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Re: Great in the air, not so good underwater

----- Original Message ----- From: "Michael Habib" <mhabib5@jhmi.edu>
To: <dinosaur@usc.edu>
Sent: Sunday, December 10, 2006 12:39 AM
Subject: Re: Great in the air, not so good underwater

All good names, and it is indeed a long-running and interesting hypothesis. I am skeptical, however, that such a difference in partial pressure of O2 is the best explanation for the large size of pterosaurs. This is the case for several reasons:

1) It is not clear how much of a performance increase a 12-15% difference in O2 partial pressure would really have for large soaring vertebrates (see below). I think the advantage may have been exaggerated to some extent.

I do too. There is no doubt that it would provide an advantage, but the large pterosaurs would be able to launch and fly just fine in today's atmosphere. And, I'm somewhat skeptical of the quantitative differences proposed for atmospheres at other times (I know they were different, but am not sure how closely the differences can be quantified at this time.

2) If the atmosphere did, indeed, make a significant difference, then I would expect to see noticeable and measurable differences in planform between modern birds and advanced Cretaceous forms. So far, I have not seen anything to suggest that the rules were different for Cretaceous birds


3) Adaptive differences in launch actually seem more parsimonious, in a sense, than evoking atmospheric effects. It seems odd to me to assume that large-bodied pterosaurs, despite being anatomically capable of using forelimb-assisted launch, used a less efficient launch system instead (less efficient for them, in any case). By contrast, simply invoking the same launch cycle used by some living quadrapedal flyers solves the problem in one fell swoop (more or less).

The suggested launch cycle is reminiscent, but not the same. the large pterosaurs would go out at a much flatter angle than the more vertical forelimb driven launch used by some bats. This would also lead one to predict relatively longer, more robust hindlimbs in the more robust species of pterosaurs. And perhaps, a relatively longer fourth metacarpal as well.

Very cool experiment. I do note, however, that 1.5-2 atmospheres is a lot of pressure (relatively speaking).

A lot.

only 1.2 atmospheres reduced the starvation rate to ~10%. My
(tentative) conclusion was that, due to unsteady-state effects, the
response of aerodynamic performance to flight medium density was
non-linear, at least in small fliers.

Seems like a decent (if somewhat speculative) conclusion to me. I suspect, however, that the major difference you saw is likely to be limited to small-bodied flyers. In particular, I agree that the relationship is non-linear, and I suspect the curve plateaus at higher Re. Just like you pointed out, the unsteady-state dynamics are probably playing a huge role in your result.

Yes, at low Reynolds numbers, and at the beat frequency expected for small fliers.

As small fliers are more
vulnerable to viscous effects than large fliers, it is my opinion that
birds may receive even larger relative benefits from small density
increases than insects.

I'm not sure about this.

I'm sure, and disagree with Don..

Granted, since birds fly in a more inertial-dominated flow regime, it seems like density changes could matter quite a bit. However, I suspect that the small insects are more sensitive to Re changes.


Testing the load-carrying capacity of
pigeon-sized birds at increased pressures might clarify the
relationship between size, flight medium density and performance in flapping fliers,
just as variable-density wind tunnels were once used to manipulate the Re
number when designing airplane wings. Astonishingly (to me), this has apparently never been done, although it appears straightforward.

That does seem like a good idea.

It does to me as well, though I see flight at reduced pressures as being a functional equivilent. But most bird flight wind tunnels aren't really designed in a way that makes them conducive to pressurization. As a aside, Colin Pennycuick has done some good work on the potential effect of both density and gravity on vertebrate flight.

I suspect that while atmospheric changes would have had an effect on ancient flyers, most of those changes would be compensated for with relatively minor changes in planform and/or kinematics. The difference in maximum observed size between pterosaurs and birds is well over 2x however, and that seems like a larger difference than can be accounted for with a 12% O2 partial pressure jump.

I note in passing that birds appear to mostly fly by continuous aerobic power (except for launch and some landing styles), while pterosaurs tend to fly with a technique that is compatible with mostly anaerobic power (flap gliding with short periods of burst power while using the non-flapping periods to restock on oxygen). I think some of the longer necked species of pterosaur may have had long enough tracheas (what's the correct plural for trachea?) to make the dead air column a factor in flight style. The flap gliding style is of course available to shorter necked animals as well, as long as their planforms are appropriate.

In any case, very interesting stuff (thanks for sharing the information on your insect flight experiments!),

Yes, I appreciated that as well, particularly the impact of changes upon survival rate.

and there is obviously plenty of work still to be done. I suspect that large pterosaurs would have been perfectly viable in today's conditions, but we'll see what further data shows.

Quantitatively, they would have.

All the best,