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Feather origins?



      For a start, lets review the five basic kinds of teleoptiles (adult
feathers:
1) Countour Feathers: consist of a shaft (composed of a calamus and rachis),
barbs extending off of the rachis in two vanes, and barbules. The barbules 
The proximal barbules extending off the proximal side of the rami
(central axes) of the barbs are flange shaped.  The barbules exteding off
the the distal side of the ramis end in hooklets, which grasp onto the
flanges of the adjacent barb.  Contour feathers are the most structurally
complex type of feather, and functions in streamlining and flight.  The
remices and retrices (main flight feathers) are usually nearly totally
pennaceous (having the interlocking barbules), but most contour feathers
have areas, usually around the base, where the feathers are plumulaceous (
in which the "barblues" are actually short, non interlocking "nodes".)

2) Semiplumes: These feathers have a rachis (central shaft supporting the
barbules), but are entirely pennaceous (no interlocking barbs), and
located underneath the contour feathers, generally around the abdomen
Semiplumes (according to my text) serve mainly to provide both insulation
and give the overlying layer of countour feathers flexibility.

3) Adult down feathers: The barbules are also pennaceous, down feathers
usually entirly lack a rachis, and serve primarily for insulation.

4) Bristles: A stiff, tapered rachis that usually lacks barbs entirely,
although they may be present around the base.  They serve a variety of
functions, mainly sensory (similar to a cat's whisker), and as filters
around the eyes and nostrils (ostrich and hornbill "eyelashes" are
bristles).

5) Filoplumes: slender, bristle like feathers, with barbs and barbules
present on the tip.  Filoplumes are closely associated with other types of
feathers, and usually have a rich supply of nerve endings, implying they
also have sensory function.  They may serve to stimulate or control
feather movements.

PROBLEMS WITH AN INSULATORY OR FLIGHT ORIGIN FOR FEATHERS:   
    The primary functions of feathers in modern birds is flight and
insulation, so the origin of feathers is usually tied in to these
functions.  I see this as problomatic for two reasons:
1) Feathers are incredibly usefull and versitile features; birds use
them for just about _everything_.  Bristles and filoplumes serve sensory
functions, other feathers may be used in courtship displays (mannikins
click the rachi together to make noise, colorful and elaborate displays
are common, to say nothing for the huge gaudy displays of peacocks.  I
think it its safe to say that they could probably fly better without those
tails; the need to get sex can supercede convenience in other areas of
survival), ducks use them for bouyancy, sandgrouse use them to transport
water for thier young, lovebirds carry nesting material with them.  Grebes
actually eat thier own feathers, possibly either to provide protetion from
swallowed fish bones or plug up the digestive tract so fish bones have
time to digest in the stomach.  Feathers are so versitile, why should we
assume thier original function was flight _or_ insulation?
     Stan Frieson has pointed out that even a little bristle can break up
air flow, but this is assuming that the main cooling problem of a small
animal comes from moving air currents (causing evaporative cooling?),
rather then simply a lower ambient temperature.  Is there any evidence
for this?  If the ambient temperature is cooler then body temperature, 
regardless of if the wind is blowing, are the bristles still going to do
the job?  As has been pointed out on this list, heat loss is a serious
concern for endotherms if the ambient temperature is even a little bit
cooler then the body.         

2) The preadaptation problem:  For a feather to trap heat, to say nothing
for providing an aerodynamic function, they have to be already be fairly
well developed.  Evolution acts on the now, not what a feature might
start to be useful for if it developed a little more.  How much heat can a
bristle trap?  Is the teeny, tiny bit of surface area covered by a
few isolated bristles _really_ enough to confer even a miniscule selective
advantage?      
    This is a relatively modest problem compared to feathers originally
evolving for _flight_.  If you are going to use feathers to fly, or even
glide, nothing but a stiff, pennaceous feather with a rachis, barbs set in
two asymetric vanes, and a complex system of interlocking barbules will
do.  Why would anyone assume that flight and contour feathers, the most
_structurally complex_ type of feathers, were the first type of feathers
to evolve?  

WHICH BRINGS US AT LAST TO BRISTLES:

     Bristles are the structurally least complex kind of feather, and it
is probably safe to assume that the earliest feathers were bristles, or
bristle-like.  Bristles, as already noted, mainly serve two functions;
tactile devices similar in function to whiskers, and filters to keep dust
and other crap out of the eyes and nostrils.  I like the idea of feathers
originally developing for these two functions for a couple reasons:
1) Even a _tiny_ little extension will help to the job enough to confer a
selective advantage.  Unlike insulatory and contour feathers, which
already have to be fairly well developed before they can do the job, even
a little layer of "goosebumps" would give a little more tactile
information, or trap a little more dust and dirt.
2) Perhaps most interesting compared with the insulatory funtion of
feathers, you don't have to be an endotherm to benefit from these designs.
Can anyone give me any reptile analogues of whiskers and eyelashes? 
     I might also point out that filoplumes, which are basically bristles
witha few barbs and barbules around the tip, also serve a sensory function
(although it is probably imporatant to remeber thier association in this
regard with other types of feathers; the point is that they are sensitive
structures).  I am assuming the barbs give more tactile information.

FEATHER DEVELOPMENT: POTENTIAL PROBLEMS

     Look as I might, I could only find information about how _contour_
feathers develop in the skin.  Feather development is probably the most
important piece of information I am lacking to back up or shoot down my
idea.  I think the little bit I have learned seems to back up my idea, but
I need more information on the growth sequence in other kinds of
teloptiles, particularly bristles.  
     Do bristles develope and epidermal collar like contour feathers do?
Unlike trees, feathers grow from the base, not the top.  Differentiation
into a contour feather begins with the devolpment of an "epidermal collar"
deep inside the follicle.  This is a crown shaped structure, and the barbs
grow off of it.  One side of the crown continues  to grow up, and the
barbs come around and attach at the base to FORM THE RACHIS.  The
imporatant lesson here, which may be problamatic to my idea, is that the
_rachis_ is formed from the _barbs_; the barbs do not "sprout" off of the
rachis like twigs off of a branch.  This implies that barbs came first,
adding credence to the insulation theory. 
     However, if you didn't form the epidermal collar, wouldn't you just
get a huge barb?  Are bristles really naked rachi, or big barbs?  More
information please!  Is that Paleocanimimus paper out or not?      
     
LN Jeff
O-
"Don't underestimate our intelligence."
"There is no way I could ever underestimate your intelligence."