Thanks to both Debbie and Ed for asking about ageing by teeth.
 
The idea of ageing an animal by it's teeth has been around for a very long
time; it is a standard method of ageing such animals as horses, sheep and
cattle.  The practice has given us such quaint expressions as "never look
a gift horse in the mouth," and "long in the tooth."  The technique is
actually a combination of three separate evaluations.  The first one is
ageing by dental eruption, the second is ageing by dental translucence,
and the third is ageing by dental wear.
 
Of the three evaluations, only the first one is accurate; that is, it is
based on testable data.  Like humans, all ferrets have a genetic "program"
which controls tooth eruption.  In animals which reach adult maturity in a
very short time, like ferrets, the tooth eruption sequence only varies by a
few days on each side of the predicted date of tooth eruption.  So, when
you see a ferret with "twin" canines (the permanent canine growing over the
baby canine, replacing it), you can be pretty confident the ferret is about
8 weeks of age, give or take 3 or 4 days.  Unfortunately dental eruption is
only a good technique for the first few months of life; after all the
permanent teeth have erupted, it is useless.
 
Dental translucence is based on the idea that the nerve canal and pulp
cavity of teeth are slowly filled with dentine (cement) over time.  This
happens to all mammals and is an evolutionary adaptation for tooth wear,
fractures, etc.  Microscopically, while the rate of filling is somewhat
variable (depending on biomechanical forces, nutrition, season, etc), there
is a seasonal aspect to the phenonmenon.  This allows the scientist to
extract the tooth, slice it, and then count the annular "rings." This is
accurate enough to be able to tell how many seasons the animal has lived
through.  Another way is to shine a powerful light through the tooth and
measure the amount of light which is absorbed (photon densitometry); the
more light absorbed, the older the ferret.  Not nearly as accurate, but
you can get to about a year of the actual age.  As far as I know, the
ferret is not well documented, but I would assume they would be similar
to mink.  Since slicing a tooth is not a technique recommended for living
creatures and few people actually own a pocket photon densitometer, we are
forced to evaluate the translucence of the tooth visually.  With a lot of
practice, this method is probably accurate to within 2 years.
 
The final method is tooth wear, which works on the assumption that as a
ferret ages, eating will grind down the teeth over time.  Well, I can tell
you for sure teeth wear, but what I can't tell you is how fast it takes,
or how old a ferret is.  Why?  Because for it to work accurately, each
and every ferret has to eat the exact same food, chew with the exact same
force, and have teeth identically constructed.  For example, suppose I am
comparing the teeth of a pet ferret from New York to the teeth of one of
my ferrets.  The NY ferret ate kibble their entire life, but the MO ferret
ate Bob's Chicken Gravy.  Now, assuming identical ages, which teeth will
have more wear?  The NY ferret by far.  My investigations suggest a diet
of kibble will wear teeth nearly twice as fast as more natural foods, an
observation also reported in the literature.  Cloth chewing, teeth
grinding, genetics, minerals in water and even grooming habits will
influence the degree of wear.  Thus, tooth wear is a very basic and
inaccurate method of ageing.
 
Having studied literally thousands of jaw sets, I can probably get with a
year or so of a ferret's true age, based on a combination of tooth wear
and dental translucence.  That ability is shared by many shelter people to
have peered into the gullets of countless ferrets.  It is unfortunate that
about the only way the technique can be taught is through OJT (On the Job
Toothing).  But you have to understand that even the best "tooth-agers" can
be as far off as one or two years simply because the technique is so full
of error.  That is just the way it is.
 
As for photos illustrating the translucence and wear, getting the picture
from a skull would be easy.  The photo would have to be an extreme closeup
with backlighting, and against a dark background.  You would need a scale
included, because even a millimeter difference would be almost impossible
to see in a photo.  But a living ferret?  Good luck!!
 
Scientifically, using such techniques to age an animal only results in a
simplified ordinal scale (items ordered, not quantified), such as "infant,
juvenile, young adult, adult, aged adult." The ability to accurately age an
animal from changes to the teeth, other than tooth eruption and cementum
annuli studies, has been recognized as ineffective by modern
zooarchaeologists for quite a long time now, and, except for using it to
create an ordinal scale, has been mostly abandoned (although many fish and
game people still do it).  So the good news is, you can do it as long as
you understand you are not really figuring out the "real" age of the
ferret, but instead are grouping them into classes based on tooth wear.
You will be frequently wrong.  C'est la vie.
 
Want to know my personal tooth aging trick?  People who have watched me age
ferrets see me open the entire mouth of the ferret.  What I am looking at
is not the front or cheek teeth, but the last tooth on the upper jaw; a
tiny little molar which sort of looks like the number "8" or two cones
connected by a narrow saddle.  I look at the saddle area, which is heavily
worn by kibble (it will actually wear through, leaving two root pegs in
it's place).  Since most ferrets I meet eat kibble, the molar is rarely
involved in non-food chewing, and the biomechanical forces at this position
are about the same in all ferrets, the wear is better correlated to time
than looking at wear in other teeth or the degree of translucence.  It's
been my secret for some time now.  Yes, I do use the other evalutation
methods, but only to get me in the neighborhood; its the top molar that
gives me the somewhat-maybe-perhaps-who knows accurate time frame.
 
Bob C and 19 Mo' Toothy Polehounds
[Posted in FML issue 2884]