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RE: Campbell's even crazier than a MANIAC? (archeopteryx



--- On Wed, 9/24/08, Erik Boehm <erikboehm07@yahoo.com> wrote:

> Just nitpicking here:
> 
> > But note the exponential increase in air speed
> > culminating in terminal velocity experienced by
> > gravity-driven animals. 
> 
> Acceleration by gravity is roughly constant (initially,
> before approaching terminal velocity), not exponential.
> Airspeed increase is not exponential. The increase in the
> force of the relative wind, and the kinetic energy of the
> falling animal, do increase exponentially however.

Nits are good to pick. The reality behind my incorrect use of terminology is 
important; the threshold size at which random structures begin to have a 
significant aerodynamic effect varies w/ air speed, and the variation is 
non-linear. Assuming terminal velocity in falling vertebrates is higher than 
the velocity they can attain by leaping, it follows that selection can act (on 
a strictly aerodynamic basis) on relatively smaller deviations from the mean 
morphology (e.g., body shape, hair, feathers, loose skin) in a gravity-driven 
scenario. 
  
> Still, a leaping animal would essentially begin with a
> ballistic trajectory, and I have a hard time imagining an
> intermediate structure that would be advantageous. I would
> think a leap would lead to low drag structures and forelimb
> positions.

Most scenarios invoke a sequential suite of advantages acting on a single 
talent. E.g., small 'hairy feather' is thermally advantageous ==> is enlarged 
by display or insect capture ==> and finally achieves an aerodynamic effect. In 
other words, in the course of staying warm, well-fed, and maintaining 
popularity w/ the ladies, we grow feathers that are large enough to be modified 
by the effects of airflow engendered by running, leaping, flapping (or all 
three). Drag removes feathers from the leading edge, lift/thrust enhances the 
trailing edge, and there you have it.

Don