbivorous
dinosaurs stayed large.  So did their predators.

If you look at dinosaur body size trends, as I have, I believe you will
find that the average herbivorous dinosaur in the late Triassic was about
the size of a deer.  By the early Jurassic this had increased to the size
of a bison, and by late Jurassic the AVERAGE herbivorous dinosaur was over
2 tons!  In the Cretaceous this declines, although herbivorous dinosaurs
remain quite large.  Predatory dinosaurs show exactly the same trend, only
the numbers are different.  Just as the prey of large carnivorous mammals
today is primarily other mammals, do predaceous dinosaurs were feeding
mainly on other dinosaurs.

Anyways, that is my happy little story to explain the difference in body
size between dinosaurs and mammals during the Mesozoic.  Predator-prey
interactions, not competition.  I believe that many of the appearances of
"dramatic" characteristics, whether large body size, elaborate
ornamentation, or bizarre morphology of one sort or another, are the result
of "runaway" selection processes.  

Okay, Dave, so explain why mammals seem to increase in body size
dramatically after dinosaurs go extinct.  

Stay tuned.

Best regards,

Dave