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Avian flight may ultimately have evolved for the simple reason
>that a controlled fall is always better than an uncontrolled fall, and
>powered unstable flight gives the maximum imaginable amount of control to a
>"falling" arboreal animal.
I must confess I doubt this.  The statement is true for fully evolved
powered flight - but for any intermediate stage I can imagine any sort of
flapping semi-flight would be a lot more energetically costly, and probably
less effective, than a gliding mechanism (and we DO have intermediate stages
in the evolution of gliders, from Costa Rican frogs that can use the webbing
of their feet to slightly alter the course of their fall, through the more
spectacular Asian "Flying" frogs, right up to the giant flying squirrels and
colugo that can glide for hundreds of metres).

To me the only locomotor advantage for an arboreal animal of powered flight
over gliding that I can imagine intermediate stages for are leaps UPWARD
from branch to branch or moving among branches in close quarters where
gliding would be useless but a fluttering semi-flight might be quite handy.
Ronald I. Orenstein                           Phone: (905) 820-7886 (home)
International Wildlife Coalition              Fax/Modem: (905) 569-0116 (home)
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