I have been asked what is the difference between a domestic ferret and a
wild polecat.  Because it is of interest to all of us, I thought I should
post the answer on the FML.  This is actually quite difficult to answer, and
it can be answered on a variety of levels.  I will do my best with such a
short answer.
 
Taxonomically speaking -- there is no difference.  Both are Mustela
putorius.  When you see 'furo,' it is a subspecies designation, meaning
there is some difference between the group in question, and the rest of the
group, normally geographical or morphological differences in populations.
However, they are still the same species.  Subspecies labels usually
designate different "races" within a group.  I find the subspecies
designation to be useless in domesticates because it is so misused.  For
example, when F&G wants to protect a species it will say that a particular
subspecies is endangered (Cottontop Tamarians, Florida (Everglades) Puma,
etc.), treating the subspecies as almost a separate species.  However,
species it wants to keep out loses such designations; the polecat and
domestic ferret become the same thing.
 
Paleontologically speaking -- the relationship is unproven.  Without the
skull, it is virtually impossible to tell the difference between many
different species of animals, such as mule deer and white-tailed deer, seals
of similar size, and fisher and pine marten.  Polecats and ferrets are so
similar in their post-cranial skeleton it becomes impossible to distinguish
one from the other.  As for the skull, the closer you come to the point when
ferrets were first being bred from polecats, the harder it is to tell the
difference.  You wind up calling the remains "Mustela sp." or "Mustela cf.
putorius." It is very difficult to identify early domesticates (if not
impossible!) Add to this the problems of sexual dimorphism (males being so
much larger than females) and other closely related species (mink, fisher,
marten, etc) and its a real mess.  Also, most bones are found broken.  There
is so much overlap in size and shape, many times bone identifications are
made using distribution charts rather than morphology.  The further you go
back in time, the harder it is to distinguish one from the other.  The
result is a "lack" of evidence even though there is certainly a close
connection.
 
Genetically speaking -- they are very similar; nearly identical.  I am not
aware of any published accounts comparing the genome of the European polecat
to that of the ferret, but I would expect a 99% or better compliance between
the two.  Does that mean anything?  Chimps and humans are 96-97% genetically
similar, and dogs are at least 99% identical to wolves, so a very small
difference can be quite important.  Also, closeness in genetics does not
necessarily mean the ferret is a domestic form of the European polecat.  The
ferret could have come from a now-extinct close relative of the European
polecat.  In addition, there is some evidence that the genetic makeup of the
domestic ferret shows variation in the number of chromosomes present, which
muddies the waters.  Without strong or compelling supporting evidence,
genetic claims alone are circumstantial, forcing such save-yer-butt terms
like "probably," "most-likely," and "the evidence seems to indicate."
 
Morphologically speaking -- there are some major differences.  Skull shape
is different, base of skull is different, teeth are more crowded and
numerically variable in the ferret, and the orbital angle is different.  The
internal structure of the eye is different, and there is some suggestion
that there are differences in the structure of the brain.  Coat colors,
texture, and durability are different.  Sound location is different.
 
Behaviorally speaking -- there are some major differences.  While there
exists a commonality of behavoral expression, the degree of that expression
is different.  Ferrets are gregarious, polecats are solitary.  Ferrets will
share space with other ferrets, polecats are very territorial (in a natural
state).  Ferrets tend to be more juvenile in behavior compared to polecats.
Most differences are not in TYPE of behavior, but of DEGREE of expression.
The same can be said for different species of polecats, so behavior does not
prove speciality.
 
