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Re: Marsupials see colors



Guy Leahy <xrciseguy@sbcglobal.net> writes of:

http://www.sciencedaily.com/releases/2006/03/060325232608.htm

> This is interesting... This would appear to knock more
> holed in the hypothesis that black and white vision is
> predominant in mammals because they started out as
> nocturnal animals in order to avoid competition with
> dinosaurs

No, actually it doesn't change much of anything in that regard.

First some context...  The molecular biological evidence that at least
four cone pigments is ancestral for terrestrial vertebrates is about
as good as you can get in reaching any scientific conclusion.  The
vast majority of placental mammals have only two cone pigments and are
nocturnal or crepuscular.  The molecular biological evidence relevant
to primates compellingly indicates that we have three pigments because
of a relatively recent gene duplication.  Put all of this together and
it is clear that somewhere between the origin of Tetrapoda and the
origin of Eutheria, two cone pigments were lost.  Whether the loss
occured prior to or after the split between Eutheria and Metatheria
has always been an open question.

Marsupials have not been adequately studied, but it has been known for
quite some time that they are not generally the same as placentals in
terms of retinal anatomy.  For instance, look at what I wrote for my
Paleontologia Electronica paper published in 2000:

     the MRCA of all tetrapods would have to have been diurnal, and
     metatherian and prototherian mammals must have been more active
     under the Mesozoic sunlight than their eutherian cousins who were
     apparently forced to spend their days sequestered in shaded
     hideouts

As for: "the hypothesis that black and white vision is predominant in
mammals"...  That is not a tenable hypothesis and really never has
been.  The number of mammals that, to the best of our ability to tell,
have no ability to discriminate lights based on wavelength content as
well as overall intensity is rather small.  Marine mammals (cetaceans
and pinnipeds -- due to convergence), procyonids (raccons), and owl
monkeys are the only ones I remember off the top of my head...

> The apparent ability to see in the UV spectrum is interesting... Any
> thoughts as to what advantage this might serve?

While common, the above looks to me like a very parochial
misconception.  Near as I can tell, the only reason people think UV is
special is because people don't see UV.  It's just another part of the
spectrum*...  Evidence from molecular biology is relatively clear that
UV sensitivity was the ancestral condition for Tetrapoda (see papers
by Yokoyama et al).  Many lineages simply retained it.  Others (e.g.,
birds) appear to have lost it as their sensitivities shifted toward
longer wavelengths and some of those (again still talking birds)
re-acquired it as their sensitivities shifted back to shorter
wavelengths.

For a partial treatment of the issues of UV, I recommend:

Kevan, P.G., Chittka, L., and Dyer, A.G. (2001).  "Limits to the
     Salience of Ultraviolet: Lessons from Colour Vision in Bees and
     Birds", _Journal of Experimental Biology_, 204:2571-2580.

"T. Michael Keesey" <keesey@gmail.com> wrote:

} It looks like only one marsupial species has been investigated:
} _Sminthopsis crassicaudata_, the Fat-Tailed Dunnart. Isn't it also
} possible that trichromacy is a synapomorphy of some metatherian
} subclade that includes _S. crassicaudata_?

Yes, and it's quite possible that trichromacy is the ancestral state
of all marsupials.  However, it is not the state of all marsupials.
Another apparently trichromatic marsupial is the honey possum
(_Tarsipes rostratus_).  But returing to "however", the tammar wallaby
(_Macropus eugenii_) and the New World oppossum (_Didelphis_) have
been investigated and both appear not to be trichromatic.

} How many mammals have been investigated, anyway? I find it
} surprising that this is the first marsupial to be investigated.

When we reviewed the relevant literature three years ago, the
marsupials I mentioned above were all we could find:

Jacobs, G.H. and Rowe, M.P. (2004).  "Evolution of Vertebrate Colour
     Vision", _Clinical and Experimental Optometry_, 87:206-216.

You should be able to pull that down for free at:

http://www.optometrists.asn.au/ceo/backissues/vol87/no4/3268

Note that it's an Australian journal, so we can't have missed any
marsupial studies and gotten away with it :-)

-- 
Mickey P. Rowe     (mrowe@lifesci.ucsb.edu)

* Granted there is a distinction to be made between ionizing and
  non-ionizing radiation, but the photochemical reaction used by
  animals to transduce UV into a visual signal is non-ionizing.  It's
  the same reaction used by animals to transduce "visible" light into
  visual signals.