I was asked off-list if I would discuss the various methods of determining
the age of ferrets when their background is unknown.  Since I have been
grilled by an international expert on this very subject (and passed with
minimal sanguination), it is probably something I can address.  NOTE: never
ask a person who does faunal analysis for a living a question so intimately
connected to zooarchaeological method and theory unless you are willing to
be bored to the point of tears.  A lesson for us all.
 
There are three basic methods of ageing ferrets; those involving inspecting
anatomical landmarks and how they change over time, those which deal with
the physical and psychological state of the ferret, and those which use
disease-states specific to life stages.  The first method includes the
eruption, translucence and wear of teeth, morphology of the Os penis, lens
weight, cranial suture remodeling, bone growth and diaphyseal/epiphyseal
fusion, and to a certain extent, the amount of arthritis and bone wear
present.  At a microscopic level it also includes Haversian remodeling
and the layering of dental cement in specific teeth (dental annuli).  The
second method is concerned with judging a ferret's behaviors, physical
condition and general state of activity.  A third method is to look for
specific diseases which tend to present at specific ages.  Some of these
methods are accurate to a specific range of time (example: 4 years =B1 2
years), others can define broad categories (example: Neonate, Juvenile, or
Adult), and a few have no real value at all (example: A is older than B).
I will discuss each method from least to most accurate.
 
Note: this post is not meant to establish a chart of ageing criteria.
While I plan on posting such a chart in the future, I feel it is important
to understand the strengths and weaknesses of each method first, rather
than seeing the chart and assuming it was meant to be taken at absolute
value.  It is important to understand that all ageing techniques rely on
statistical averaging to work, but individual ferrets have a good chance of
being an exception.
 
To start off, ACCURACY is the ability to get the age correct, while
PRECISION is the ability to get the same age for similar criteria in
multiple trials.  If you ALWAYS overestimate a ferret's age by 2 years,
you are precise but you lack accuracy.  ABSOLUTE AGE is the actual age of
the ferret (example: 2 years), RELATIVE AGE is an approximation of life
stage (example: Neonate, Infant, Juvenile, Sub-Adult, Adult, Old Adult),
and GENERAL AGE is a broad age determination (example: Young or Old).  For
*THIS* discussion on ageing ferrets, I define a Neonate as a kit not eating
solid food (roughly 0-4 weeks).  An Infant is an unweaned kit eating some
solid food (roughly 4-6 weeks).  A Juvenile is a kit more or less weaned,
but not having all adult teeth (roughly 6-12 weeks).  A Sub-Adult has all
the adult teeth, but the bones are not mature (roughly 12 weeks to a year).
An Adult has mature bones, minor bone remodeling, "normal" bone density and
minimal wear of the teeth (roughly 1-5 years).  An Old Adult has extensive
bone remodeling, some loss of bone density, and moderate to major wear of
the teeth (roughly 5+ years).  You may not agree with these categories (one
arbitrary method is no more valuable than another), but they are useful
because they contain definable behavioral and anatomical markers useful
for the scientist as well as the lay person.
 
There are some diseases which tend to strike at certain life stages, such
as specific organ diseases, cancers, and cataracts.  From an evolutionary
perspective, such diseases occur late in life; that is, after prime
reproductive age, so they are considered to be diseases of old age.  While
there is oft-repeated anecdotal evidence some diseases tend to strike at
specific stages of life, I know of no scientific study which documents
such evidence with hard data (If anyone knows of such a study, I would
appreciate the information).  However, even if such data existed, they
would only be useful in determining two basic age categories: those ferrets
that are old and those that are not.  This method cannot be used to
establish a specific age because the onset of diseases of ageing is usually
so broad there is a considerable degree of error (having a wide deviation).
They generally strike when the ferret is old, which is why they are
more-or-less absent in wild populations where elderly animals are rare.
This often erroneously leads some to think the disease is one of
domestication or inbreeding; is the disease absent in the wild because it
only occurs in older animals which do not survive, or because of genetic
changes caused by inbreeding during domestication?  A serious complicating
factor is the tendency for diseases of age to rely on environmental
triggers to occur; some cancers being prime examples.  This results in a
great deal of variation (deviation), which lessens precision and accuracy.
For example, if the median age of the onset of a specific disease has been
shown to be about 4 years, but it can occur as early as 1 year and as late
as 7 years, then you would typically express the age as 4 years =B1 3
years.  True, MOST ferrets with the disease may be 4 years old, BUT, since
you cannot tell if they are 1 or 4 or 7, you have to assume the ferret's
true age lies within that range.  The problem is, that range of age (1-7
years) is near the entire life span of many ferrets, making the method
useless except in the most general of cases (Ferret A is probably older
than ferret B).  It has no real value in absolute or relative age
determination.
 
