can of worms

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

As I said, I didn't want to open a can of worms, so this will be my last
post on this subject for a while.  Others may feel free to have the last word.

As body size increases, and the same shape is maintained, the surface
area/volume ratio decreases.  This makes it inevitable that both heat
dissipation and heat uptake will be slower.  In any environment, an
elephant loses heat more slowly than a mouse.  (Yes, I realize they're not
the same shape.)

Greg Paul was correct in saying that the air temperature must be higher
than the animal's body temperature for heat to flow into it.  But it is not
necessary for heat to flow in in order for an endotherm to overheat.  If
this were true, then no human could ever overheat in the shade as long as
the air temperature were <98.6 degrees.  If the internally-generated heat
cannot dissipate quickly enough, the animal will overheat.  This is why
large mammals tend to be naked, and why they spend so much time in the
water, especially in hot climates.  None of this, of course, precludes
dinosaur endothermy.

It is true that big mammals in cold climates have thick fur.  But this is
because they are continuously exposed to very cold temperatures.  Even a
large body, if exposed, will eventually cool down under such conditions.
But it takes a while, especially with an endotherm, because their metabolic
rate increases as the temperature drops, generating more internal heat.

It is possible for a large endotherm to overheat even in a cool
environment.  It may take a while, but it can happen.  Yet sauropods are,
to me, the LEAST likely dinosaurs to have ectothermic metabolisms.  How can
such large animals avoid overheating?  Because they have a body shape that
increases their surface area dramatically.  Of all the giant animals that
have ever lived, sauropods have by far the greatest surface area/volume
ratio.  An animal the size of a large sauropod, shaped like an elephant or
a rhinoceros, is an impossibility.  Any animal of that size with an erect
posture is an endotherm, and any terrestrial endotherm that large must have
an elongate body shape.

Now that I have probably provoked everyone into a frenzy, I will shut up on
this topic.  For a while.

Best regards,

Dave