Q: "...When trying to determine [my ferrets] ages, [my vet] said that
you told him that the method used to determine age is not really
accurate, that there is no good way to tell age. I know everyone goes
by the translucency of the teeth, etc, but according to [my vet] you
said that wasn't really accurate. Thus I just wanted to hear it from
you, that that's true? You can't really determine the age of a ferret
by their teeth? Or did I misunderstand something?"

A: We all misunderstand comments from time to time. I once
misunderstood the phrase, "bite me," to mean something positive.

Your vet did not misunderstand. There are two generalized ways to tell
a ferret's age. One set of methods uses invasive techniques, such as
directly examining the skeleton and individual teeth, cutting them,
and looking at various structures under a microscope. These require
invasive laboratory tests and are not suitable for use in living
ferrets. The other set of methods uses non-invasive techniques, such as
x-rays of the skeleton, the visual inspection of teeth, body condition,
and even of the behavior of the ferret. It should be obvious that the
invasive methods provide a better determination of age than the
non-invasive techniques. Even so, MOST techniques -- even the best -- 
can only be as accurate as plus/minus one year, provided everything
goes correctly.

What that really means is that you are determining a range of ages, not
an exact age. For example, if your lab test said the ferret's teeth
were 4 years of age (+/- 1 year), then you are actually saying the
ferret is from 3 to 5 years of age: (4 - 1) to (4) to (4 + 1). Labs
that do tooth ageing will provide an error rate based on their overall
performance, but it is hard to get that quality of work done on a
living ferret, and they just might miss their teeth.

Non-lab tests have a larger error rate than the results from a good
lab. These tests look at things that change over time, such as the
deposition of cement and/or dentine on the exterior of the tooth and
within the pulp cavity. Dental translucence is based on the phenomenon
that canines in older ferrets allow light to pass through the tips more
easily, making them seem more transparent. This is because the pulp
cavity has aged and has been filled with dentine that transmits light
better than the blood and tissue that originally filled it.

Because the filling rate of the pulp cavity tends to be seasonally
uniform, dental translucence can actually give a good age estimate,
but not the way it is done by most ferret people. In one technique,
the tooth is cut into sections, sanded down to a specific thickness,
a light with a known luminescence is shown through the slice, and a
sensitive and calibrated light meter is used to read the transmitted
light. That process gives you a result that is generally considered
to be plus/minus 1 year in most quality labs. In shelters, the ferret
is generally held up and the tooth studied with the naked eye to make
an age estimate. There is no control for observer training, light
intensity, size of the tooth, way to read the light, or even the angle
of the tooth to the light, so the technique is marred by a high error
rate.

Even if a person was a really, really good estimator of age using tooth
translucence, the error rate would probably be at least 2 years, and it
could be up to 3 years or more (most people don't test themselves with
large numbers of unfamiliar ferrets of a known age to judge their error
rate and range over multiple observer conditions, so they can't give an
accurate rate of error). What that means is instead of having a 3-year
age range, it is now significantly longer (4 +/- 1 = 3 to 5 years), (4
+/- 2 = 2 to 6 years), and (4 +/- 3 = 1 to 7 years)! Think about it; if
you are good enough to have a 2-year error rate and you estimate the
ferret is 4 years of age, then what you are actually saying is the
ferret is somewhere between 2 and 6 years of age! You could guess that
well without even opening the ferret's mouth!

The problem isn't so much the technique; any method that can get you
within 2 years (+/-) of actual age is actually not such a bad test.
The problem is that ferrets live short lives. This makes the error rate
extremely significant, so non-invasive techniques that have error rates
that would be considered small in a long-lived species have little
determinative value in short-lived species. For example, there is a
world of difference in saying a horse is between 4 and 8 years of age
compared to saying a ferret is between 4 and 8 years of age.

What this type of significant error rate does is to make the
determination of a ferret's age using non-invasive techniques no better
than a guess, and it is actually worse than many guesses. I have found
most experienced ferret owners can guess a ferret's age to within a
couple of years of the true age, indicating that an educated guess is
just as good as using dental translucence.

