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_Sinosauropteryx_ fibers




> From: Toby White <augwhite@neosoft.com>
> Date: Friday, October 31, 1997 7:02 PM
> 
> Ralph Miller said:
> >
> >Polar bears, it seems, have black skin and clear, hollow fur.  The
> >transparent fur conducts the warmth of the arctic sunlight to the
> >heat-absorbent black skin, much as man-made fiber optics convey light
from
> >one end of a transparent fiber to the other.  So polar bear fur is cold
on
> >the surface, but the animal's skin is warm.  Moreover, the hollow fibers
> >trap air inside, providing additional insulation.  I find these
adaptations
> >most remarkable, but it is the implications of the "hollow fibers" that
I
> >will discuss here.  (Unfortunately, I have no references; please correct
me
> >if I'm misrepresenting the functional morphology of polar bear fur).
> 
> I'm not sure why this would work in the polar bear.  Optical fibres
depend
> on having a medium surrounding the fiber (e.g. air) which has a lower
index
> of refraction than the fiber.  Only in this way can you get the total
> internal reflection on which optical transmission depends.  If the hair
or
> feather is hollow, then the transmission medium is air.  Its hard to see
how
> the fur could have a lower index of refraction than air.
> 
>   --Toby White

It appears paradoxical, doesn't it?  The foregoing data I reported on polar
bear fur is consistent across more than one source, but did not come to me
directly from scientific journals, so the potential for error exists.  My
<guess> is that polar bear fur is coarse enough that it may be hollow and
yet retain sufficient thickness so that it is certainly the fiber (rather
than the entrapped air) that transmits radiant energy from the distal tip
of each hair to the proximal end where it penetrates the skin.  By
"thickness" I am referring to the difference between the inside diameter
and the outside diameter of the cross-section of a fiber.

To offer a simplistic analogy: would a glass tube such as a laboratory
pipette, for example (although admittedly geometrically dissimilar to the
fur fiber at the distal end), be capable of conveying light or heat from
one end to the other in spite of its hollow aspect?  Perhaps you could
picture a polar bear fur fiber as a long, hollow acrylic cone for the sake
of comparison, then stretch the length of the cone further in your mind's
eye until it is a clear, hollow fiber which would still perform optically
as the original cone model in certain respects.  Would this shape be
capable of transmitting radiant energy?  I am asking; fiber optics are not
my specialty.

The slide of the _Sinosauropteryx_ fiber cross-section which Philip Currie
projected at the SVP meeting had the appearance of being hollow, yet
substantial in the thickness of its "wall."  If the polar bear fur fibers
were similarly "robust," then perhaps the transparent polar bear fibers
could perform in a similar way to the pipette or elongate cone in the above
analogies, and perhaps they could still be compared to fiber optics, though
the geometry would dictate a much lesser degree of efficiency than that
achieved in the conventional man-made variety.  If you like, discard the
reference to "fiber optics," and call them "quasi-lame-not-so-hot-fiber
optics."  Technically, we <are> still speaking of the "optics" of "fibers."
 I can't see Disneyland using polar bear fur for their Light Magic parade,
but none-the-less the transparent, hollow fur and absorbent black skin
pigmentation have been cited as being among the adaptations which enable
the endothermic polar bear to efficiently absorb and retain heat in the
extreme cold of the arctic environment.  These features help keep the
animal warm, or so the story goes.  

Of course, the optics of polar bear fur was <not> the main topic of the
letter I posted on October 30, 1997.  Lest I be misunderstood, I am not
suggesting that small, endothermic theropod dinosaurs utilized the polar
bear adaptations of black skin and clear (as opposed to merely white)
fibers as a means of insulating themselves.  (Such a proposition would not
seem provable, in any case).  Rather, I am suggesting that the <hollow>
fibers evident on polar bears (which are hypothetically useful in retaining
heat in these extant endothermic mammals) may provide a functional
equivalent to the apparently hollow fiber structures observed on fossils of
_Sinosauropteryx_.  That is, the hollow property of the _Sinosauropteryx_
fibers may likewise have served as a significant aid to heat retention,
implying that thermoregulation requirements may have applied the selective
pressure which drove the development of the fibers to reach the morphology
observed in these specimens.  I further propose that such hollow fibers as
we see in this non-avian theropod may reveal a glimpse of an early stage in
the evolution of the hollow rachis seen in the more derived feathers of
modern birds.

I am open to criticism, and would most appreciate the input of any experts
in polar bears, thermoregulation, integument anatomy and function, and the
physics of insulation.  I am offering here speculation, mindful of the fact
that my information, and hence, my hypotheses, could be unfounded.

Ralph Miller III <gbabcock@best.com>

"Patience is a virtue of the gods."