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.
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
Agreed.
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).
Very cool experiment. I do note, however, that 1.5-2 atmospheres is a lot of pressure (relatively speaking).
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.
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.
Yes.
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.
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.
In any case, very interesting stuff (thanks for sharing the information on your insect flight experiments!),
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, JimC