🚀 go-pugleaf

>[color vision  widespread in teleost fish as well as in birds]
Thanks, I wasn't sure how widespread it was in teleosts (I knew many 
teleosts had color vision but not if it was very common or not.)

>>Mammals on the other hand usually don't have very good
>>color vision -- especially nocturnal mammals.  Diurnal mammals, like squirrels
>>and primates, especially fruit-eaters, tend to have very good color vision
>>but a lot of the other mammals have some poor excuse for color vision or
>>none at all.  
>
>Actually squirrels don't have really good color vision, but they do
>seem to be better at making "color" discriminations than many other
>mammals.  However this impression might be due in part to people
>looking for color discriminations in the wrong wavelength bands.
>Within the past couple of years, evidence of UV detection has been
>discovered in three different rodents (mice, rats and gerbils if I
>remember correctly).
Ah, thanks for the clarification.  One of my texts (I think it was 
Schmidt-Nielsen) gave squirrels as an example of a diurnal mammal with
a fairly high concentration of cones and therefore probably good color
vision, but didn't give any more details.

>>Usually careful studies reveal *some* sensitivity to color but
>>it's usually not comparable to what we're used to.  For instance cats can
>>distinguish pink from grey, but that's about it. (Of course they can 
>>distinguish all different shades of grey, too.)
>
>Apparently it's difficult to get cats to make discriminations based on
>"color" even though they have all the necessary ingredients for it.
What do you mean "necessary ingredients"?  Do they have a high 
concentration of cones?  I'm curious...

>>A general rule of thumb for figuring out if an species has color vision is,
>>do they eat brightly colored things (fruits, flowers) or communicate with
>>each other with color in some way (bird plumage, lizard dewlap
>>displays)?  In other words would color vision be useful for them in some way?
>
>Careful on the plumage thing... I take it you've heard of the pitohui
>by now?  Bright coloration also signifies "I'm poisonous; don't eat
>me!"  Since distinctive coloration is nearly ubiquitous in poisonous
>animals (and plants?) one might argue that color vision is useful to
>just about *any* animal.  

I was especially careful to say "communicate with EACH OTHER with color"
for exactly this reason.  Obviously there are many animals that are
brightly colored for entirely different reasons than communication with
each other -- most notably, bright coloration to advertise toxicity, and
bright coloration as some form of camouflage (e.g. green caterpillars on
leaves).  There is no particular reason to expect such animals to have
color vision.  What I had in mind, instead, was animals such as sexually
dimorphically colored birds, in which the male (usually) has bright colors 
that he displays in some way that has behavioral significance for conspecifics. 
An example would be the red-epaulet display of male red-winged blackbirds.

At any rate, I only meant the "do they need color vision" argument as a
rough rule of thumb, to get people thinking about the costs & benefits of 
color vision.  Clearly, to really demonstrate color vision you need to do 
a lot more than see if the animal has colored feathers or eats fruit.

And since you mentioned the pitohuis.... One thing that immediately leapt
to mind when I read that paper was "Are these birds monomorphic?"  You'd
think that both males and females would want to advertise their toxicity,
so that both males and females would have the bright coloration.  I can
imagine, though, that females might still end up duller during the 
breeding season to keep the *nests* cryptic (since I doubt the eggs or 
nestlings would be toxic.)  (Assuming that only females incubate.  If both
sexes incubate, again, they should be monomorphic.  Unless sexual selection,
and toxic advertising are *both* affecting plumage.)  Anyway, it was an
interesting issue that they didn't address at all -- they didn't mention
whether any of the three species were dimorphic in coloration, and didn't
mention the sex of the adults they captured and analyzed.  I'll check out
"Birds of New Guinea" as soon as my advisor remembers to bring it in...I
have a hunch they're monomorphic but who knows.

It was the prettiest cover Science has had in a long time, IMHO. :-)

>Also be careful about what "brightly colored" means.  To us there are
>a lot of drab white flowers.  To bees there are some drab black ones
>(e.g. roses that appear red to us) but probably no drab white ones.
>Those that appear "white" to us have interesting patterns of
>reflection of light with wavelengths too short for us to perceive.

Yes, this is a good point.  It turns out this may also be true of
"monomorphic" birds that are hard for humans to sex -- some birds are
now known to have UV patterns in their feathers that presumably other
birds can see.  It is possible (just possible -- I know there are some
people studying this but I'm not sure what the results have been) that
apparently "monomorphic" birds may have sexually dimorphic UV plumage
patterns that are perfectly obvious to conspecifics.  Has anyone heard
anything more recent about this work?    Hmm, I wonder if this could 
dovetail into the toxicity thing -- there could be toxic insects out there
with dramatic UV patterns, that look fairly drab to us but are "colorful"
to avian predators.  Someone must have looked into this...?

>
>>Many animals can detect the plane of polarized light...
>
>I *asked* you not to get me started about that :-) Polarization vision
>is well established in crabs (including the non-crab horseshoe crab
>:), octopi and a variety of other invertebrates.  Vertebrate
>polarization sensitivity is somewhat more problematic.  Aside from
>anchovies, polarization vision in fish is probable, but not well
>established (Please *PLEASE* don't get me started on that :-).  In
>pigeons it may well be that previous reports of polarization
>sensitivity result only from artifacts of the stimulation paradigms.
>That's the direction that the jury is currently heading.  I have
>independent reasons for anticipating that polarization sensitivity
>will be found in other birds, but the pigeon case is iffy.

Go ahead, get started on it!  :-)  I'd like to hear your thoughts
on avian polarization vision particularly.  What's the deal with pigeons,
and why do you expect other birds to have polarization sensitivity?
And -- dare I ask -- what's the deal with fish?

>>Rattlesnakes & their relatives have directional sensitivity to infrared
>>(heat) radiation, so in essence they can see in infrared.    
>
>That's not fair because their sensors are not in their eyes.  

Well, that's why I said "in essence"...I know, I know, they just use "pits",
not their eyes.  But to me, directional sensitivity to EM radiation is the
essence of vision.  So you could make a case for the pits being a type
of "eye".  Please don't push me on this analogy or it'll fall apart. :-)

>The one place where color vision does not seem to be worth the effort
>is in nocturnal animals.  In fact it is most likely because of this
>that elaborate color vision isn't well represented in mammals--during
>the mesozoic era, mammals were restricted to a few nocturnal lineages
>and this is reflected in the visual systems of those currently extant.
>
>My impression is that previous misconceptions about "the ladder of
>life" have lead people to think that because mammals don't have much
>by way of color vision, color vision isn't widespread at all.  In fact
>quite the opposite is true.

Good point.  Another example of mammal-centrism -- anything mammals do,
or don't do, becomes the "standard" by which people think about *all*
animals.

>>Kathleen
>
>Mickey Rowe     (rowe@pender.ee.upenn.edu)
>
>P.S.  I'm working on some posts outlining the evolution of color
>vision.  They'll be posted to talk.origins when I complete
>them--e-mail me if you'd like a copy mailed to you directly.
>

Yes, please send me a copy.

Kathleen


-- 
Halfway round the course, up spoke the noble rider:  
 "I fear we must fall back, for she's going like a tiger."
Up spoke the noble horse:  "Ride on, my noble master,
  For we're halfway round the course, and now we'll see who's faster."