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comparative color vision



Randy King <randyk@ims.com> writes (first quoting me):

>> The only fish that are likely to be completely colorblind are those
>> that live at great depths in the ocean.
>
> My refernces disagree with you on this point.  (Maybe I need better
> refernces?)

Yes, you need better references.  However, I will add one other group
of fish that tend to be completely color blind.  Those that live in
caves and have consequently lost all visual function.

> my understanding is that most are basically colorblind as in
> black-gray-white vision.

As I stated before, this condition is only probable for deep sea fish.
Oceans of evidence on a wide variety of species indicate that if there
is any significant amount of light in the fish's natural environment
then that fish has some capacity for color vision.

> The color in lures that fishermen use is mostly for the fishermen.

That I'll *partly* grant you because the Centrarchids (bass) that I
know about have limited color vision compared to us.  They have only
two cone classes and hence likely see the world something like a human
with red-green color blindness.  It's a bit more complicated than
that, though, because they can see a lot better than we can at longer
wavelengths -- the region of the spectrum we would call "infra-red".
A bright red lure will probably thus seem even brighter to a bass than
to you.

> Aquatic mammals tend to have better vision.  

You *definitely* need to get better references!  As a general rule
mammals (at least Eutherians) have the least facility for color vision
relative to other animal groups.  Ironically, recent reports have
indicated that some marine mammals are among the only animals known to
have absolutely *no* capability for making color discriminations.

> Both livestock and fish are awfully large groups, 

Actually livestock is a fairly small group (tens of species at most
I'd guess) whereas "fish" generally encompasses some number in the
tens of thousands of species.  Nevertheless comparative molecular
biology indicates that the most recent common ancestor of
Crossopterygian and Actinopterygian fish had four cone classes (i.e.
potentially tetrachromatic vision).  That split predates the evolution
of Tetrapoda (which derive from the Crossopterygia), so it encompasses
almost all extant fish.  Mammals lost two of those cone classes.
Primates re-evolved a third.  We've thus almost caught up to our
ancestors...  Many fish are known to have retained all four cone
classes, and very few are known to have fewer than three.

> I believe that cattle are also strictly black-white colorblind.

Sorry to gore another ox, but given the available data the most
probable condition for cows is that they (like most mammals) have two
cone classes and hence dichromatic color vision (as I stated in my
last message, their color experience is probably somewhat similar to
that of a human with red-green color blindness).

Nick Pharris wrote:

] If I remember correctly, some of the older studies that suggested
] that mammals (cats, in particular) were completely colorblind did
] not control adequately for differences in intensity in their color
] samples.

This is a problem with *all* color vision experiments, though it
generally leads to the false conclusion that an animal has an ability
that it may not have.  People naively think that if an animal can make
a discrimination between two things (i.e. it "tells" you that two
things look different) then the two things look different to the
animal in the same way that they look different to us.  But that's
just out and out wrong.  Two objects which look equally bright to us
will generally *not* look equally bright to any other animal.  Good
color vision experiments control for that difficulty.

] More recent studies indicate that many mammals do in fact see in red
] (well, technically yellow) and blue, like me :-).

It's probably escaped notice, but I've tried not to use color names
except when explicitly referring to human psychological experiences.
I'm going to steal something I wrote about six or seven years ago to
end this.  It also sort of addresses:

] Right.  Many extant dinosaurs actually see in four colors (red, green,
] blue, UV), correct?

Many (if not most) birds have four cone classes.  So do at least some
lizards, turtles and alligators.  As I indicated above about the
Actinopt/Crossopt split, this is the plesiomorphic condition (I'm also
now telling you a good bit of what was in my GSA talk...) for most
extant vertebrates (and hence all of the dinosauria).  However, I'd
recommend that you don't use the word "color" like Nick just did.  In
describing our own experiences, most people use the word color to
describe hue.  A color-normal human sees a lot more than three hues
(typically we talk of seven colors in a rainbow, but that's certainly
not complete since it doesn't note some strangenesses like that what
*could* physically be thought of as a dark yellow we perceive as an
altogether different hue... brown.)  So to end, my plagiarization of
myself:

-------------

Let's compare the color vision systems of two animals that both have
three photopic (e.g. active under bright illumination) photoreceptor
classes. One is the human, the other is the honey bee (specifically
the worker--I don't know how the other castes are endowed). Does
anybody here think that what a bee sees when it looks at a rainbow has
the same appearance as what we see? We'll ignore optical polarization
(which the bee is sensitive to and we're not) and focus on what we can
infer about "color" based on, among other things, our knowledge of the
bee's receptor classes. To begin with, at the inside of the rainbow
where the violet-appearing light fades off to invisibility for us, the
bee will still see more rainbow. On the outside, where we see red, the
bee would see nothing for although bees have an ability to see what
for us is UV, we have the ability to see what bees might call
infrared.

Now picture that rainbow: what you see appears to have discrete bands
of color. Don't for a minute think that those bands arise from there
being anything discrete about the radiation emanating from that patch
of sky. If you measured the radiation with a spectrophotometer, you'd
find that the wavelength of maximum intensity as a function of the
radial distance across the rainbow would decrease smoothly and
monotonically from the outside to the inside of the bow. The apparent
discreteness is an artifact of our photopigments (chromophore + opsin)
and the neural processing of our photoreceptors' outputs. The bee too
would probably see discrete bands (We can't ever really know how the
world appears to a bee, but given what we can infer from doable
experiments -- I actually chose the bee in part because its color
vision has been studied about as much as any other animal's, excluding
the human's -- the supposition that it would see discrete "color"
bands from a rainbow is reasonable.) However, just as the outer and
inner borders would be in different locations for us and bees (as
described in the preceding paragraph), the borders of each "color"
would be placed differently by the bee as well.

-------

Now I've really got to get to work...

--
Mickey Rowe     (mrowe@indiana.edu)