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More on respiratory turbinates (long)
In his reply to my comments on RT Ruben says "it is highly disingenious of
Paul to assert that the water retention capacity of RT is not generally
accepted". Disingenuous? I don't think so, because in fact I never said any
such thing. What I did say was that the experimental protocal used by
Hillenius did not prove that RT function as water saving devices, and that
the simple RT of some birds and mammals do seem to function about as well as
more complex RT. Never said nothing about general acceptance, nor that the RT
water conservation hypothesis has been falsified. I do think the hypothesis
is not as strong as is often asserted.
After all, Varene et al 1986 Clynical Physiol 6:405 found that people who
have simple RT lose only about 5-15% more water when breathing through the
mouth than through the nose (Hillenius 1992/94 does not cite this paper).
Since these differentials are much less than observed in mammals with complex
RT, and because respiratory water loss in humans is similar to other mammals,
perhaps this shows that animals can use alternate means of saving water after
all. Fact is that at this time no one really nose (my sincere apologies for
such a bad yet pun). We can find out whether RT really save water by testing
kids which occasionally lack them, see if they are dehydrating faster than
kids with fully developed ones, and then see if both are losing water faster
than in similar sized mammals with complex RT. Any young volunteers out
there? (perhaps the budding dinosaurologist Rachel C. is willing.)
Ruben says that sea birds without RT "have well developed salt glands that
enable them to drink quantities of water sufficient to compensate for
increased rates of respiratory water loss". Maybe, but do we really know if
they drink so much water, or even if they have increased rates of water loss?
What if they conserve water as well as other birds?
Ruben asserts that RT are good metabolic indicators because over 99% of
endotherms have them, and no ectotherm does. The numbers are probably true,
but Ruben did not bother to mention that about 99% of big brained animals
have RT, while no small brained animal does. This is very important because
the evidence that RT serve as brain coolers in at least some mammals is
arguably better than the evidence that they serve as water savers (in some
mammals the RT blood vessels are directly connected to a sophisticated
cerebral counter-current heat exchanger). So how can we tell whether RT
evolved to cool brains, to save water, or both (as well as for other
functions suggested by various researchers)? At this point, it remains
possible that small brained endotherms (possibly therapsids, pterosaurs, and
dinosaurs) will not have RT. Ruben (and Hillenius) have so far failed to
discuss this matter in detail.
Nor does Ruben seem consistent when he uses statistics. That 99% of mammals
and birds have RT, but no reptile does, does imply that the presence of RT
suggests high metabolic rates (but even this correlation is weakened by the
brain cooling complication). However, this does not mean that the absence of
RT are also good evidence for low metabolic rates, because there are notable
exceptions (and because it remains at least possible that water can be saved
by means other than RT). Take fur and feathers. 90-something percent of
endotherms have them, no ectotherm does, a pattern very similar to RT. Yet
while Ruben promotes RT as the best metabolic indicator, he rejects
insulation as a metabolic indicator. One might suspect this is because Ruben
wants dinos which supposedly lack RT to be reptiles in the metabolic sense,
while he wants Archaeopteryx that has feathers to a reptile too. I try to be
objective by assigning assign a consistent diagnostic value to insulation and
RT - i.e. the presence of soft insulation (and RT)
is evidence for high metabolic rates because they are found only in birds
and mammals (and becasue insulation reduces the rate at which environmental
heat can be absorbed, while saving body heat). The absence of insulation (and
RT) is consistent with any metabolic level because some endotherms lack these
features (but very small endotherms probably need insulation).
Ruben says that sirenians and elephants do have RT. Maybe so, but Scott 1954
(same year G Scott P was born) stated that in "dugongs and elephants, the
maxillary turbinate is absent or poorly developed". Sikes 1971 states that
"maxillo- and naso-turbinals are absent" in African elephants. Sure enough,
in Matthes 1934 fig 751 shows the nasal tract of an Asian elephant with no
maxilloturbinal (though a simple nasoturbinal is indicated). It is possible
that elephants and sirineans occasionally have a little RT, and that is what
Negus 1958 is citing. Or perhaps these researchers are wrong, and evidence to
the contrary can be provided.
Ruben's argument that RT are the ancestoral solution to respiratory water
loss remains speculative. They have not been identified in Mesozoic birds
(nor are they ever likely to because avian RT are usually made of cartilage).
The supposed RT ridges in therapsids are questionable. Similar naso-maxillary
ridges appear to be present in some dicynodonts (fig. 6C,D in Barry 1967),
and even in some mammals in which they do not support a turbinate. The only
way to firmly establish the presence of RT in therapsids is by finding
ossified examples. Until then it is all interesting speculation. Same goes
for dinosaurs. Ruben commented about the "apparent total absence (of RT) in
all dinosaurs" as a reason for thinking they had low metabolic rates. This
statement has little meaning because it is a classic example of relying on
unreliable negative evidence; unrelable because it is possible that many
dinosaurs had cartilage RT that can never be found (unfortunately Ruben did
not comment on Maryanska's identification of ossified RT in ankylosaurs,
which has yet to be confirmed or denied). An obvious and basic requirement
for a good metabolic indicator is that it be readily preserved and measured
in the fossil record. In this respect the delicate and often cartilage RT are
a nightmare. No one has shown how we will ever detect cartilage RT in
fossils. The RT-metabolic hypothesis is built on the sand of many assumptions
Ruben is quite right that whales (and presumably elephants) have alternative
means of saving respiratory water. And there is the rub. Perhaps various
dinosaurs also had alternate means of saving water. How will we ever know?
And how can we A) show that a sauropod with its bizarre basal cavity
(nostrils may be behind the internal nares) did not have cartilage RT, and B)
even if it did not have RT, why could it not have a high metabolic rate when
there are elephants, whales and kids without them either?
Please understand that I am not saying RT have no value as metabolic
indicators. I am saying that claims that they are the first and best
indicator of metabolic rates are very exaggerated and premature, because of
their poor preservability, their poorly understood & multiple functions, and
absence in some mammals and birds.
Someone asked how I know all this stuff. Well, I read a lot. Very
importantly, my coworker Guy Leahy is very good at ferreting out obscure
references which he sends my way. Guy is not on-line yet, if and when he does
hook up things might become even more interesting around these here parts.
I see in Ruben 1995 a ref for a Bennett paper in Adv. Veter. Sci. Comp. Med
38 listed as 1994 in press. I presume this is the one that has the data on
exercise capacity in big moniters. Anyone have a complete ref?