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



Thomas R. Holtz, Jr. <tholtz@umd.edu> wrote:

> That IS basically their argument. Bristles (for sensory or display function), 
> modified by density &
> structure for insulation afterwards.

After initially raising it, the authors then go cold on a display
function for the origin of feathers: "However, sexual display is an
explanation that can be theorized to have played a role in the
evolution of nearly any outwardly visible feature. As such, sexual
display constitutes something of a default explanation that can be
offered whenever a fossil taxon is found with a structure for which no
other function is apparent. The case of early feathers has been such
an instance, and there is nothing particular to the form of simple
feathers that supports the sexual display theory."

The sensory/tactile hypothesis proposed by Persons & Currie proposes
an exaptive pathway whereby dense bristles on the face happened to
provide insulation for part of the head; these bristles then spread to
the rest of the body to form a heat-trapping pelt.  A similar
hypothesis exists regarding the origin of mammal fur, with proto-hairs
originally having a sensory function (sparse, but distributed over the
body) before becoming exapted for insulation.  (As an analog, naked
mole-rats have sparse vibrissa-like hairs that serve a sensory
function, not insulation.)  As Persons & Currie point out, sparse hair
lacks the density to be useful for insulation.

However, there is a hypothesis that sparse hair serves a
thermoregulatory function - but not insulation.  Instead, sparse hair
can actually be used to help *shed* body heat, not trap it: Myhrvold
et al., 2012 PLoS ONE 7(10): e47018. doi:10.1371/journal.pone.0047018.
This study concludes: "Since proto-hairs are believed to have
developed from single barbless shafts (similar to elephant hair) that
increased in density over time, our results are cause for further
investigation as to whether hair could have initially evolved to
increase, instead of decrease, heat transfer" thereby "resolving the
prior paradox of why hair was able to evolve in a world much warmer
than our own".

Maybe the same was true of proto-feathers?  No sensory function -
instead, sparse and filamentous proto-feathers served as wicks for
shedding heat.  So proto-feathers initially evolved in archosaurs
living in a hot environment - and were later exapted for insulation
(via increased density) in cooler environments.  Functions in display
and aerial locomotion came later.




Cheers

Tim