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color vision and croc-sighting



I know I pretty much called for a moratorium on the croc thread, but
this seems to me to be veering away from the crocodiles and more
towards visual ecology, so I'm letting myself send it.  In response to
one of my editorial comments, Stan Friesen wrote:

> Humans also have good color vision.

However, our color vision is probably not as good as the dinosaurs'
was.  Many (if not most or even all) of them could probably see much
shorter wavelengths than we can, and they could probably make finer
wavelength discriminations in some of the wavelength range that we do
see.

> For one thing, as often as not the water is quite opaque.

"quite opaque" is a function of the visual system doing the observing
as well as a function of the environment being observed.  This is
probably the first time that the list has actually given me the
opportunity to talk about something I've researched directly.  Using a
polarization analyzer and a liquid crystal to rotate the plane of
polarization of incident light, my colleagues and I demonstrated that
under some conditions it is possible to detect targets much farther
away than you can with a polarization blind system.  We were
principally interested in fish seeing through water with a lot of
suspended particles, so what we found is applicable to the situation
of a hadrosaur trying to see through murky water.  We used scattering
agents that scatter very well but don't absorb much (which is
different from silt and decaying plant matter which over certain
wavelength ranges will absorb much better than they scatter), so I
can't give you hard numbers for the specific case in question.
Nevertheless, from those experiments it's safe to conclude that if our
hypotheses about polarization vision in extant animals are correct,
then hadrosaurs and tyrannosaurs would almost definitely have been
much better at seeing submerged crocodiles than are wildebeests and
zebras.

I'm sure John Bois won't like that appraisal, but c'est la vie.  And I
won't get into what my analysis has to say about whether or not
hadrosaurs and tyrannosaurs were good croc-fodder because if I did I'd
probably have to reject this message :-)

--
Mickey Rowe     (mrowe@indiana.edu)