From: pmb2@ellis.uchicago.edu (Peter Bell)
Subject: Re: Color vision in snakes?
Reply-To: pmb2@midway.uchicago.edu
Organization: Yale Uni
Date: Wed, 6 Jan 1993 18:48:46 GMT

In article <1993Jan6.165436.5466@sci.ccny.cuny.edu> christ@sci.ccny.cuny.edu (Chris Thompson) writes:
>A while back, during the thread discussing various snakes' gustatory
>preferences, several people asserted that snakes posessed color
>vision, as evidenced by the fact that they would only eat certain
>colored prey.

given that the discrimination in question was between black and white, the 
color vision issue was separate.  

>  I checked this out with a herpetologist friend of mine,
>and also received confirmation on a snake special on The Discovery 
>Channel the other night.  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.

Either you don't understand what we were on about, or your friend did not 
understand your explanation.  Ask him if snakes have good enough vision to
discriminate white from black.  While it is possible that snakes in low 
light do use heat signatures due to differential transmission characteristics
of hair, ONLY THOSE SNAKES WHICH ARE SENSITIVE TO HEAT COULD DO THIS.

Curiously enough, in one group of those  snakes that have been studied 
the map of space perceived by the heat organs (defined by recording central 
electrical activity while presenting an infrared stimulus) is precisely 
aligned with the map of space perceived by the eyes (defined again by 
making electrical recordings in the visual portions of the central nervous 
system while presenting visual stimuli).  The fascinating physiology here is 
subtended by an anatomical system in which the cranial nerve bringing in infor-
mation from the heat organ brings its information into register with the 
visual map starting at the level of the optic tectum and converging again at 
the level of visual cortex (if I remember correctly, the retinogeniculate
projection, in parallel with the retinotectal projetion, is not paralleled
by input from the heat organs, which catches up again through the 
tectal projections to the cortex.  Thus, the geniculocortical path
is exclusively visual while the tectal path is mixed).
 
>  Snakes
>have very limited vision as we define it,

Nonsense.  Simple and utter nonsense.  they have areas of greater and lesser
visual acuity, a "foveal" region within the retina, etc.  They have both rods
and cones as photoreceptors, and I believe they have, as do turtles, oil drops
in their cones.

> and rely much more upon
>the data gathered by the vomeronasal organ, which senses heat only.

Did your herp friend tell you this, or did the TV?  whichever,  you were not
awake.  The vomeronasal organ primarily detects chemical odorants brought
into contact with it by the motion of the tounge through the air and then onto
its sensory mucosa.  It is an exquisitely tuned gustatory organ, but 
not much for sensing heat, as it is buried within the skin of the 
face and much of it inside bone as well.  

The heat organs are the small pits in Crotalinae (rattlesnakes) and Boidae
(boas and allies) and probably a few groups I don't know off the top of my
head.

>They can (obviously) also detect ground vibrations from quite a ways
>off.
>
>Chris Thompson
>
>-- 
>(Help!  I've fallen and I can't reach my beer!)

Sorry if some of this has bordered on flameage.  The next to last statement,
about the vomeronasal organ detecting heat, woke me up......

Peter
bell@minerva.cis.yale.edu (posting off another account for now)

reference:  Dorsal Ventricular Ridge, Philip S. Ulinski.  Wiley Interscience,
1986 or thereabouts.  An excellent review textbook of central sensory 
processing in reptiles and in birds, with extensive direction to the primary
literature.


Newsgroups: rec.pets.herp