Scientific American feathers paper

["Tim Williams" <twilliams_alpha@hotmail.com>]

Yesterday I read "Which Came First, the Feather or the Bird,", the 
Scientific American (March 2003) article by Richard Prum and Alan Brush.  
(Thanks to Mary for passing it on.)

This is a concise, engaging discussion of the development of feathers, and 
its implications for evolution.  Nice stuff, not dumbed-down, with great 
illustrations that complement the text beautifully.

The article did raise a few questions in my mind.  (Alan, are you out there? 
 :-) )

First, a run-down of the individual stages.

Stage 1: Hollow cylinder.

Stage 2. Tuft of unbranched barbs.

Stage 3. Two alternative stages...
Stage 3A: Planar feather with unbranched barbs (i.e., no barbules) fused to 
a central rachis.  This is a pinnate feather, but not bipinnate.
Stage 3B: Feather with barbs and barbules, with the barbs attached at the 
base to a short calamus.

Stage 3A+B: Planar feather with branched barbs and open vane.  Thus, this is 
a double-branched (bipinnate) feather.

Stage 4: Feather with closed pennaceous vane.  The feather is "closed" 
because hooklets on one barbule attach to grooves on barbules of the 
adjacent barb, producing a tightly interlocked planar vane.

Stage 5: Closed asymmetrical vane.  Needed for airfoil production and 
flight.

According to the article, the hypothetical "Stage 3B" in feather evolution 
is essentially the structure of modern down feathers (plumules).  My first 
question: If a down feather is a "jumbled tuft of barbs", what are the 
barbules there for?  Does this second level of branching provide extra 
insulation?

My second question also concerns the barbules.  In Stage 3A+3B (planar, 
open, bipinnate feather), what is the adaptive utility of the barbules?  Do 
they provide coherence to the vane via some level of interaction between 
adjacent barbs?  In other words, although the barbules do not yet have the 
hook+groove system to produce a tightly-closed vane, do the barbules help 
hold the barbs together in a planar structure.

Finally, is it possible that the presence of barbules in down feathers is a 
vestige of passing through a Stage 3A+3B (open pennaceous feather) or even 
Stage 4 (closed pennaceous feather)?  In other words, modern down feathers 
are 'trimmed down' pennaceous feathers, not a more primitive intermediate 
stage.

Again, thanks for a great article.

Cheers

Tim





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