Note: Due to the loss of my hard drive, the following questions are
not direct quotes, but paraphrases made from memory.  If my memory was
faulty, or for some reason I have failed to answer a question, please
feel free to email me and I will resolve the situation.
 
Q: Why does coat color affect vision?  Can you explain the difference
between human and ferret eyes a bit better?  Are ferrets really color
blind?
 
A: These questions are getting away from the topic of enriching ferrets,
however, understanding WHY a ferret is like it is goes a long way to
understanding why enrichment is so important.  It is also quite
satisfying to see so many questions asking for clarification and
elucidation of ferret physiology--it shows people really care about
their ferrets and want to make a better life for them.  I'll work in
some enrichment at the end of the answers.
 
Some argue the eyes are the only outwardly visible part of the brain,
developing out of the same embryonic neural tube that eventually gives
rise to the brain, spinal cord, and other parts of the nervous system.
Because of this, ANY defect in the neural tube at the very front part of
the structure can potentially cause problems with vision.  Neural tube
defects can impact virtually ANY neurological structure or function,
and many result in severe disease or the death of the individual.  It
is likely most defects result in the death of the kit prior to or
immediately after birth and are never recognized.  There are no data
that quantify the number of neural tube/crest defects, or the number
that impact vision.  It is unlikely most vets would recognize neural
tube/crest problems unless gross anatomical defects were apparent, or
the vet performed a detailed necropsy that included specific microscopic
studies.  Therefore, it is probable most neural tube/crest disorders are
vastly underreported.
 
Domestication has a significant impact on the development of structures
rising from the neural tube and neural crest (develops out of the neural
tube), changing the TIMING of migration for specific pigmented cells
(argued to be the origin of the differences between the vision of the
polecat and the ferret).  This change in timing also impacts the coat
color of the ferret, causing dilution effects (excluding breeding for
melanism, domesticates are lighter in color than their wild progenitors)
and piebaldism (white spotting and striping).  Domestication also causes
the conservation of traits, such as those seen in depigmented and albino
individuals, that would--in nature--result in the death of the individual
from various natural selectors.  For various reasons, albinism has a
marked impact on the function of the eye, not the least being that some
structures require pigments to work efficiently.
 
For this discussion, color changes in ferrets can be lumped into five
basic categories: albinism, depigmentation (dilution), hyper-pigmentation
(melanism), temperature dependant coloration (pointing), and striping or
spotting (piebaldism).  Albinism is an inability of the ferret to produce
pigment, and is not associated with the genes for coat color.  Depending
on the exact cause of albinism, an albino sable passes the genes for
sable coat color, and, if mated to a sable ferret that does not carry
genes for albinism, will produce sable kits.  Depigmentation is the
overall lightening of all or parts of the coat, such as with dark-eyed
whites, champagnes, cinnamons, and chocolates.  There can be a number
of reasons for the depigmentation, ranging from defects in pigment
production, loss of guard hair, overall reduction in the amount of
pigment produced, or reduced density of pigmented cells.  Melanism is
the opposite of depigmentation, and is (as yet) not seen in ferrets; the
only color coat coming close is the black sable, but that only approaches
the color of the wild polecat and is not really melanistic.  Eventually,
it is possible an all-black ferret can be produced, which if sold for
medical research can be called the, ah, "black lab".  Pointing, as seen
in Siamese coat colors, is temperature dependent pigmentation, with
cooler areas of the body producing darker coloration because more
pigments are produced.  White striping or spotting (piebaldism) is a
type of coloration most commonly associated with a suite of neurological
problems, including various types of hearing disorders.  Piebaldism
results in a continuum of coat colorations that in ferrets includes
blazes, pandas, and the like.  Commonly, other areas of the coat are
depigmented, such as the face, throat, upper chest, feet, knees, and
paws.  Because this type of coat coloration is associated with neural
crest disorders (piebaldism is thought to be caused by delayed migration
of specific pigmented cells in the neural crest), hearing, behavioral,
and other problems are common, but rarely is vision is impacted to a
significant degree.
 
Bob C
[Posted in FML issue 4256]