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Re: Why insulation = endothermy



From: Jeffrey Martz <martz@holly.ColoState.EDU>
 > ...  Did those first few little
 > pre-feather bristles offer even a TINY bit insulation for increased
 > development and isulating ability to be selected for?

Yes.

Even a very slight roughness or discontinuity on asurface is enough
to break up air flow, and thus the currents that enhance heat
exchange.  We all know that blowing on a hot item cools it faster,
and blowing on a cold item warms it faster.  This is the effect that
is disrupted by small bristles.  Such bristles also reduce evaporative
water loss - and they do that even more efficiently than their reduction
in heat exchange.  This is probably the original reason why cacti
evolved thorns (reduced water loss, that is).  The defensive function
almost certainly came later.

Perhaps the first step was the formation of small bristles in a desert
animal???

 >     Which brings me back to a question I asked a while ago; does the
 > presence of a rachis, barbs, and barbules on plumaceous feathers make
 > them better insulators then hairs?  Are the nodes on plumaceous
 > feathers atrophied barbicels, or are barbicels on contour feathers
 > derived nodes?

This whole issue is complicated by the fact that the development of
flight and body feathers are probably governed by much the same genetic
program.  Thus it is quite possible that features that are selected
for in one kind of feather may show up incidentally, and without
any direct use, in the other kind.

This is why human males have nipples, for instance.  They are of
no selective advantage per se.  However, their presence in *females*
is a decided advantage.

Thus the structure of *modern* body feathers may be an indirect result
of selection on the flight feathers.  In this case, even though
body feathers may have preceded flight feathers, it is the latter
that are determining the current shape of the former.

 >  When IS that frigging Sinosauropteryx paper coming out?
 > 
Soon, I hope, though I will probably be delayed some weeks in
reading it.

swf@elsegundoca.ncr.com         sarima@ix.netcom.com

The peace of God be with you.