Many long-time ferret owners, shelter operators and veterinarians
intuitively use a ferret's behaviors, physical condition and general state
of activity to arrive at an estimate of the ferret's age.  For example,
a ferret that is very active, has good muscle tone and likes to play is
considered younger than a ferret with little interest in play, poor muscle
tone, and who sleeps a lot.  This is relative ageing, where the comparison
of one ferret to another allows you to determine the relative age of both
(younger vs older).  While I think that experience can easily assign
ferrets into age categories (Neonate, Infant, Juvenile, Sub-Adult, Adult,
Old Adult), there is a great danger in assigning specific (or absolute)
ages.  Why?  Because each determinant is greatly influenced by environment.
In other words, two ferrets of the same age might display different
behaviors, physical condition and general state of activity simply because
of degree of bonding, level of boredom, caging conditions, presence of
disease, or nutritional levels (to name but a few).  Case in point: caging
ANY animal reduces physical and aerobic conditioning, slows the mental
state and even reduces the density of bones.  Also, this method is more
accurate in younger ferrets because the life stages are so short compared
to to those of older ferrets.  For example, the neonate life stage only
spans a few weeks of time, but the adult life stage lasts for years.  This
means you can document a neonate to with a few weeks of their true age, but
can only document an adult ferret to within a few years.  From a scientific
point of view, this method is not very accurate in terms of actual age
(Absolute Age), but can be quite accurate in terms of life stage categories
(Relative Age).
 
The above methods rely on personal judgment in determining age categories,
so each determination is dependent upon the experience and knowledge of
the individual observer.  But observers can (and frequently are) fooled,
which means the results are only as good as the individual and may not be
reproducible.  The third method is an attempt to escape that problem by
introducing a degree of standardization to the observation.  For example,
when inspecting the upper canines, you can observe just a deciduous tooth,
both deciduous and permanent teeth, or just a permanent tooth.  These are
standardized observations which can be easily reproduced by different
observers AND they occur at approximately the same age in different
ferrets.  These observations result in absolute values; usually a median
age expressed with a deviation.  For example, if both the deciduous and
permanent canines were present, the age of the ferret would be expressed
as 8 weeks +/- 1 week (this is a scientific way of saying the age of the
ferret is probably 8 weeks, but can be anywhere between 7 and 9 weeks old).
While this method is very accurate (compared to general and relative
ageing), there is a serious problem; most techniques rely on changes which
occur while the ferret is still maturing.  Once mature, absolute ageing
techniques are limited, which means that they are most useful when the
ferret is younger than a year of age.  For example, if you saw a permanent
canine, all you really know is the ferret is older than 8 weeks, but not
HOW much older; it could be 8 years old.
 
There is another very important aspect of absolute ageing techniques which
is common in ferrets, but rarely investigated or considered.  That is the
impact on ageing techniques caused by early neutering.  Most absolute
ageing techniques rely on bone growth to fix age, but bone growth is
strongly influenced by sex hormones.  This is especially true with male
ferrets because of their high degree of sexual dimorphism.  What I have
discovered is early neutering has a minimal effect on the ultimate SIZE
of females, but it has a great effect on bone density and can delay the
fusion of long bones by weeks to months.  Male ferrets, on the other hand,
are seriously impacted by early neutering.  Their bones are smaller, fuse
slower and are less dense than in late neutered animals.  In most cases,
the skeleton of early neutered males are quite effeminate, and I have a
hard time telling the difference between the non-skull bones of early
neutered males and whole females based on morphology alone.  The end
result is a great deal of confusion has been injected into absolute
ageing techniques, which in some cases has made them impossible to use.
 
Odontology is the scientific study of teeth, and there are four basic ways
to use a tooth to determine a ferret's age (EAAT: eruption, attrition,
annuli, and translucence).  First, teeth typically erupt in a specific
sequence at a specific age.  For example, if you saw both a deciduous and
permanent canine in a ferret's upper jaw, you would know the ferret was
about 8 weeks old (=B1 1 week).  This method is easy to use (the tooth is
there or not), and it is pretty accurate in determining absolute age
(deciduous maxillary canines are replaced between 7 and 9 weeks of age in
95% of all ferrets).  The problem is, it is only useful for the first
12-14 weeks of a ferret's life, before the final teeth have erupted.
(Early neutering MAY cause a minor change in the timing of eruption"a
couple of days to a week"but not the sequence).  After that point, the
technique is useless.
 
