Re: Pterosaur size

[don ohmes <d_ohmes@yahoo.com>]

Thanks for taking my point about the implications of long wings and ground 
clearance on launch.  I have taken the liberty of numbering your comments and 
giving my replies the appropriate number. This is to prevent "infinite 
interspersal" and the concomitant confusion about what who said. It won't work, 
but it is worth a try. Anyone who weighs, please do the same? 

PLEASE DON'T WRITE BETWEEN THE LINES... }: D

Don

----- Original Message ----
From: MICHAEL HABIB <habib@jhmi.edu>
To: d_ohmes@yahoo.com
Cc: dinosaur@usc.edu
Sent: Thursday, December 14, 2006 2:20:02 PM
Subject: Re: Pterosaur size


> "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.

1). 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

1). What you say about the ptero launch technique seems to make sense
if the image I have taken from context is correct. I am sure that I am
behind on my reading. Do you have a reference that delineates this
launch cycle in detail? That would help a lot. All the descriptions I've read 
are real vague.            No, herons are nowhere near any limit. Yet they use 
most of their potential stroke amplitude till they get going. For A. mag 
launch, Campbell relied on a climate regime w/ daily wind patterns (10 mys
worth, IIRC) for Argentavis launch/landing and evolutionary matrix. Sort of a 
modified box kite scenario. If I remember rightly... corrections received w/ 
gratitude.            As to my skepticism relative to mega-volants and standard 
atmosphere, it is intuitional and rural in origin. I was aware of extinct 
mega-volants from a very early age, and the
liberal use of potential wingstroke amplitude on takeoff by buzzards
and herons caught my attention early on. Also, we had mallards and geese that
periodically got their wings clipped, and as they slowly regained their
flight ability, the difficulties of highload vs lowload flight were
obvious. Not that ducks are ever really lowload. As marginal takeoff and
level flight ability were regained, landings were pretty much a poorly 
controlled crash, the last
 fair amount of time in the library and some
experimentation have increased rather than decreased my skepticism.            
My take is most easily expressed by re-stating two of
your sentences. Quantifying launch abilities _from a skeleton(s)_ is a
mechanical problem. The _probability_ of achieving the phenotype for a
given launch technique through natural selection is critically affected by the
"ecological part". Were mega-volants the only phenomena difficult to explain 
relative to current conditions I would be much less hard-headed (by the way). 
--Don



> 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.

2) 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

2) I agree entirely that limits vary per flightstyle, but there are absolute 
limits, per medium. It is not possible to infinitely improve locomotive 
phenotype. Further, I posit that, because any living system must engage in 
multiple processes, theoretical limits for a given process can be approached 
(rarely), but not achieved.             High altitude migratory birds aren't 
optimized for sealevel flight, even if they nest there, as high altitude 
selection obviously occurs. Those examples demonstrate very little about 
density effects. (Heh. Wind blows like hell up there. V^2 and all that.)        
          At _any_ weight, the effects of medium density change are NOT easily 
compensated for if the other critical variables (circulation, temperature, and 
composition) are controlled. The effects are easily observable in lab in both 
wing kinematics and various metrics of power, especially lift generated per 
power expended. Further, the flight morphologies of birds that are optimized at
 5500' are measurably different relative to sealevel birds (per 
flightstyle/species), ditto w/ insects. (See Feinsinger P, Am Nat v 113, #4, 
481-497 for Andean hummingbirds).             As to the relative effect curve, 
I want to think about that some more. By the way, any testing of that for a 
given bird can only be done  _while controlling for circulation, temp, and 
composition_. And at positive pressures, of course. Testing at negative 
pressures and extrapolating is distinctly non-empirical in this case. --Don

 
> "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.

3). I see the relevance.  The point is that there is not an observed change i
g 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

3). I don't. This conversation started out w/ mega-volants. Now, you seem to be 
saying that because a 27 lb goose can fly in uncontrolled conditions, that is 
support for a 150 lb whatever flying in standard air. I am surprised you put 
forward these 'in vivo' anecdotes. Although it would be interesting to see how 
a giant albatross does in Denver. Also-- Controlled conditions, blah, blah 
(please see comment 2).                 I haven't read any author that feels 
that the extant maximals show any potential for doing well or better (as you 
seem to imply) at larger sizes. I grant you, they continue to be volant, but 
just barely (in still air). Also: Reference on Quetz. performance, please. 
(Heh. You sound like you got one in the backyard. Kin I see him?  }: D.)        
          As to size limits, please allow me to communicate by re-statement 
again. I think of the mega-volants as _approaching the practical size limits 
for their flight morphology at that time_.  And I feel you should say,
 "_theoretical_ mechanical size limits" no matter how much modeling you've 
done. --Don


> 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"....

4). 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

4). I like "maximal volants". Actually, "... the largest observed volant 
species of their time intervals..." is an excellent definition of what maximal 
volant refers to. Plot wingspan vs time,
are the minimums... it is a math thing, as you know. As the specimens are 
obviously "observed", the burden of distinction falls on those who use the term 
"maximal" in the theoretical sense.             In the same vein, the declining 
trend of maximal wingspan exists, and as such is real, unless new data destroys 
it. Relevance or statistical significance may not exist, but I don't think the 
term "real" should be used as a substitute for "relevance" or "significant". It 
may sound cool, but it limits the language, and causes confusion. --Don

Cheers,

--Mike H.