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

[Tim Williams <twilliams_alpha@hotmail.com>]

Don Ohmes wrote:


> It is not too surprising that there are no gliders currently making the 
> transition to powered flight, 
> considering the fact that they would have to invade niches occupied by birds 
> and/or bats as they did so. 
> Seems a lot to ask...


It sure is.  However, I'm setting the bar much lower.  I'm not asking that 
gliders have to make the transition to powered flight.  All I'm pointing out is 
that no glider that I know of (not amphibian, reptile, mammal) shows the 
slightest interest in flapping their arms.  If flapping confers advantages to a 
glider, why is this the case?


Also, the inception of flapping should not be assumed to be equivalent to 
"moving in the direction of powered flight".  In order to be selected, this 
behavior (gliding+flapping) should be beneficial in its own right, rather than 
just be a stepping stone to flapping flight.


There is a swathe of empty morphospace available to gliders who want to engage 
in occasional flapping.  This ecomorphology would put them outside the vast 
morphospace currently occupied by flying vertebrates (birds and bats).  So 
competitive exclusion shouldn't hold for flapping gliders, given that it's only 
one step up from passive gliding, but a long way from powered flight.


BTW, I'm not arguing that bats didn't evolve from patagial gliders.  I'm just 
saying that the transition by which this might have occurred is not as simple 
as it sounds.  It'd be interesting to know what circumstances led to the 
gliding-fluttering flight inferred for _Onychonycteris_ (the most basal known 
bat), given that lack of echolocation would make aerial hawking behavior highly 
unlikely.  _Onychonycteris_ seems to be have been very well-adapted to life in 
the trees (especially scansorial and suspensory behaviors), so there's little 
doubt that the evolution of flight in bats had an 'arboreal' component.


> Competitive exclusion logically explains the paucity of gliding predators, 
> too. 


I don't see things quite the same way.  The paucity of gliding predators may 
have more to do with the paucity of specialized arboreal predators, and/or the 
demands imposed by predation and using the forelimbs for prey capture (damage 
to the patagium by struggling prey; reduced mobility when chasing prey when the 
fore- and hindlimbs are joined by a patagium; etc).


> And that in turn explains 
> why most extant gliders are 'just moving from tree-to-tree'.


Most extant gliders are 'just moving from tree-to-tree' because that's what 
gliding is for in these taxa: for commuting. 


> BTW, are there ANY cursorial vertebrates currently "moving in the direction 
> of powered flight"?


None that I can see.  So that's zero cursorial vertebrates currently "moving in 
the direction of powered flight"?  Thats' the same number (zero) as arboreal 
gliders currently "moving in the direction of powered flight"?  So it's a level 
playing field.


Cheers

Tim

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