Feather tracts (was Re: Life of Birds (vertical running))

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

Ken Kinman wrote:

>  But in the meantime, exaptation for
> brooding would have driven the evolution of feathers from just the tail,
> forward to the back and arms as well.  Evolving up the tail and back would
> also help to explain why feathers occur in "tracts" (more so than if they 
> had originated on the arms in the first place).

What Ken is referring to is the pterylae (sing. pteryla), the neat rows into 
which feathers are arranged over the body in most birds.  The term usually 
(but not exclusively) relates to the contour feathers (the feathers that 
define the body shape, including flight feathers).  The "naked" gaps in 
between the pterylae are called apteria (sing. apterium).

However, the arrangement of contour feathers along tracts is not a universal 
feature of bird plumage - even among flying birds.  Ostriches, rheas and 
penguins do not have feather tracts, nor do the screamers (Anhimidae) which 
can fly.  In these forms, the feathers are uniformly distributed over the 
body surface.

Even in birds that do have pterylae, the intervening areas are not 
necessarily featherless.  In many water birds (ducks, divers/loons), the 
areas separating pterylae on the body are packed with warmth-giving downy 
(plumulaceous) feathers.

If one posits that insulation was the primordial function of feathers (and I 
personally think this is a safe bet), then I see no reason why feathers (or 
their precursors) should have been originally organized into tracts.  The 
uniform arrangement of feathers seen in ratites and screamers, rather than 
neatly organized tracts, may represent the primitive condition.  To be 
honest, I don't know of any developmental or paleontological evidence that 
supports one theory over the other.



Tim


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