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Re: RTs (lengthy)
> From: "Van and Kathy Smith" <email@example.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
> 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
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
Do you know the embryological origin of bird lungs and air sacs? Are
RT's embryologically derived the same in birds and mammals?