Domestically speaking --they are different species by the same rules that
make dogs, cats, goats, etc, different species.  A domesticate has been
controlled or adapted by humans to be used for work, food, or companionship
(pets).  Scientific nomenclature classifies pigs, rabbits, horses, llamas,
camels, ferrets, ducks, geese, and chickens as the same species as their
wild kin, but fails to do so for cats, dogs, cows, goats and sheep (short
lists).  All are clearly domesticated, but the same rules of nomenclature
should apply to all.  If the ability to go feral is a requirement for
separate species status, then consider this.  Of all the animals listed
above, all have formed feral populations in island ecosystems, and all but
one have formed feral populations in mainland ecosystems.  Domestic ferrets
have not established scientifically verifiable feral populations in any
mainland ecosystem that I can document.  Furthermore, the feral fitch
populations that do exist in island ecosystems were expressly and
artifically established by humans.  Finally, it has not been reliably
demonstrated that the feral populations were in fact pure-blooded domestic
ferrets; they could have been hybrids, which changes things considerably.
 
Reproductively speaking -- they are very similar.  While it is true that
polecats and ferrets can interbreed forming viable offspring, that alone
does not prove they are the same species.  Wolves can interbreed with
coyotes and domestic dogs, and form fertile offspring, yet they are
classified as separate species.  Different species of felines can likewise
interbreed forming fertile offspring, as can cattle and bison (different
genera), mule and white-tailed deer, most polecats, and many, many others.
Ernst Meyer says such interbreedings are mistakes, otherwise the two species
would merge into one.
 
Specifically speaking -- they are very similar.  However, even when two
different species share the same genetic make-up, can successfully
interbreed forming fertile offspring, but fill different niches or live in
different geographic areas, which form a barrier to reproduction, they can
be classified as separate species.  The designation is supposed to show
reproductive isolation has taken place, and that speciation is occurring.
While the domestic ferret and the European polecat are clearly of the same
genus, it has yet to be demonstrated that they share the same species
designation.
 
Bob speaking -- it is my opinion that domestic binomials are all screwed up,
with one set of rules naming one group while another set names the other
group.  (BTW, this is the key to why some states classify the ferret as
domesticated and others classify it as wild.) I personally would like to see
all domesticates take the binomial of the species of origin, with a
subspecies designation to indicate domestication.  Thus dogs would become
Canis lupus domestica (or familiaris), cats would be Felis sylvestris
domestica/catus, horses would be Equus caballus domestica, and ferrets would
be Mustela putorius domestica/furo (I prefer domestica over any other
designation for the clarity it provides.)
 
Also, and I admit this is a small trick, I use the term "feral fitch" to
designate feral ferret populations, such as in Britain and New Zealand,
reserving the term "domestic ferret" for pets and hunting animals.  Those
involved with ferrets know I am speaking of the same animal, but others may
not.  The idea is to create a pyschological separation between the two
populations, so they are seen in different lights.  Let the uneduated
beware!
 
One last thought.  Just because one authority cites a specific binomial
doesn't mean all do.  There is considerable disagreement at times.  For
example, the relationship between bobcats and lynx has been described as
Lynx lynx and Lynx rufus, Lynx lynx lynx and Lynx lynx rufus, Felis lynx and
Felis rufus, and Felis lynx lynx and Felis lynx rufus.  Which one is correct
if all are in use?  Ground sqirrels had the genus name "Citellus," but it
was changed to "Spermophillus." Pick up Petterson's "Field Guide to
Mammals", and you will find the old name in use, then pick up Hall's "The
Mammals of North America," and you will find the new name in use.  Which one
is right?  Hall is, but tell that to Burt and Grossenheider who wrote the
Petterson's Guide.  I say, lets use the designation of Mustela furo, cite
the experts who use it, and if anyone argues, say, "Prove they are the same
species!"
 
You can see the issue is NOT very clear, mostly because no one has studied
the relationships between polecats and domestic ferrets in a depth
sufficient to fully answer the questions. Someday, these questions will be
answered, but as for now?....
 
Bob (with one "o"--the other was removed)
Moose, Stella, Daye, Tori, Bear, Apollo, Foster, Buddy, and Razz.
Moose says, "All I know is I want some jerky..."
[Posted in FML issue 1390]