From: rowe@pender.ee.upenn.edu (Mickey Rowe)
Newsgroups: rec.pets.herp,sci.bio
Subject: Re: Color vision in snakes?
Date: 7 Jan 93 23:44:55 GMT
Followup-To: rec.pets.herp
Organization: University of Pennsylvania

In article <1993Jan7.161214.24646@dvorak.amd.com>
   melanie@cayman.amd.com (Melanie Typaldos) writes:

>Over in rec.pets.herp we've been having a discussion on whether
>snakes have color vision or not.  

Last fall I put together a series of 4 posts dealing with the
evolution of color vision.  I'll e-mail them to anyone who's
interested.  I didn't deal much with snakes, because to the best of my
knowledge there isn't a whole lot of work that's been done with
studying their visual capacities.  I can point people towards some
papers about garter snakes--they're interesting because they have very
large pupils.  Apparently they have some fairly unique oil droplets on
their photoreceptors, and these oil droplets help to focus light down
the axes of the receptors increasing the snake's visual acuity
relative to what one might predict based on the rest of the eye's
optics.

As for color vision, I wouldn't be surprised to find that snakes
weren't very well developed there.  Current scenarios indicate that
the ancestors of snakes lost their legs when they went underground.
Vision is de-emphasized in subterranean environments (e.g. look at
moles).  Some snakes may have re-evolved some aspects of color vision,
but I doubt their vision would compare to that of fish and birds (or
even turtles--there is a great deal of information about the color
vision of emydids).

>Also, I remember hearing a while ago that it may not be true that
>most mammals lack color vision.  What's the current story on this?

Most mammals have some ability to make "color" discriminations.
Primates (like us) are better than most (or possibly all) other
mammals in this regard.  There are other animals (e.g. some fish,
birds and probably even emydid turtles) that are better than us
though, so don't go bragging too loudly :-)

>Hope you don't mind the cross-posting...

I wasn't aware of rec.pets.herp.  I'll have to come over and check it
out, thanks!

>----Melanie

>In article <1993Jan6.190722.22628@progress.com> gerard@progress.COM
     (Gerard Bras) writes:

>>christ@sci.ccny.cuny.edu (Chris Thompson) writes:

>>>...  While snakes do not posess color vision
>>>at all, they can PROBABLY detect shade differences by the varying
>>>amount of heat that different colored fur or hair emits.  Snakes
>>>have very limited vision as we define it, and rely much more upon
>>>the data gathered by the vomeronasal organ, which senses heat only.

Ouch.  No, the vomeronasal organ (also called Jacobson's organ) has
absolutely nothing to do with heat or vision.  The vomeronasal organ
is the sensory epithelium that lines the roof of the mouth of a lot of
animals (including some mammals), and is involved in the "smelling" of
non-volatile chemicals.  Snakes and varanid lizards "sniff" the air
with their tongues, and then push their tongues against the
vomeronasal organ to get an idea of what sorts of proteins are
floating around them.  

As Gerard said (in text I'm about to delete; I hope I got the
attribution correct), the heat sensors to which you're referring are
the labial pits in boids and pit vipers.  That's a snake of a
different color (sorry, I couldn't resist :-)

>>Snakes, have highly evolved eye structures, capable of both focusing
>>and dilation/contraction to control the amount of light admitted.
>>In fact, it the means of focusing that is the one characteristic of 
>>snakes which is totally unique from all other animals.  

Are you sure about this?  If I recall correctly, some boids have
multiple stenopeic pupils (i.e. when the pupils are open they're
vertical, but when they're stopped down they look like a series of
pinholes one on top of the other--Tokay geckos definitely have this
sort of pupil; look at one in bright light to see what I mean), but
the garter snake I referred to above has an immovable pupil if I
recall correctly.  I can look that up pretty easily if you want.

>>gerard

>Melanie (Melanie Typaldos)        melanie@cayman.amd.com  

Mickey Rowe     (rowe@pender.ee.upenn.edu)
Newsgroups: rec.pets.herp