Message Number: YG2571 | New FHL Archives Search
From: RRC
Date: 2001-04-15 10:39:00 UTC
Subject: Bob C: Ageing Ferrets

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 ± 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 ± 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 (± 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

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 ±2 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