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Re: flapping from gliding



Dinogeorge writes:

>It is "conceptually easy" because the force of gravity pulls downward, not
>upward; the energy keeping a gliding creature aloft comes from its fall
>through the earth's gravitational field (in still air, that is). Arboreal and
>other acronomic animals require mechanisms to keep themselves from injury
>from falls, and every arboreal theory of the origin of flight starts out from
>this point. Cursorial flight-origin theories, however, invariably work
>_against_ gravity; there is no way to keep the animal aloft unless it already
>has the very wings you're trying to get it to evolve.

The "force of gravity" is a red herring.  It pulls downward with the
same force on _everything_ regardless of whether you are cursorial or
arboreal, and the same force must be overcome in all cases.  BTW, cursorial
animals also have to protect themselves from falling injuries (remember
the recent paper about tripping being fatal for a running T. rex?)

I believe the real argument you are trying to make is that gliders
already have the necessary equipment for powered flight, and that
cursorial animals cannot possibly have them since wings are of
no use unless you are already in the air.  Of course, neither one
is necessarily true.

First, yes, gliders have something resembling a wing, but it takes
a lot more than a flap of skin to achieve true powered flight. 
Such as: a strong breastbone for flight muscle attachment,
shoulder support, great muscle coordination, high intelligence, 
high metabolism, lightweight construction, good vision, etc., etc.
A glider would have to evolve all of these things in much the
same way a cursorial animal would.  Except, the cursorial flight-origin
candidates already have a head start on most of these things.

Second, there can be other uses for "proto-wings" (wing-like
structures that are not yet capable of fully powered flight).
For example:  insulation, using them as a net to catch prey (Ostrom),
sexual display, using them as "spoilers" for ground-effects
to aid in tight ground maneuvering, using them to gain additional
height in hopping or jumping, etc.  None of these require being
airborne before the benefits can be realized.

BTW, _Exocoetidae_ (flying fish) is another counter-example to
your gravity argument.  It has to overcome not only gravity,
but also the viscosity of the water in order to become airborne.
But I have never seen a fish climb up a tree first :-)

>So, I can't see how you could possibly think that evolution of flight from
>the ground up is "conceptually easier" than evolution of flight from the
>trees down.

Both are _evolutionarily_ possible.  Which one is "conceptually easier"
depends more on psychological predisposition than biological feasibility.
 
>> ...Consider extant flightless birds. I believe all of them are good
>> runners.  If you believe that modern flightless birds are descended
>> from previously flighted forms, then you acknowledge that the
>> reverse transition has taken place.  Why then is the transition in
>> the other direction conceptually hard?

>Some transitions are simply irreversible, for all practical purposes.
> [rest of entropy argument deleted]

Of course I know that some transitions can theoretically be irreversible. 
I took thermodynamics in college too.  What I am claiming is that
the running-flight transition specifically _is_ reversible;
in fact, that both transitions have occurred at least once.
To make your irreversibility argument, you will have to show how the
specific changes in the genotype at the DNA level are irreversible.
And showing an increase of entropy in one direction is insufficient
since organisms can forcibly reduce entropy locally (at the expense
of increasing entropy even more globally).

>We do not know how pterosaurs evolved, just like we do not know how birds and
>bats evolved. In fact, we do not yet know just how well pterosaurs were
>adapted for walking and running; as with birds, it is more likely that
>pterosaurs evolved from climbing forms, not running and jumping forms.

Then why do pterosaurs, especially primitive ones, show no signs
of being adapted for climbing?  A climbing pterosaur would be an
awkward sight indeed.  The structure of the feet was unsuited for
grasping tree limbs, and lacked the claws needed to dig in.  They
would therefore have had to rely on the three fingers of their hands
to do the grasping.  This would further be made more difficult by the
fact the hands are attached to the front of the wings, so the wings
would open up as hands stretched out to climb.  This means it would
be unable to keep its huge wings tightly folded as it climbed.  Further 
awkwardness in climbing would be caused by the extremely large heads
(up to half the body size).  Hopefully, the tree doesn't contain many
branches or leaves, or the hapless pterosaur would be unable to get
through them without catching its wings.  And of course it would do
no good to climb only part way up the tree and jump off, risking 
that space between the foliage would be insufficent to fully open
its wings.  No, it would have to climb clear to the very top of the
tree, in order to be sure to clear the whole forest canopy.  I don't
know about you, but I find this whole scenario pretty absurd.

On the other hand, there _is_ a lot of evidence for pterosaurs being
adapted for running.  In fact, very well adapted.  The shin bone/femur
ratio, often considered a reliable indicator of running speed is
nearly 2, putting it in the class of the very swift agile runners.
The articulation of the leg bones shows that it could stand fully erect.
See the wonderful exhibit at the Museum of the Rockies showing
a Quetzalcoatlus life model standing fully erect (thanks to Ellen
in the Museum lab for giving me a lot of info about this exhibit).
Not to mention all the pterosaur ancestors we can find are cursorial.

I believe this whole climbing pterosaur idea was originally motivated
by the (now obsolete) idea that pterosaurs were awkward fliers; once
grounded, the only way the poor thing could get airborne again would
be to climb up a tree, or find a cliff and jump off.

We now know pterosaurs to be supremely competent fliers.
Pterosaurs probably ran quickly on open ground (or water) to
reach takeoff speed, in much the same way the waters birds do
today.

>_Scleromochlus_ could well have been a flightless form descended from
>climbing and gliding forms ancestral to pterosaurs.

Oh sure, it's possible.  Find me evidence of such ancestral forms and
then we'll talk.  For now, all we have to go on is _Scleromochlus_,
which was clearly non-arboreal.

>It is entirely incorrect to state that "the further back you go in the
>pterosaur family tree, the better adapted they are for running." There is no
>evidence for this whatsoever, unless you go way back to the original
>quadrupedal forms when they first became arboreal.

The evidence is there, you just have to look for it.

-----------
Homer Simpson in Jamaica:       Daay-oh!

Achut Reddy                    
achut@troodon.isp.net