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Re: feather asymmetry



 Mike Habib <biologyinmotion@gmail.com> wrote:


> Indeed. So far, there is little evidence that low feather asymmetry has a 
> measurable advantage.
> However, as Tom Holtz noted, if limited asymmetry and symmetry are 
> functionally the same,
> there isn’t selection promoting symmetry, either.


Nevertheless, as Speakman & Thomson's study makes clear, flightless
birds generally have primaries that are highly asymmetrical, whereas
flightless birds tend to have less asymmetrical primaries (or even
symmetrical ones). In terms of the degree of asymmetry, there is
certainly an overlap between "flightless" and "flighted" birds, such
as (for example) doves that have vane ratios of 2.5:1, which are
within this range.

But _Archaeopteryx_'s weakly asymmetrical primaries lie well outside
the range, and are smack bang in the flightless zone for extant birds.
Then again, there is the caveat that _Archaeopteryx_ (and any
non-ornithothoracean bird) might have had a flight style so different
to modern birds that a one-to-one comparison of its remiges to those
of modern birds is irrelevant (or at least inapt). For one thing,
these 'proto-birds' likely had a much more posterior (caudal) center
of mass compared to modern birds.

It may be that _Archaeopteryx_ was secondarily flightless - it would
fit with its island habitat.  Or it may be that it had extremely
limited flight abilities that were primitive (nascent), which only
required limited vane asymmetry (or none at all, in its ancestors).
IMHO, the morphology and arrangement of _Archaeopteryx_'s  feathers
fits with the use of its wings and tail as control surfaces, in lieu
of powered flight.  But I happily concede that this is not the only
plausible hypothesis.


>> Putting aside full powered flight for a moment, does this also mean that 
>> other aerodynamic advantages of feathers, such as running up near vertical 
>> slopes, is impeded until 4:1?
>
> Not necessarily. Behaviors like wing assisted incline running might be brief 
> enough that feather torsion is of less concern. There are also other ways of 
> reinforcing feathers (such as
> overlap) which allow some living birds to fly well with less asymmetric 
> feathers - this could apply to WAIR, too.


As noted here on the DML and elsewhere, the objections against WAIR
are based on skepticism over whether basal avialans like
_Archaeopteryx_ (and related winged theropods) were capable of the
highly refined flapping required for WAIR.  In other words, the
feathers may not be the problem; it's whether the pectoral skeleton
and musculature were 'advanced' enough to execute the WAIR wing
stroke.





Cheers

Tim