Eric wrote: "I'm afraid I don't have the actual citations, but I recall
that ferrets have zero color vision and do not have binocular vision,
whereas actual polecats have at least some color perception and do have
binocular vision."
 
Hey Eric!  There has been a lot of work done on this exact thing, most done
in the 60s and 70s.  Ferrets do have cones, so they can see some color,
mostly reds and blues.  BUT, they have a lot more rods, so mostly they
see in black and white and all shades of gray in between.  Comparative
anatomical studies have shown animals that tend to be most active during
dusk and dawn (crepuscular) tend to have some color vision, but mostly
black and white.  Reportedly, polecats see a little more of the reds than
ferrets, and up into the greens.  So there is a slight difference between
the two animals, but between you and me, I'm not sure WHY those changes
have occurred.  Perhaps it was random chance or a sort of founder's effect
during the domestication process, but no one really knows the "why"
ferret's color vison is less pronounced than polecats.  I don't think there
was human intent behind it; I think it was accidental.  Still, assuming the
domesticated ferret's ancestry lies with the European polecat, the change
in color vision is one of the pieces of evidence that the ferret is indeed
domesticated.  It would be interesting to see how the structure of the
steppe polecat retina would compare to the domesticated ferret.
 
Both animals have binocular (or steroescopic) vison to be sure.  Both
ferret and polecat eyes point forward and a little to the side.  Those who
are sensitive to animal issues, please skip down to the next paragraph.
No, really, please skip down.  You down?  Now if I get any hate mail,
I'll know you cheated.  No peeking!  Ok.  Out in CaCaLand, I helped teach
comparative animal anatomy and human anatomy for 6 semesters, so took apart
quite a number of mink and cats and people and other stuff.  I was an
expert at removing all the bone and tissue from a mink's head to expose the
eyeballs and optic nerves as they feed into the brain.  This was extremely
important for the students to learn these pathways and structural
relationships (all pre-med and pre-vet), as I am sure they would also
testify.  The eye attaches to the skull in specific areas, and the optic
nerve and vessels pass through the skull in very easy to identify
locations.  Once you learn these landmarks, you no longer need to see the
animal in the flesh to figure out the degree of binocular vision; you can
do it for the most part with cleaned skulls.
 
The point is, once you know the angle of the eye in the eyesocket, the
position of the retinal fovea, the position of the center of the pupil, and
the distance of the lens from the retina, it is simple to figure out the
area of coverage and the degree of binocular vision.  That is how they
figure it out for dinosaurs and other extinct animals.  The degree of
binocular vision (based on skull landmarks) is one of the measurements I am
doing in my ferret-polecat research.
 
Polecats have a slightly narrower field of vision compared to ferrets,
which means they have slightly better binocular vision.  I believe this is
due for several reasons (I say believe because I haven't tried to test it
yet).  First, the ferret brain is smaller that the polecat's, which is
probably the driving force behind the narrowing of the post-orbital
constriction found in ferrets.  Both the changes in brain structure and
corresponding skull shape can influence the eyeball angles.  Second, the
polecat eyeballs seem to be slightly closer together than in ferrets.
Again, this influences the angles of the eyeball.  Last, ferrets have a
skull that is more "juvenile" in proportions than polecats, which can
change a whole lot of eyeball angles.  While I know of a lot of research on
eye and angles, I know of nothing that explains "why" ferret stereoscopic
vision would be different than in polecats.  Its probably some combination
of the above reasons causing the changes.  The bottom line is, while
polecats have slightly better binoccular vision than domesticated ferrets,
they both primarily have binocular vision.
 
Bob C and 16 Mo' Fartin' Ferts of Freedom
[Posted in FML issue 2750]