So, what can you do for ageing? Simple; simply stop trying to guess age
in years (which is hard) and start assigning age categories (which is
easy). These are the categories I use:

Neonate: birth to start of weaning: 0-4 weeks.

Kit: start of weaning to replacement of milk teeth: 4-8 weeks.

Juvenile: replacement of milk teeth to the first birthday (generally
the onset of sexual maturity in wild polecats): 2 months to 12 months.
The onset of adulthood is easy to determine with a single x-ray; if the
long bones are fused to their epiphyses, it is an adult. If not, then
it is a juvenile. Long bones in immature mammals are composed of the
main shaft, the diaphysis, and several end pieces, each called an
epiphysis. These bones are connected to each other with a plate of
cartilage, commonly called a growth plate. As the mammal matures and
the bones stop growing, the diaphysis fuses to the epiphyses and the
bones stop growing. This is the stage between being a juvenile (still
growing) and an adult (no longer growing), and it occurs in ferrets at
about the same time as sexual maturity, roughly at one year of age.

Adult: first birthday to onset of geriatric characteristics: 1 year to
5 years (for early neutered American pet ferrets that are fed kibble
and housed in indoor cages; different husbandry can shift the onset of
geriatrics and even prolong lifespan). Specific characteristics are not
required to determine the status of "adult;" if the ferret is not a
juvenile or younger, or if it is not obviously geriatric, then it is
an adult by default.

Geriatric: Any age past the onset of geriatric characteristics: roughly
5 years and older. Geriatric characteristics are sort of nebulous, and
there are certain to be exceptions anyone can cite. Still, there is a
definite pattern of changes that demark geriatric ferrets from younger
ones, and my research shows those characteristics statistically
increase at about 5 years of age. These include (but are not limited
to) a decrease in play, becoming less social, reduction in muscle mass,
retreat from playful juveniles, less tolerance to handling, decreased
immunity, and medical changes such as the thinning of the skin,
formation of cataracts, and the loss of body fat. There may be
locomotor problems, or a lack of suppleness in the vertebral column (I
call this a "hitch in the get-along").

If you carefully consider these age categories and compare them to the
age estimates from dental translucence and factor in the error rates,
the ages are not only as good, but, in some cases -- such as the very
young and old -- better than can be determined by dental translucency.
For example, dental translucence will not give any or little accurate
results in ferrets younger than 3 or so years of age.

As part of my research into ferret dental wear and domestication
changes, I constructed something called a life table. Those of you who
were at Portland heard me discuss a finding from this table during my
husbandry lecture, and heard me say that the ferret husbandry in North
America is shortening the lives of our domesticated ferrets. I know
this because the life tables of North American ferrets do not match
the life tables of those from Australia, Europe, or Britain, and nor
do they match those of polecats. Our ferrets are simply not living as
long. The Gompertz-Makeham law of mortality can make some extremely
good predictions of life span (when you have the appropriate data),
and my research suggests North American pet ferrets are dying before
they reach "normal" life expectancy (based on polecats and European
pet ferrets). The life table also results in curves that show the
population numbers for different ages, which could be used to
statistically predict how many animals would be found at a specific
age within the population.

I have to admit the life table was very inadequate in Europe, where I
consistently under-estimated ages by about two years. That was when I
realized the life tables of North American pet ferrets was different
from life tables of ferrets from other areas. That is the value of
failure; if I didn't make those aging mistakes, I wouldn't have started
trying to figure out why. That one simple set of mistakes led me to
understand better the influence of husbandry on the life spans of
ferrets.

The bottom line is that there is currently no method that both
accurately and non-invasively determines the age of a ferret past the
juvenile stage. Non-invasive tests in short-lived mammals, such as
dental translucence in ferrets, results in a range of ages that are
no better and often worse than an educated guess, even if the rate of
error is small. Non-invasive age determination techniques simply cannot
yield an accurate and reliable age estimate. Using an age category
rather than an exact age may not be comfortable to some, but it is at
least more scientifically sound, and in many ways is more accurate
than the dental translucence method.

Bob C [log in to unmask]

[Posted in FML 5876]