Using tooth wear (dental attrition) to measure age is based on the often
erroneous assumption that teeth wear down at about the same rate, and
that rate can be correlated to age groups.  Once erupted, teeth cannot be
repaired to any degree, so over time, they are slowly worn away.  There
are literally hundreds of references detailing how to use tooth wear to
determine the age of various animals, some even including drawings of the
teeth with dashed lines representing wear at specific ages.  While there
are a lot of game wardens who rely on such charts to age animals coming
through their checkpoints, the technique is actually quite limited and
prone to error.  Wear depends on multiple factors; total chewing, hardness
of water, presence of specific minerals in the diet, hardness and texture
of food, general nutritional state, and genetic variability to name a few.
Even individual behaviors can complicate the question.  Some ferrets will
chew cloth, grind teeth, or mouth the wire on their cages, and so forth.
All these factors (and more) greatly influence the wear rates on the tooth.
Of all the factors, perhaps diet is the greatest influence on tooth wear.
How?  Because eating hard dry foods causes teeth to wear down much faster
than those in ferrets eating animal carcasses.  There are a variety of
other problems related to using dental attrition to age ferrets, the least
of which is the fact that ferret teeth are SO small.  The difference
between a year and 3 years of wear may be a fraction of a millimeter; hard
to accurately measure even with electronic calipers.  IF the teeth being
measured belong to ferrets from the same family group, eat the same food,
and chew environmental objects about the same, THEN you can tell by tooth
wear that one ferret is older than another.  IF you then measure hundreds
of ferrets of a known age who belong to this same group, THEN you can write
a formula which assigns specific ages to specific amounts of wear.  When
that happens, you can probably judge a ferret's age to within a couple of
years.  Until then, about the best you can do is say one ferret is probably
older than another, even though you are actually saying one ferret has
teeth which are more worn down than another.
 
Teeth are designed to last a lifetime, which is easy to say since once the
animal's teeth wear down, they generally starve or are killed by predators
because of their weakened nutritional state.  Teeth will wear down to the
gumline and even the exposed tops of the roots can be used to grind food
(this is more common than you might think in artiodactyls like deer and
elk).  How is this possible when each tooth has a central pulp cavity,
complete with nerve, artery and vein?  Why don't animals need root canal
specialists as they get older?  Easy; as the tooth wears, biomechanical
forces on the tooth stimulate the deposition of dentine on the internal
walls of the pulp cavity.  Slowly, the tooth's nerve and blood supply is
withdrawn from the tooth and the canal is filled with dental cement.
Because this process is seasonal, it results in dental annuli within the
roots of the teeth (presumably it is a photoperiodic process, but it MAY be
due to seasonal differences in nutrition).  These dental annuli are sort of
like tree rings; you just count the number and add one to get the age.  The
trouble is, first you need to extract the tooth without damaging it, then
you need someone who can cut, decalcify, stain, slice and observe the root
to see the annuli to determine the age.  Not so easy in a living ferret
who still wants their teeth.  Some teeth are better than others in forming
complete annuli rings.  There is evidence that nutritional state, disease
and environment can greatly influence the formation of annuli.  While some
annuli research has been done on European and steppe polecats, I do not
know of any done for the ferret.  Dental annuli is currently the technique
favored by fish and game departments and is pretty accurate (within limits)
if done properly, but for the ferret owner it is pretty useless.
 
As associated method is to use dental translucence to determine age.  As
the dental annuli are deposited, the root canal is filled with dentine.
This changes the way light passes through the tooth, giving the appearance
of translucence to those areas filled with cement.  In ferrets, this is
best seen in the canines, where the tips seem to turn transparent.  Making
the assumption that dental annuli are seasonally deposited at about the
same rate, and that the farthest points within the pulp cavity are filled
first, it it commonly thought you can measure the degree of translucence
to determine age.  The problem is, if you look at the upper right and left
canine, translucence can be different between the two; one tooth might
yield an age of 3 years, while the other indicates 5 years.  This happens
because the internal diameter of the root canal is variable, even within
an individual.  Also, the deposition of dental cement is influenced by
the biomechanics of tooth use; heavy chewing results in thicker deposition
of cement, and ferrets"like humans"tend to favor one side over the other
for chewing.  Finally, factors such as nutritional stress, disease,
photoperiodism, and injury can all have a major influence on the degree
of translucence.  It's like this; unless you know the history of tooth
use, injury and individual nutrition (among others), you cannot rely on
translucence to determine age.  Its a Catch-22 situation; you can only
determine absolute age using translucence IF you know the tooth history,
but if you know the tooth history, you already know the age.  This
technique should only be used for relative ageing, and only then with the
awareness that it is inaccurate and imprecise.
 
