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Re: Fw: Dinosaurs and birds



Well, I disagree.

An animal that increases it's maximum speed by generating thrust w/ it's 
forelimbs as it runs gains an advantage. If it is predatory, it gains advantage 
in both resource acquisition and escaping predation. An animal that is adapted 
to assist running w/ forelimb-generated aerodynamic thrust, receives an 
advantage should it produce lift, or even alter the direction of thrust w/ 
respect to the ground (advantageous exploits would include perching in trees, 
'leaping' over obstacles, catching/escaping similarly talented animals, etc). 

The logical end point of your argument is the conclusion that a ground-up 
evolution of flight could not evolve on a smooth planet... no doubt inclines 
could be helpful, just that they are not essential to a selective scenario that 
produces flapping flight. 

Don

----- Original Message ----
From: Michael Habib <mhabib5@jhmi.edu>
To: dinosaur@usc.edu
Sent: Wednesday, April 4, 2007 9:00:06 PM
Subject: Re: Fw: Dinosaurs and birds


On Wednesday, April 4, 2007, at 08:08  PM, don ohmes wrote:

>
> Not sure I understand, from the perspective of a 'ground-up' selective 
> process that can transform a terrestrial mud-lover into a barn 
> swallow, where the line between volancy and various forms of 
> wing-assisted running is (inclines are NOT necessary, in my opinion). 
> Once Archies ability to flap synchronously is established, then the 
> question of volancy in Arch. requires drawing that line, does it not? 
> So my question is, has that been done?

The incline is necessary, because without it there is no requirement to 
produce a lift force towards the substrate, which is the critical 
aspect of wing assisted running.  In the WAIR dynamics, the flapping is 
not utilized to generate better climbing speed via thrust; the bird is 
generating increased vertical momentum indirectly by elevating the 
frictional force on the hind limbs.  If the animal is running on a 
level substrate, then producing a lift force is usually not 
particularly helpful, and can actually be counter-productive under some 
scenarios (as would probably be the case for basal birds).

Many living birds utilize running takeoffs, and some of them flap 
heavily while launching.  However, most of those birds are semiaquatic, 
and the wings are important in the early to mid launch mostly because 
the lift elevates the bird slightly from the aquatic medium and allows 
for the animal to run on the water surface to reach its launch window.  
Only late in the running launch is the oscillation of the wings really 
helping the bird get into the air directly.  Most birds use a leaping 
launch.  In that case, the wings contribute more heavily to reaching 
flight speed, and are brought to bear in full earlier in the launch 
cycle.  Also note that a running launch is derived in semiaquatic 
birds, and not the basal condition.

Cheers,

--Mike H.