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RE: MY TRIP TO THE ZOO



> GSP1954@aol.com wrote:

>Speaking of which, I also got to see a 4 year old Komodo monitor close up. A
>most impressive beast in some ways. But it moved with that somewhat slow
>motion speed typical of reptiles. It also showed that these big reptiles grow
>very slowly, being no bigger than a medium sized dog after 4 years. It is
>also being observed that the growth of the captive oras is highly dependent
>upon food consumption. Captive reptiles can also be kept constantly warm,
>further boosting growth rates. As a result captive reptiles - especially
>those grown on farms where rapid weight gain is a priority - can grow at
>least twice as fast as their wild counterparts, even those that live in
>tropical locations with abundant food. This exposes a serious problem with
>the work of Owerkowicz on growth of reptiles in his laboratory. Just because
>reptiles can grow rapidly and deposit mammal-like bone when fed lots of food,
>kept warm, and exercised by humans does not mean that the same can be
>achieved under natural conditions. In the wild finding large amounts of food,
>keeping warm, and exercising require the expenditure of large amounts of
>energy both on an immediate basis, and in the long term energy budget. This
>is probably why only birds and mammals can grow rapidly. As for dinosaurs, at
>least some seem to have grown more rapidly than living reptiles, and none
l>ived at Harvard, so it is probable that they did not have reptilian
>energetics.

It's good to see Greg taking the trouble to look seriously at what modern
ectotherms
can and can't achieve - many people seem to have a stereotyped, negative view
of
lizards etc.

However, this is an example of how we can interpret the same data in different
ways
according to our viewpoints.   I don't think anybody is putting forward the ora
as a
serious model for dinosaur physiology/metabolism.  The significance of oras as
I see
it is

a)  They are a good bit larger than nearly all their modern relatives.
b)  They have a greater aerobic scope (with associated modifications to lungs &
      circulation) than their relatives.
c)  A degree of thermal stability associated with size has been demonstrated
for
      oras.

This seems to indicate that it is possible that as an ectotherm evolves towards
larger
size, selection can favour increased aerobic activity associated with thermal
stability
without associated endothermy.   This is consistent with the inertial
homeothermy
model (not proof, not even great evidence, just not inconsistent).

Unless you take the view that oras have gone as far in this direction as it is
possible to
go then it is reasonable to infer that larger, active, thermally-stable
ectotherms can
exist.    To refute this model you need to say why such an ectotherm could not
stroll
at 4km/h; it is not sufficient to say that oras can only manage 2km/h.

With respect to growth rates - the increase in growth rate of domestically
reared
lizards & snakes can be pretty dramatic - increases of 4 to 6 times the
'normal'
growth rate.   The main factor associated with such growth is temperature
stability.
Not constant heat, but the provision of a suitable thermal gradiant so the
animal can
maintain it's optimum body temperature with minimal effort.   It is not
necessary to
provide a large increase in food supply.  Accelarated growth occurs on a diet
that
wouldn't keep a mammal alive for a week (due, obviously to ectothermy).    What
you have done in this situation of course is to create that mythical beast, the
ectothermic homeotherm.   It's true this is not natural, but the important
point is that
ectothermy per se is not what restricts growth rate at a physiological level.
  One
would expect a (theoretical) ectothermic homeotherm to have a high growth rate.

I'm not arguing strongly here in favour of any particular model, but I think
this is what
Mickey R.  was talking about referring to 'evidence' and data which are
compatible
with either of the 2 main metabolic models, and  therefore proof of neither...

Tony
tonyc@foe.co.uk