One of the better methods of ageing males is the morphology of the Os
penis.  Most male mammals have a small bone within the penis which
partially surrounds and supports the urethra (contrary to urban legend,
it does NOT support the erection, but instead maintains the opening of
the urethra, preventing the tube from becoming kinked which ensures the
delivery of sperm).  In the ferret, the distal tip of this bone, commonly
called the baculum or penis bone, has a strongly deflected tip, forming a
hook.  There is a groove on the baculum that goes down the length of the
shaft, which supports the urethra.  The base of the baculum attaches to a
thick ligament, which is attached at the other end to the base of the pubic
bones forming the Os coxae (pelvis).  Bone increases in size as a result of
stress, so over time the base of the baculum gets larger, and will continue
to do so until the death of the individual.  This is a constant growth, so
if one baculum has a larger base than another, the individual is generally
older.  In some well studied species like the raccoon, the baculum can be
weighed and the age determined to within a year (X +/- 1 year).  Of course,
to weigh the baculum, you must have it in hand, which may cause some degree
of protest from the ferret.  But the base of the baculum can be measured
from X-rays, so it does have some use in living individuals.  Also, with
practice, you can use your finger to feel the size of the base of the
baculum and get fairly accurate results; I have an 90+% success rate for
determining bacular age within a year of the known age, based just on using
my thumb to determine size.  Simply find the midpoint between the anus and
the exposed tip of the penis, and place your finger or thumb on the abdomen
over that point.  You should feel a hard thin bone; that's the baculum.
Follow the bone towards the anus, and before you get there, you will feel a
thickened hard spot.  That is the base of the baculum.  If it feels about
the size of a match head (4 mm), the ferret is about a year old.  If it
feels the size of a baby pea (7 mm), the ferret is about 4 years old.  In
whole male ferrets, the base f the baculum seems to thicken between 0.5
to 1 mm per year.  The greatest problem is the effect of early neutering.
Bacula in early neutered individuals are much smaller and have tiny bases
in comparison to whole males.  Since no study has attempted to correlate
the size of bacula between whole and neutered ferrets, direct comparisons
are useless.  For those vets who take a lot of full body X-rays, this
method can hold promise once practiced.
 
As individuals age, the lens of their eyes change.  Over long periods of
time, the lens increases in density.  It is possible to weigh the lens
of the eye to determine absolute age, although I am unaware of any study
which has documented the age differences in ferrets.  This is not a typical
ageing technique, and one which cannot be effectively used by ferret
owners.
 
Bone may be hard, but it is a dynamic, living tissue.  There are two
major components of bone; a living matrix of collagenous proteins which
is impregnated with hard inorganic bone salts (hydroxyapatite).  Bone may
look solid, but it is actually full of holes, some so small they can only
be seen with electron microscopes.  Because all living tissue dies and
needs to be replaced, the bone you are born with is not the stuff you die
with.  In small, fast living mammals, old age occurs before the bone is
replaced, but in longer living animals, bone is replaced at more-or-less
a constant pace.  Ferrets are sort of at that dividing line, and it is
possible to age older ferrets by the amount of bone remodeling
(replacement) which has taken place.  The bone in ferret long bones are
organized into Haversian systems, which is remodeled in such a way that
allows for accurate age determination.  Unfortunately, the details of
ageing by Haversian remodeling haven't been worked out for ferrets, so
it remains a potential technique only.  Also, because it requires bone
sections, it is not useful for ferret owners.
 
As bone remodels, those places where two pieces of bone fuse together (at
the sutures or growth plates) will change.  Gaps are filled in between the
bones, and the sutures take on a worn or smoothed appearance.  The degree
of fusion and smoothing in cranial bones has long been used to determine
age in many species, but it is limited in ferrets because the skull fuses
at such an early age.  While this technique can be used with X-rays, for
the most part it is limited to visual inspection of skulls because only
with the inspection of the degree of fusion for multiple sutures is the
technique accurate.
 
The growth of bone is a common ageing technique, and one which is commonly
used in conjunction with X-rays.  It is especially useful when looking at
the length of specific long bones (tibia, femur and humerus) and the bones
of the pelvis, but other bones have limited value.  However, this technique
is limited to animals that haven't reached adult size.  Because the
skeleton of ferrets reach adult size at approximately six months of age,
the use of bone growth to age ferrets is extremely limited.
 
