Re: Bristles before down: A new perspective on the functional origin of feathers

[don ohmes <d_ohmes@yahoo.com>]

On Thu, Mar 12, 2015 3:09 AM EDT Tim Williams wrote: 

>I can only refer you to how Myhrvold &c address this question
>regarding hair: "At low densities, hair has almost no effect on air
>flow and does not trap an insulating air layer near the skin, but the
>extended hair acts as a pin fin that increases thermal exchanges with
>the surrounding air. Thus, as the hair density decreases from that of
>very furry animals, a break-even point is reached where the hair
>function switches from an insulator to a heat exchanger."
>
>
>I don't know if the same principle would apply to bristle-like and/or
>filamentous feathers.

There is no such principle. Hair, bristles and feathers have low thermal 
conductance. If hair could conduct heat well enough to act as a "pinfin", or 
heat exchanger, at low density, it would dump even more heat at high densities.

As Orenstein points out, disruption of laminar flow around the body might 
affect radiative balance, to some degree...

A vascularized sail could pull heat out of a warm environment, or dump heat 
into a cool one, but Myrhvold's idea is completely whacked.