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Re: PLEUROCOELOUS VERTEBRAE IN SAUROPODS



Jeffrey Martz wrote:

>Greg Paul wrote:

>> 1 - Giant animals are not in particular danger of overheating, they
>> store it in the daytime and dump it at night.
>> 2 - Large animals cannot use their deep respiratory tract to shed
>> heat.
>
>     Then exactly how DO they dump heat at night?  This seems to imply 
>that the night will be so much cooler than the day that an 80 ton animal 
>will have as much heat seep through its total surface area during the night 
>as it picked up during the day.      

It seems to me that we need to keep in mind that the assumed goal here
is to maintain an optimum body temperature. The differences between the
ambient high and low and the optimum body temp are probably not equal.
That is, the drop of the ambient temp BELOW the optimum may well be
much more than the rising of the ambient temp ABOVE the optimum.
        Suppose we say that our hypothetical ectothermic sauropod (that's
gonna get me flamed, I think!) is comfortable at, say, 35-38 C, (just off the 
top of my head; anybody else's guess is better than mine). If the ambient
temperature at night drops to, say, 30 C, then the differential between that
and the low end of the preferred range is 5 C. If the temperature rises to,
say, 40 C in the heat of the day, then the differential between that and the
high end of the range is 2 C. So the heat gain, relative to the preferred body
temperature, would be slower than the heat loss at night.
        Of course, this assumes many things: first, that my off-the-wall temp-
erature estimates are in the ballpark, or at least usable for guesswork; second,
that the animal is either ectothermic or uses mass homeothermy (my guess!);
third, that the animal just stands there and takes no measures for thermal
regulation, like standing in the sun/shade, rolling in water or mud, or curling
up to minimize heat loss. AND we have to keep in mind the tons of digesting
plant matter Jeff (I think) mentioned, and the climatic variations with
geography and season...it gets pretty complicated...: )

        In my last comments on this, I said:

>Seems to me the airsacs would only help with cooling if they had a way to
>either radiate heat outward (not inside vertebrae!) or a means of exchanging
>air, i.e., being an extension of the respiratory system WHERE THE AIR IS
>CIRCULATED.

I then put forward the idea that they were for structural lightening. Greg Paul
responded that they were not part of the cooling system (I didn't say they
were!), and stated that they probably weren't for lightening, since large
mammals didn't have them. He explained that they were part of the respir-
atory system, allowing air to be drawn completely through the lungs for
maximum respiratory efficiency -- like those little dinosaurs, birds.
        To which I say: Large mammals (I assume he's referring to guys
like Indricotherium, the biggest land mammal I know of) may not have
vertebral air sacs. They didn't have sails on their backs, either. Nor
did they have really long necks or tails, or scales. The point is that
mammals have often approached problems differently from reptiles (or
dinosaurs, if you will!), and need not have the same set of
solutions. While Indricotherium's estimated weight of 30 tons is in
the same ballpark as Apatosaurus, it wasn't built the same: short
neck, and a negligible tail. And it sure doesn't approach the mass of
the big brachiosaurids.
        Finally, any time you hollow out a bone and put in an air sac,
without weakening the structure to a serious degree, you have
lightened the animal considerably. Even if it serves another purpose
(and I knew about the respiratory idea, though I wasn't sure the
sauropods did it that way), the lightening would be significant.

Wayne Anderson