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Re: RTs (lengthy)



> From:          "Van and Kathy Smith" <vksmith@dwx.com>
> Agreed.  My point was made mainly to counter Mr. Jones's -- if his
> argument has merit then perhaps the most important place to 
> measure would be the narrowest part of the main air passageway.  I'm
> still very anxious to see the results of his latest study -- Mr. Jones
> replied to my original message saying that it looks like there _is_ a 
> correspondence between tracheal cross sections and energetics 
> among modern animals.

There is a linear relationship between alveolar surface area and O2 
uptake in mammals, reptiles, birds (passerine and non-passerine), and 
fishes.  A separate curve for each.  Also comparing surface area with 
size(weight).  I haven't looked for data relating cross sectional 
diameter of trachea toVO2, but the trachea MUST be large 
enough to do its job.  There is no gas exchange there.  It's only 
conduit as far as gas exchange goes.  I would be very surprised for 
someone not to find that trachea size is related to VO2.  In the 
same vein, nasal passages cannot interfere with air flow.  It will 
evolve to do its job. 

> I believe Mr. Jones argues that without RTs, an animal on an expensive
> energy budget would lose so much H20 and heat as to render that animal
> incapable of moderating either.  I suspect that RTs exist in both 
> modern mammals and birds because, for small forms (and all modern 
> mammals and birds evolved from very small ancestors), they are indeed 
> important for heat and moisture retention (ventilation surface area/body
> volume
> as well as ventilation rates both become higher and higher for smaller and 
> smaller animals).

I would agree that it is important for smaller animals with high 
ventilation rates that live in dry environments to recapture as 
much water as possible. However, even todays small desert 
animals must use behavioral mechanisms to avoid excessive water loss. 
They are generally noctural.  Burrows have cooler daytime temps 
than their core temperature and so is the night environment when 
they are active.  This allows them to avoid large losses of water 
even with their RT's.  Also, they have other mechanisms to 
compensate for ventilatory water loss.  Deriving water from 
processing food directly and metabolically allows some the luxury of 
not even needing to take in water at all.  Large animals have more of 
a problem with heat than water so employ different behavioral 
mechanisms.  

So, not having a turbinate that is effective in warming and 
humidifying inhaled air and recapturing heat via moisture on 
exhalation does not mean the animal was not endothermic.  A RT is 
merely one mechanism to recapture heat and water.  There are salt 
glands that help in some reptiles.  The entire upper airway does the 
same thing that the RT does.  The RT just adds to the efficiency.  
And if dinosaurs had bird like respiratory apparatus, they would have 
benefited from higher tidal volumes and lower ventilation rates.  
Long necks equal long trachea which allows more recapture of 
water(and heat if necessary).  It also increases dead space in 
proportion with the length.  This increases tidal volumes and 
decreases ventilation rates thereby also decreasing water losses. 

The argument here is that RT=endothermy and that may be right.  
However the fact may be that RT=better endothermy. And I point out 
that it is uncontroverted that dinosaurs had turbinates.  How do we 
know how efficient or inefficient they were in recapturing heat and 
water.  We cannot presume that they were just sensory.  Nor can we 
presume that they could not use other mechanisms to overcome heat and 
water problems if they had inefficient or absent respiratory 
turbinates.

Do you know the embryological origin of bird lungs and air sacs?  Are 
RT's embryologically derived the same in birds and mammals?

Michael Teuton
803-732-2327 Phone
803-749-6191 Fax