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Re: Dinosaur color vision and evolution of feathers
Yep, the word "asymmetric" has been very much used and abused. To
give a historical perspective, Feduccia and Tordoff's 1979 study on
the primary feathers on _Archaeopteryx_ set off the notion that vane
asymmetry indicated flight ability. In the immediate aftermath of
this paper, there was pushback from some paleontologists who argued
that vane asymmetry did not necessarily imply that _Archaeopteryx_
could fly, and (further) that the asymmetry could be the result of
non-aerodynamic functions. This was because the skeleton of
_Archaeopteryx_ was interpreted by these paleontologists as having
absolutely no flight adaptations and no arboreal adaptations
whatsoever.
Nonetheless, the idea that _Archaeopteryx_ was capable of at least
some degree of flight ability seemed to catch on. The asymmetry of
the primaries were therefore assumed to play a role in flight. It's
only relatively recently that this entrenched idea of a volant
_Archaeopteryx_ has been the subject of renewed scrutiny. Again, the
impetus for this revision is that the skeleton of _Archaeopteryx_
appears to have no incontrovertible adaptations for sustained flight.
The same applies to a host of other pennaraptorans that have large,
planar pennaceous remiges. The possession of asymmetric vanes of
_Archaeopteryx_ sets it apart from many of these other pennaraptorans.
But as Mike has pointed out, anatomical asymmetry is not the same as
functional asymmetry as deployed during flight.
It could also be that vane asymmetry had other functions unrelated to
flight in _Archaeopteryx_ and co. As Dann suggested, it could enhance
folding ability of the "wings" - such as by improved imbrication of
serially adjacent remiges. Or the remiges (and rectrices?) could have
served an aerodynamic function, but one that was not directly
connected with aerial locomotion - stability flapping during
predation; maneuverability on the ground; or display - such as
vigorous arm-waving during display rituals, as Ronald proposed. The
kagu, for example, uses its wings during territorial displays (and
duels), as well as to aid terrestrial locomotion when fleeing
predators. The kagu is a flightless bird, and its remiges are weakly
asymmetrical.
On Tue, Oct 28, 2014 at 9:00 AM, Mike Habib <biologyinmotion@gmail.com> wrote:
> Just to reiterate a point I’ve made previously, we also need to be careful
> with what we call “asymmetric”. Do we just mean feathers with anatomical
> asymmetry (i.e. one vane at all wider than the other)? If that’s the case,
> then it could probably appear via basic drift and/or pleiotropy. A feather
> with a vane ratio of 1.1 to 1 will not behave, mechanically speaking, any
> differently than one exact symmetry in a fluid interaction sense (though the
> wing folding idea and other such mechanical functions are different and, to
> the best of my knowledge, untested). The more extreme asymmetry seen in some
> (but not all) living volant birds should have an effect on aeroelastic
> stability and stall thresholds, but probably not steady state functionality.
>
> Cheers,
>
> —Mike
>
>
>> On Oct 27, 2014, at 2:51 PM, Dann Pigdon <dannj@alphalink.com.au> wrote:
>>
>> On Mon, Oct 27th, 2014 at 7:21 PM, Dr Ronald Orenstein
>> <ron.orenstein@rogers.com> wrote:
>>
>>
>>
>>> A thought: is it possible that the aerodynamic properties of asymmetrical
>>> feathers could be
>>
>>> related not to flight or gliding, but to display? In other words - if
>>> rapid movement of banks of
>>
>>> pennaceous feathers in the course of display has any significant energy
>>> cost associated with it
>>
>>> (eg vigorous arm-waving displays) then an adaptation that would reduce the
>>> effect of air
>>
>>> resistance on the cost of display movement might be selected for
>>> (especially if the display
>>
>>> included leaping, parachuting etc).
>>
>>
>>
>> Or perhaps feather asymmetry improved the ability to fold display fans away
>> when not in use.
>>
>>
>>
>> --
>>
>> _____________________________________________________________
>>
>>
>>
>> Dann Pigdon
>>
>> Spatial Data Analyst Australian Dinosaurs
>>
>> Melbourne, Australia http://home.alphalink.com.au/~dannj
>>
>> _____________________________________________________________
>>
>>
>