Re: Pterosaur size

[MICHAEL HABIB <habib@jhmi.edu>]

> "Why is wingstroke amplitude a problem?  The quetz shoulder that I 
> have lying 
> here on my desk doesn't have a wingstroke amplitude problem.  
> Available 
> amplitude is far greater than articulated stripwise flapping 
> calculations 
> indicate would be needed to maintain level flight in no-lift conditions."

> On take-off or other flight close to the ground. As in the wingtip 
> running into the ground. Even blue herons have to take a pretty good 
> jump when taking off to get ground clearance. A 11m wingspan implies a 
> wing of 5m, right? Even a few degrees down from horizontal and the 
> wing is on the ground. I'm sure you've got that all worked out in 
> theory, but from the ecological perspective I continue to be skeptical.

One important aspect of the pterosaurian forelimb dominated launch cycle is 
that the wing is deployed relatively late in the launch cycle. Another effect 
is that the animal gets up to high speed before the wing is deployed, meaning 
that it is within the flow regime for a high speed gait by the time the wings 
deploy.  As such, required wing amplitude is much less than I think you're 
imagining.  The wingtips would have plenty of clearance; and the wings would 
not be particularly close to vertical during the flight stroke.  Herons are not 
launching like a pterosaur.  They require a strong leap because 1) they need to 
use this leap to reach their launch speed window and 2) herons deploy the wings 
early in the launch cycle.  These are both common traits of avian launch, and 
are the reasons why birds often need to take a powerful leap to launch, 
complete with a very distinct hind limb preload phase.  Note that even small 
herons take a strong leap, and they are obviously not mass l
imited.  Large herons are not loading limited, either, actually. They have low 
wing loadings and a highly vertical launch sequence (though the launch speed is 
low).  Skepticism is healthy, but I'm not sure I follow what you mean here by 
an ecological perspective.  The ecology is important, but quantifying launch 
abilities is more of a mechanics problem.  The selection for launch 
techniqueswould be the ecological part. --MH


> You are changing the subject. What does a 15lb goose flying in 50 mph 
> winds at 59% have to do with a 110lb bird with a full crop trying to 
> make a getaway from a carcass on flat ground on a damp, still day? Or 
> a Rueppels vulture at 35000 ft, for that matter? Koford (1966) says 
> modern condors have limited success without a downhill slope and 
> headwind.

Yes, and frigatebirds can barely takeoff without gravity assistance and/or 
favorable winds becuase they cannot run.  However, birds larger than either 
frigates or condors can take off in a still wind from the ground without 
difficulty, because they launch differently.  The comparisons must be 
controlled for mechanics.  The minimum requirements for volancy vary by 
morphology; there is not global rule.  The examples of high altitude flight 
demonstrate that a thinned atmosphere makes very little difference to 
vertebrate flyers.  Even at 15 lbs, the effects of atmospheric density are 
easily compensated for by individuals, to a such a subtle extent that the 
compensation is not evident or easily observable.  The relative effect will 
probably taper more at larger sizes and faster flight speeds.  As you already 
pointed out, the effects of altered Re via density change is non-linear; but 
it's likely non-linear in the opposite direction that you seem to imply. --MH

> 

> 
> "That occurs at an elevation of about 5500 feet, where the density 
> ratio is 0.85 (a 15% reduction from sea 
> level).  Does that mean that a modern bird suited for flight near sea 
> level 
> would be ground bound at 5500' MSL ?  Does that hold in practice?  
> Swans (a 
> group that includes the heaviest individual bird known to flap by 
> means of 
> continuous flapping) spend a lot of time near sea level, but certainly 
> 
> aren't limited to elevations less than 5500 MSL."
> 
> Sorry. I don't see the relevance to Quetz, Argent, et al. Or any 
> argument I have ever put forward.

I see the relevance.  The point is that there is not an observed change in 
performance in modern flying birds across very different atmospheric densities. 
 You suggested previously that a 15% atmospheric difference would be 
significant for large vertebrate flyers.  It turns out that such differences to 
not have a significant impact.  It is important to note that Quetzalcoatlus, 
being larger and faster than the anserids in question, would probably be 
affected even less.  It is also important to note that Quetzalcoatlus is not at 
the maximum size for pterosaurs; it's launch performance was almost certainly 
quite stunning, really, rather than borderline.  I get the impression (though I 
cannot say for sure), that you are thinking of the largest observed flying 
vertebrates as examples of the size limits for their clades.  In fact, none of 
the mega-volants known are likely to have been at a mechanical size limit, with 
the possible exception of Argentavis.  All others presumably fe
ll at a selection size limit; ie. further increase in size was not particularly 
advantagous.  Quetz., despite being impressively large, was not only below the 
size maximum, but it was still within what we might consider a "high 
performance" range for it's particular morphotype.  --MH


> Fact. Plot maximal volants in timeslice fashion. Stick to birds if you 
> want. The trend is there, relevance is debatable. Although the 
> correlation on the chart I did 15 years ago is -.75 (past to present), 
> not -.95. Sorry about that. If it were only birds, or only volants I 
> would say " random chance"....

I'd be careful here; I'm sure that trend looks very distinct, but I don't think 
it's real.  For example, sticking to birds as you suggest, the largest volants 
form about a plateau from the Eocene through the late Miocene.  There's a 
little peak in the Late Miocene (for Argentavis), then plateaus again until the 
late Pliocene (loss of pseudodontorns).  At that point the max volant size 
drops to essentially the modern observed.  There is a little trend signal 
there, but it's mostly driven by that last sequence (Miocene, Pliocene, 
Modern), which is actually two extinction events, and not really a trend.  
That's without correcting for clade.  If you take into account the fact that 
there are several bird clades involved, and correct for phylogeny (and thus 
clade-specific effects), then the trend will very likely evaporate altogether.  
I could actually crunch that phylogeny-corrected trend this weekend, if you 
want, though the power is going to be very low with such a small number
 of samples (same problem with the raw data). --MH

I would also caution against use of the term "maximal volants".  The species in 
question are not maxima; they are the largest observed volant species of their 
time intervals.  Most are not actually near the quantitative mechanical maxima 
for their particular morphotype. --MH


Cheers,

--Mike H.