[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index][Subject Index][Author Index]

(desert iguana) nasal respiratory water recovery



Although I'm trying to phase out of getting into the middle of these
things, I think the following should enlighten the discussion a bit.
It was written before I received Guy's other message.  I've forwarded
that message to Terry also, so we can probably expect another
response.  Since I am trying to extricate myself, though, Guy I'd
appreciate it if you (or anyone else) would forward copies of your
messages to Terry (or whomever else might be appropriate).  I've
suggested to Terry that he send his response to Guy and have Guy
forward it to the list.  Since he's not currently subscribed, Terry
can't send messages here directly.

Back to the party -- MR:

_____________________________________________
SUBJECT: Leahy's recent claim that lizard (desert iguana) nasal respiratory 
water recovery functions as well as RTs in many endotherms.

Schmidt-Neilsen (1972) states that "The principle that underlies the
cooling of exhaled air is the same [in reptiles] as that in mammals
and birds, but the efficiency of this exhange [in reptiles] is much
lower, and hence the water [and heat] savings as well" (see
Schmidt-Neilsen,1972, p.21).

It IS true that the % respiratory water loss per unit air breathed
(=%RWL/cc air breathed) in desert iguanas apparently overlaps that in
humans.  However, such a comparison is virtually meaningless.  Desert
iguanas, and the dimensions of their nasal passages, are orders of
magnitude smaller than humans (desert iguana mass <100g compared to
70,000g in humans). In fact, % RWL/cc air breathed in the desert
iguana is actually far greater than in the cactus wren, an endotherm
of similar mass. This relationship holds true over a broad range of
masses, i.e., at any comparable mass, avian and mammalian %RWL are
significantly less than in reptiles.  Moreover at masses greater than
about 5kg, %RWL/cc air breathed in reptiles approximates that in
MOUTH-BREATHING bird and mammals (Geist, in press).

Why these relationships?  Reduction of %RWL/cc air breathed in nasal
passages of ANY air-breather is a function of the nasal passage
temperature and the ratio of nasal passage (cavum nasi proprium)
surface area to nasal passage volume (=NPSA/NPVol). Obviously, the
higher the ratio of NPSA/NPVol, the greater the contact (and
opportunity for heat/water exchange rate) between nasal passage
linings and inhaled/exhaled air.  Due to mass and surface area/volume
relationships, ANY very small air breather must, by geometric
necessity, exhibit high ratios of NPSA/NPVol. Thus, it's hardly
surprising that a 50g desert iguana would have nasal passages
sufficiently narrow to possess some countercurrent heat/water exchange
function. Given these relationships, wrens and desert iguanas would
undoubtedly exhibit similar %RWL/cc air breathed except that wrens,
have, by virtue of their RTs, possess greatly INCREASED relative nasal
passage surface area (NPSA/NPVol), and heat/water recovery capacity of
their nasal passages.  Given these relationships, it is remarkable
that even the reduced RTs located in the relatively broad (and
foreshortened) nasal passages of humans function to cut down on RWL
about as efficiently as the extremely narrow nasal passages of tiny
lizards. Of course, most endotherms with masses similar to humans
possess relatively more elaborate RTs and, consequently, exhibit
%RWL/cc air breathed far SMALLER than even those in tiny desert
iguanas.

All of this has profound implications regarding the necessity for
dinosaurs to have possessed RTs had they been endothermic.  Inasmuch
as surface area/volume increases exponentially (SA=M^0.67), it is not
surprising that, all other factors remaining constant, RTs in larger
endotherms tend to function less efficiently than do those in small
endotherms. Similarly, %RWL/cc air breathed MAXES OUT (at >85%) in
reptiles greater than a few kg in mass (Geist, in press).  For these
reasons, it is logical that most large endotherms possess
well-developed RTs.  Without them, given the high absolute volumes of
air breathed by endotherms in the field (about 17X that of ectotherms
of equivalent mass), TOTAL (whole-body) daily water balance would be
chronically out of whack (by about 30-50%). The implications here for
dinosaurs are obvious-- had dinosaurs been endothermic and lacked RTs,
their large mass and large diameter nasal passages (compared to small
lizards) would have virtually precluded any respiratory tract
mechanism whereby they might have cut down on respiratory water/heat
loss.-- a process which seems a necessity in virtually all extant
endotherms.

REFERENCE

Schmidt-Neilsen, K.  1972.  How Animals Work.  Cambridge University
Press.  (I would reccommend that anyone wanting to get a better
understanding of respiration, metabolic rates, scaling, etc. in
animals.  The book is written for both physiologists and
non-physiologists alike).

Terry D. Jones
Nick Geist

TDJ
:-{)
-------------------------------------------------------------------------
    Terry D. Jones                             Voice:  541/737-6120     
    Oregon State University              Fax:      541/737-0501          
    Dept. of Zoology                         JONEST@bcc.orst.edu
    3029 Cordley Hall
    Corvallis, OR  97331-2914
-------------------------------------------------------------------------