🚀 go-pugleaf

In article <1992Nov16.193918.19728@u.washington.edu> jespah@carson.u.washington.edu (Kathleen Hunt) writes:

>One of my physiology texts here says "Color vision has been demonstrated
>in all classes of vertebrates", but I think the only class it is
>really widespread in is the birds.  

No, it's very widespread in teleost fish as well.

>Most birds have superb color vision, with
>tons of cones, visual pigments, and special colored oil droplets for 
>contrast, in addition to UV sensitivity.  Gill's "Ornithology" text says flat
>out "The richness of avian color perception is probably beyond that of 
>human experience."   

Definitive studies haven't really demonstrated this yet, but judging
from retinal anatomy this is a good guess.  I can point to a few
papers on color vision in pigeons if anyone is interested.  Pigeons
have a variety of pigments in their oil droplets and in their cones.
The bizarre thing is that different pigments are localized in
different retinal regions.  Throw in the fact that pigeons have two
fovea in each eye...  The only thing it seems reasonable for me to
conclude is that the visual world of the pigeon resembles ours not at
all.  Many other birds have similarly strange retinas (and don't even
get me started on the subject of the perception of optical
polarization :-)

>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).

>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.

>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.  

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.

>  Lizards may have good color vision, as they have
>a lot of cones in their retinas.  

A little bit of care has to be taken with the cone/rod distinction,
since there is evidence that the two photoreceptor subtypes
"transmutate" into each other.  For example the "rods" of the banded
gecko are most likely homologs of the "cones" of other vertebrates.  I
think there are animals where the opposite is true--perhaps
sharks--but I can't recall for sure off the top of my head.

>I don't know about amphibians.

I don't know as that anybody knows a whole lot about amphibian color
vision, but the tiger salamander has three cone classes, including one
with UV sensitivity (peak wavelength something less than 400 nm).  

>Many animals can detect the plane of polarized light, most famously the
>honeybees (navigation, the waggle dance, all of that), but also horseshoe
>crabs, octopi, some fishes, pigeons, lizards, & more.  Seems to be important in
>orientation in a lot of animals.

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.

>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.  Boid
snakes have independently evolved a similar set of receptors, however.

>Stop me or I'll start babbling about homing pigeon experiments and bee
>cognitive maps and toad movement detectors.  :-)

Me too.  I want to babble about one other thing, though :-)

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.

>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.