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RTs



I know it has been awhile since the thread on RTs petered out, but there
are a couple of very important issues that did not get resolved.

Mr. Terry Jones argued in December about why nasal passage size would be a
direct indication of an animal's energetic capacity.  Since it has been
such a long time, I've included his comments here:

----------
> From: Terry D. Jones <jonest@ava.bcc.orst.edu>
> To: dinosaur@usc.edu
> Subject: Re: [GSP1954@aol.com: MORE DINOSAUR ENERGETICS]
> Date: Tuesday, December 17, 1996 2:52 PM
> 
> If an animal has high metabolism (high O2 consumption) and therefore high
> lung ventilation rates, the nasal passage proper must be large enough to
> accomodate increased airflow.   In order to house the RTs (which as has
been
> pointed out many times, >99% of endotherms possess), without increasing
> resistance to airflow, the nasal passage of endotherms must be larger
still.
> For the same reason ectotherms have narrower nasal passages since they
have
> lower metabolism and correspondingly low lung ventilation rates.  This is
> not a result of our work, but it is a law of physics (Poiseuille's law:
> flow rate is proportional to radius^4  -- in other words,  decreasing the
> radius of a tube by 1/2 increases the resistance to flow 16x).  <...snip>
 It may
> appear that some endotherms have relativeley narrow air passages (or that
> some ectotherms have large air passages), but when compared with body
mass,
> the relationship holds true--there is an approximately 4-fold gap between
> the nasal x.s. area of modern endo- and ectotherms when compared to body
> mass.  I'm waiting eagerly for the results of Greg's work to come out in
> print...I hope there is an explanation included for the apparent
> contradiction of Poiseuille's law.
>

I, Mr. Paul, and several other members of this list took issue with his
arguments, but the best posting on this matter came from Guatam Majumdar. 
Mr. Majumdar brought up several important facts, but one in particular
stood out:

Gautam Majumdar wrote:

>Another point (again in humans, I am sorry - this is the only animal I am
>familiar with) - the narrowest part of the respiratory tract is not the
nasal
>passages, but the trachea.   

Obviously, Mr. Majumdar's argument renders much of Mr. Jones' statement
irrelevant if other animals are similar to humans -- and I believe that
nearly all are.  About a month later Mr. Jones seemed to concede that this
is at least an issue and promised to have the results of a study into the
comparative tracheal cross sections in about a month from his posting (I'm
sorry, but somehow I lost this posting from Mr. Jones and I pray I have not
distorted the details of his letter  -- Mr. Jones, please correct me if I'm
wrong).  A comparison study of the narrowest points in the respiratory
passages of various animals would be most relevant to Mr. Jones' original
argument.

Another issue important to Mr. Jones' study was the problem extreme of
water loss possible in large  animals lacking RTs.

I believe that the reason why at least some dinosaurs lacked RTs is that
they used evaporative cooling through their complex air-sac laden
respiratory system as their primary method of dumping heat.

Dinosaurs were big and needed some type of heat dissipation apparatus for
times when the animal was very active, and/or the ambient temperature was
high.  Sweating, our method of evaporative cooling, becomes less efficient
as the mass of the animal increases (surface area/volume ratios decrease
rapidly).  As I've mentioned previously on this list, I suspect that
evaporative cooling via dinosaurs' respiratory systems might have been
their primary method of cooling.  Several of you on this list stated that
the air taken in would soon reach 100% humidity and become incapable of
producing any more evaporation, but if RTs are indeed organs necessary to
moderate water loss in the animals that possess them, then surely animals
without them would have no problem losing enough water to stay cool.

Mr. Jones, if you have any results, I'm sure we would all be very
interested to see them.  If indeed there is any correspondence between
tracheal cross-sections and metabolism, I suspect that dinosaurs will fall
on the line with other warm-blooded animals.

Van Smith

vksmith@ipa.net

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