Re: New bird /pterosaur flight paper in PLoS ONE

[Michael Habib <habib@jhmi.edu>]

On Apr 29, 2009, at 8:23 PM, don ohmes wrote:
>I do not know if their conclusions are valid, but I certainly applaud the 
>empirical approach...


I applaud the empirical approach, as well, but there are a few notable problems 
plaguing the conclusions and use of the results.  They do not invalidate the 
analysis, just some of the conclusions.

The first issue is the assumption that a global maximum exists for size in 
flying animals.  The authors start by using such an assumption, then relax it a 
bit later in the discussion.  However, they implicitly keep such an assumption 
by indicating that the scaling curves they generated should apply to animals 
far outside the morphology and phylogenetic bracket of their sample.  What the 
authors probably have is the maximum size for a procellariiform seabird - not 
the maximum size for a soaring animal, or even the maximum size for a coastal 
soaring animal.  The authors state that:

"Some studies have proposed that large pterosaurs such as Pteranodon and 
Quetzalcoatlus may have had narrow wings similar to those of albatrosses, and 
used slope soaring and dynamic soaring [18]. However, our study of living 
Procellariiformes as model animals suggests that if pterosaurs larger than 41 
kg (or 5.1-m wingspan) had the narrow wings, they could not have attained 
sustainable flight in environments similar to the present."

There seems to be a bit of confusion here, as the authors have apparently 
assumed that their calculated scaling patterns should hold for a given aspect 
ratio regime.  Flapping limits also vary according to relative muscle mass, 
launch type, wing efficiency, etc.  As such, the reconstructed AR has little to 
do with the problem at hand.  I also find it strange that the authors consider 
their paper support for Darren and Mark's bit on semi-terrestrial hunting in 
azhdarchids.  Darren and Mark put together a fine paper, don't get me wrong, 
but poor launching ability and the inability to fly without external lifting 
conditions would argue *against* such a mode of life, as the animals would be 
stuck on the ground after feeding.  Semi-terrestrial predators need a strong 
ability to launch and gain altitude, at least for bursts.


The other major issue, in my mind, comes from the assumptions regarding launch. 
 Still, after all the years and fine work that have gone into launch mechanics 
(Tobalske, Earls, Rayner, etc), we have biologists that are confused about how 
launch works in birds at a general level.  The authors suggest that:

"Takeoff is the transition from being supported by something that is 
essentially part of the earth's surface to being supported entirely by 
aerodynamic forces in flight, and these depend on air flowing over the wings 
[7]. Takeoff seems to be the most crucial task for flying birds and requires 
more active flapping than level flight because the flight speed is zero at the 
beginning and the birds must raise their body mainly by muscular effort."

This is, technically speaking, true.  However, further reading indicates that 
the "muscular effort" indicated is meant to be rapid flapping - but most of the 
launch impulse comes from the hindlimbs!  I have to admit that I feel a bit 
like a broken record on this one these days - birds do not flap themselves into 
the air, for precisely the reasons indicated in the paper (ironically enough): 
at low flow speeds, there is limited fluid force.  Thus, speed needs to be 
generated *first*, then the wings can engage to good effect.

Overall takehome message (in my opinion, anyway): Nice data, neat analysis, 
possible limit for procellariiforms.  Application to pterosaur is, however, 
quite weak.

Cheers,

--Mike



Michael Habib, M.S.
PhD. Candidate
Center for Functional Anatomy and Evolution
Johns Hopkins School of Medicine
1830 E. Monument Street
Baltimore, MD 21205
(443) 280-0181
habib@jhmi.edu