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Re: Why insulation = endothermy




On Thu, 6 Feb 1997, Stan Friesen wrote:

> 
> 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???
> 

Nice idea but water loss through the skin shouldn't have been a problem 
for the ancestor of the first feathered archosaurs (at what ever node 
feathers originated) if it retained the impermeable, non-secreting skin 
that is a synapomorphy of the clade Reptilia. Is modern bird skin under 
the feathers still reptilian in this respect?
Your idea might hold for the initial evolution of bristles within the 
synapsida, though. Perhaps one day there will evolve a bristled desert 
dwelling amphibian, we could call it the velvet frog.

"All major battles in history have been won by the side with the shortest 
haircut"

Adam Yates