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Re: RTs (lengthy)
> 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:
> > 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.
O2 consumption(MO2) is O2 used at the cellular level. This is
balanced by ventilation (VO2). Maximum ventilatory capacity (MVC) is
a function of the entire ventilatory apparatus. Not just trachea
size. I do not know if cross sectional measurements of tracheae
across classes are going to tell us anything. MVC under exercise or
demand is going to be dependent on a number of factors within a
species and even for different classes of animals. MVC can be
increased in a number of species with training. It is not directly
related to turbinate size as far as I know.
I also don't think it is fair to compare restrictions in ventilation
via turbinates. Most animals breathe thru the nose and mouth with
exertion. Only the maxilloturbinates are debated as being
diagnostic of endothermy. I have not researched the embryology of the
turbinates in mammals or birds yet, but I see this as
possibly important in trying to determine some answers about
I'm still a little confused about RT's and endothermy. The fact that
mammals and birds have more highly developed maxilloturbinals than
reptiles and dinosaurs would not seem to be totally convincing
evidence of endothermy. I think it is difficult to isolate structures
in an animal without looking at the whole and say this is what makes
this animal homeothermic.
First, the ventilation systems in reptiles , birds and mammals are
very different. Even among reptiles. Among extant animals both birds
and mammals are homeothermic and have higher O2 consumption rates(Vo2)
than ectotherms. However bird and mammalian ventilation are very
different. Birds have several large inflatable air sacs which act as
bellows and where gas exchange is neglibible as well as small constant
volume lungs. The lungs are attached to the ribs and are rib
and breast plate driven. Mammals have expandable lungs diaphragm and
thorax driven. Birds have parallel flow, higher ratio of exhange
surface area to lung volume, and thinner blood-air barrier than
mammals. Additonally, birds have a lower ventilation rate and higher
tidal volume than mammals. So...
Are advanced maxilloturbinals necessary for high O2 consumption or
has the multifunctional turbinates evolved independently in birds and
mammals as an adjunct to high ventilation rates as well as their
other functions? Are they primarily for warming inflowing air,
trapping particles, fighting disease, catching some water
vapor on exhalation and not allowing high ventilation rates to
dessicate the mucosa? If the ancesters of mammals had turbinates
would it surprise us that most mammals had them?
> 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.
That is an interesting thought. The air sacs are divereticuli off the
primary and secondary bronchi. While I don't think they would use
evaporative cooling there is a way for birds (?dinosaurs) to use
their ventilation system to cool efficiently. Birds are unique in
being able to shunt ambient air (?cool) thru the trachea into the
posterior air sacs and back out without vascular exchange. This
allows temperature regulation without hypocapnia.
Do you think that dinosaurs had the ability to shunt warmer core
blood to the skin and cooler extremities as an additional way to cool
- From: "Van and Kathy Smith" <email@example.com>