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Re: Birds again



> -The more important muscles involved in the flight stroke are those that 
> lowere
> d the wing, because is in this phase when the force to go up and forward is 
> ori
> gined. So you have to have strong pectoral muscles in order to make enough 
> forc
> e to fly, and strong bones in whose insert those muscles. _Archaeopteryx_ (and
> especially the Early Cretaceous birds) has an osified sternum and great 
> coracoi
> ds to perform this function. The upward movement of the wing is the movement 
> th
> at allow to make another downward movement (the recovery and the powered 
> stroke
> s, respectively). So, if you don't have strong muscles to recover the wing, 
> you
>  still can fly, although slowly, because you can'tmake the movements as fast 
> as
> you should to fly faster.
> -Metabolism: you said it's necessary a high metabolism to fly. That's is true 
> i
> f you fly for long time. If you don't, you don't need it (it's like run; if 
> you
>  run fast for long distances, you need a high metabolism). But maybe the early
> birds had high metabolic rates (if the feathers were developed as an 
> insulation
> structure, and then exapted to fly).


     I just finished Ruben's 1991 paper in Evolution on Archaeopteryx, in 
which he makes a very interesting argument that implicates ectothermy in 
Archaeopteryx.
     First of all, he discusses  the 1984 Eichstatt conference on 
Archaeopteryx.  The general conclusion of the conference, based on the 
assumption that Archaewopteryx was a endotherm, was that Archaeopteryx 
was a largely cursorial animal that was capable of flight due to:
1)  the asymmetrical, airfoli shape of the feathers
2) the well developed humeral deltopectoral crest, indicating stresses 
   produced by the muscles involved in wing flapping.  
But to fly, it had to climb up into the trees, and was incapable of 
taking off from the ground becasue the pectoralis muscles were apparently 
much less developed than in birds.  These muscles apparently provide 
additional power needed for a ground takeoff.  Without them, a bird can fly
"from the trees down", but can't generate the extra lift needed for a "ground
up" takeoff.  
      Ruben's argument begins with a discussion of reptile physiology 
focusing on the short, extremely high bursts of energy certain reptiles can 
produce due to anaeorbic glycolosis that produces ATP at high rates, 
combined with a high concentration of muscle fiber.  Ruben states that 
as as far as short bursts of activity, some reptiles can produce twice 
the energy of modern endotherms.  He goes on to suggest that the 
pectoralis muscles of an  ectothermic Archaeopteryx could have provided 
the burst of energy needed to take off from the ground.
      Ruben also makes note of the fact that this sort of activity 
results in fatigue after a short time, a major impediment to sustained 
flight.  He suggests that Archaeoptery could have flown for bursts 
interspaced by brief resting periods.
     He furthemore suggests that the Early Cretaceous neornithean 
adaptations of a keeled sternum, stronger corocoids, and a triosseal 
canal (all of which implicate larger pectoralis muscles), accomanied 
the development of endothermy to allow birds to fly for extended 
periods of time (endothermy providing more STAMINA, while ectothermy 
allows higher, shorter bursts of energy).     
     It sounds like a persuavive argument, but not having investigated 
endothermic/ectothermic metabolic capabilities that much, I'd appreciate 
any perspectives.

LN Jeff