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Re: [Re: Insulation does not = "Warm-blooded"] (long)
> This does lead me to a side question: How do birds (especially large
> active flyers like geese) dump excess heat? If I remember correctly, birds
> don't sweat.
Correct. As HP Larry Febo just wrote, the air sacs are the primary cooling
mechanism of birds. Diverticula reach into all large muscles and dissipate
heat directly from there into the air that is exhaled. Pat Shipman's book
"Taking Wing" mentions that the big sternal keel of birds has the primary
function to anchor the individual bundles of the pectoralis &
supracoracoideus to prevent the air sac diverticula between them from
collapsing during muscle contraction, and offers the hypothesis that
pterosaurs did the same, while bats, not having air sacs, use their
well-vascularized and large patagia for dumping heat via the blood. It seems
to be in dispute (I don't know the primary literature) whether there is
evaporative cooling in the air sacs; anyway, exhaled water is regained in
the nose -- the big difference to sweating where all the water is GONE --,
while hot air is exhaled.
> > > So if your a fast living ectotherm covered in heat retaining feathers
> > > and your flitting about, then all the heat generated by those muscles
> > > (be they ecto or endo)would get trapped in the body, effectively
> > > removing the need to warm up ectothermically. The only problem then
> > > would be in keeping from heat exhaustion.
> > Indeed -- imagine an isolated ectotherm that has chilled out trying to
> > warm up in the sun. Will take quite some time, probably too much to let
> > insulation be an advantage.
> I think you might have missed my point. I'm saying that with the heat
> trapped on the body, while it would allow for a warm body, it could also
> lead to heat stroke if not used carefully.
I did misunderstand that (I was too lazy to look "heat exhaustion" up in the
dictionary and guessed its meaning wrongly :-] ). Air sacs are surely a way
around the problem, and there is enough evidence for their presence in
Thanks a lot for the information on thermophysiology -- I didn't know the
leatherback turtle is endothermic.
> > > About bones... some people think that enantiornithines were
> > > ectothermic because of the LAGs (lines of arrested growth) in their
> > > bones. This conclusion has turned out to be nonsense, at least in
> > > hadrosaurs.
> > > I'm sure this has been discussed ad nauseam onlist; if you want, I can
> > > dig up some papers on this subject.
> > Well, I dont' know if Rob would want them, but I certainly would. If
> > anythingall these bone histology studies have shown is that they are
> > useless in determining endo/ectothermy (or poiki/homeothermy,
> > brady/tachymetabolism; whichever is more appropriate).
Wait a minute -- if you think they are useless in determining
thermophysiology, which they are indeed to a large extent, then why do think
enantiornithines were bradymetabolic?
Refs -- Off the top of my head, there is an article in Padian & Currie's
Dinosaur Encyclopedia that mentions that there are different numbers of LAGs
in different bones of the SAME individual, which means that whatever LAGs
are, they are not seasonal, and that crocs bred under constant temperature
exhibit cyclical growth, so this rhythm is genetic rather than dependant on
the weather. And there is
John R. Horner, Armand de Ricqlès & Kevin Padian: Long bone histology of the
hadrosaurid dinosaur *Maiasaura peeblesorum*: Growth dynamics and physiology
based on an ontogenetic series of skeletal elements, JVP 20(1): 115 -- 129
"ABSTRACT -- Ontogenetic changes in the bone histology of *Maiasaura
peeblesorum* are revealed by six relatively distinct but gradational growth
stages: early and late nestling, early and late juvenile, sub-adult, and
adult. These stages are distinguished not only by relative size but by
changes in the histological patterns of bones at each stage [shown in color
illustrations!]. In general, the earliest stages are marked by spongy bone
matrix with large vascular canals. Through growth, the cortical bone
differentiates into fibro-lamellar tissue that tends to become more
regularly layered in the outer cortex. By the sub-adult stage, [...] (LAGs)
begin to appear regularly. Resorption lines and substantial Haversian
substitution in many long bones also begin to appear at this stage, and the
external cortex has a lamellar-zonal structure in some bones that indicates
imminent cessation of growth.
Judging by the rates of similar bone tissue in living amniotes, and by
the number and placement of LAGs, these patterns suggest that young
*Maiasaura* nestlings grew at very high rates, and at high and moderately
high rates during later nestling, juvenile, and sub-adult stages, slowing to
low and very low growth rates in adults (7 -- 9 m total length). The nesting
period would have lasted one to two months, late juvenile size (3.5 meters)
would have been reached in one or two years, and adult size in six to eight
years, depending on the basis for extrapolating bone growth rates.
The histological tissues, patterns, and inferred growth rates of the
bones of *Maiasaura* are completely different from those of living non-avian
reptiles, generally similar to those of most other dinosaurs and pterosaurs
for which data are available, and much like those of extant birds and
mammals. No living reptiles (except birds) grow to adult size at these
rates, nor do they show these histological patterns. We conclude that
*Maiasaura* did not grow at all like living non-avian reptiles, which cannot
be considered informative models for most aspects of dinosaurian growth (or
physiology, to the extent that growth rates reflect metabolism). The use of
[...] (LAGs) to infer dinosaurian physiology has never been tested and is
not supported by independent lines of evidence; their use in calculating age
is also more complex than previously suggested and should not be based on
>From the conclusions:
"4. The significance of [...] (LAGs) in dinosaurs must be regarded as
entirely inconclusive at this time. Although some independent evidence is
consistent with annual rates of deposition of LAGs in some dinosaurs at some
stages of growth, LAGs by themselves cannot be said to determine with
confidence an animal's ontogenetic status, metabolic regime, or
> > > Jura - probably the only listmember who advocates bradymetabolic
> > > dinosaurs and enantiornithines.
> > I'll dig up HP Paul and Leahy's Dinofest 1994 paper...
> Terramegathermy in the time of the titans right?
> In a perfect world, all papers would be online :)