A better technique is to look at diaphyseal/epiphyseal fusion.  A long
bone, such as the femur, is not actually a single bone.  Depending on the
specific bone, it can be composed of three or more pieces which slowly
fuse together as the animal matures.  For example, the humerus has a medial
diaphysis, a proximal epiphysis, and a distal epiphysis; a total of three
bones.  The femur, depending on the species, can have a total of 8 bones.
It has two or three proximal epiphyses (one for the head, one for the
greater trochanter, and sometimes one for the lesser trochanter), a medial
diaphysis, and up to four distal epiphyses (sometimes the right and left
condyles are a single bone, sometimes two, often the right and left
epicondyles form separate epiphyses).  Between each epiphyses and the
corresponding diaphysis is a layer of cartilage called the growth plate.
As the ferret grows, the growth plates expand and ossify where it contacts
other bone.  When the skeleton is mature, the growth plates fuse, and
the diaphysis and epiphyses form a single bone.  At a year of age (+/- 2
months), the ferret skeleton is mature, BUT not all bones have completely
fused (some take years to completely fuse).  By looking at which bones are
fused and which are not, as well as the degree of fusion, you can determine
the age of the ferret to within a year or two.  This technique holds a
great deal of promise for vets who can safely determine age with a single
X-ray, but it has no real application to the average ferret owner.  The
problem is, while some work has been done to determine ages of fusion
for polecats, nothing major or statistically valid has been done for
domesticated ferrets.  And I know of NOTHING done on neutered animals (sex
hormones greatly influence bone growth rates, so they almost react like a
different species).  These are two of the things I am currently working on
in my skeletal research on ferrets.
 
The final method of age determination is to look at the presence and
severity of arthritis and bone wear.  Since these are highly variable
between individuals, about the best you can do is say one looks older
than the other.
 
The bottom line is, there are no magical formulae or techniques which can
effectively determine age in ferrets with unknown backgrounds.  The most
reliable and accurate methods involve invasive or destructive techniques
and cannot be used in living animals.  As for the rest, because of the
short lifespans of ferrets, any educated estimate is nearly as accurate as
the results obtained by the less invasive techniques.  For example, if you
know a ferret is older than a year because they are adult-sized and have
permanent teeth, and you know they are not excessively old because of their
demeanor and level of activity, then you have already aged them between 1
and 5 years old.  In terms of relative ageing, that means they are "Adult"
and your guess is probably quite accurate.  In terms of absolute ageing,
then you could say they were 3 years old =B12 years, and you have probably
made a reasonable determination.  You just can't get better results with
most of the noninvasive ageing techniques.
 
Ferrets have a different ageing curve than humans.  Initially, they mature
rapidly, then go into an extended period of slow change, then a rapid
deterioration.  This results in an ageing curve that has a very rapid rise,
a long, slowly falling plateau, and then a rapid fall (although not nearly
as rapid as the initial rise).  The more rapid the rise and fall and the
more level the plateau, the harder it is to age the individual.  Because
of this, ferrets hide their age diligently, making any ageing technique
problematic.  The most accurate and precise way is to use as many of the
ageing techniques as possible, then average the results.  Or, if you have
read this mini-dissertation closely, you can simply make the best guess
possible and realize you are as close to the mark as most of the
non-invasive techniques.
 
Bob C
 
This is an addendum to the large post which I also sent to Bill's
address in case the server spits it back at me.
 
 ---
 
Sorry, my joke was ruined by my word processor and I didn't notice until
too late.  Instead of "sanguination", I should have had "exsanguination."
Somehow the "ex" was stripped off, making me look foolish.  Not that I
can't do that well enough on my own.  If you cut a zooarchaeologist, does
he not bleed?
 
One aspect I didn't mention was a short discussion on why it is so easy
to age humans compared to ferrets.  The reason is actually related to
lifespan; when the lifespan is eighty years, an age determination having
a five year range is insignificant, but when you only live eight years,
that same five year range is more than half the lifespan and becomes very
significant.  Consequently, it is extremely difficult to age animals with
short lifespans with any degree of accuracy.  Assuming an eight year
lifespan for the average ferret, it is at about the point where precise
and accurate age determination is somewhere between "difficult" and "its
giving me a migraine," especially when using noninvasive techniques.
 
Over the next couple of days, I will post some photos showing cranial
suture remodeling, fusion, dental attrition, and bacular changes for the
FHL archive.  I may even draw a diagram of the ferret's ageing curve if I
can find the time.
 
Bob C
[Posted in FML issue 3406]