I guess the colors were still swirling.  I got 6 emails from people telling
me their ferrets lived longer than 3 years, and 2 from people asking me if
I had a program to prolong ferret lives.
 
The confusion about ferret age is natural.  In most cases, the length of a
life span is figured for animals living in a natural, or wild, state.  This
is what some define as the natural life span or the evolutionary life span.
But animals generally live longer in captivity because they have their
nutritional needs met, they are protected from predators and disease, and
they are protected from environmental disasters.  This is what some people
define as the captive life span or the maximal life span.  In some animals,
the difference between the two is minor, especially in reptiles.  But in
mammals, the general case is that the captive lifespan is 2 to 3 times
longer than the natural life span.  There are exceptions, especially in
those animals that have a low predation rate, but for smaller mammals,
like ferrets, it holds well.
 
In ferrets, you can compare the captive life span of pet ferrets and
captive European polecats to the natural life span of feral ferrets (in New
Zealand) and of polecats.  What you discover is the natural life span for
both ferrets and polecats is less than a year.  That certainly doesn't mean
some ferrets and polecats don't live longer; all it means is the mean (or
average) age at death is just under a year.  This only tells part of the
story; the real answers are found in the species demographics.  Wild
ferrets and polecats have a very steep curve, just like any other small
predator, and very few live past 2 to 3 years.
 
Now, here is the hard part.  It doesn't matter if a species in the wild
only lives 2 years, but in captivity it can live 20 years.  It is
genetically adapted to the lower number because if it wasn't, it would
become extinct.  Remember, in stable populations the birth rate is about
the same as the death rate.  Reproductive rates in established species
factor in predation, disease loss and death rates.  A biologist can look
at the birth rate of a species and extrapolate the predation it must face.
Ferrets (and polecats) have a high birth rate simply because so many die so
early, mostly from predation and starvation (which is why the mean life
span is so short).  To insure positive reproductive success, a species MUST
pop out enough babies prior to the end of the natural life span so that at
least one female survives (ideally, a male and female survive, but in
animals that do not pair-bond, the emphasis is on female survival).  To do
this, mammals reach a physiological "peak" which generally occurs prior to
the end of the natural life span at a period roughly equal to the length of
time it takes to raise replacement offspring.
 
This peak is actually the top of a bell curve, where everything on the
ascending slope is developmental (or the growth period) and everything on
the descending slope is deteriorative (or geriatric).  The top of the curve
is the physiological peak, or when the ferret is in it's best physical
condition.  Now pay attention, because this is where the difference between
the natural and captive life spans takes place.  In the wild, any animal on
the downward slope is at a great disadvantage and doesn't survive long.  In
captivity, HUMANS support the ferret in this part of it's lifespan, so it
continues to live.  Now it just so happens that the bell curve isn't
symmetrical; the ascending portion is very steep, while the descending
portion is much shallower.  So, a ferret can live for a very long time
after it's physiological peak, assuming humans provide support during that
time.  That is why a ferret is "engineered' to only live 2 or 3 years, but
they can live 8 or more years as pets.  That is also why most ferrets are
considered geriatric after 4 or 5 years (about the mid-point on the
descending slope), few live past 7-8 years (the toe of the slope), and
there are always a few living a dozen or so years (the extended portion of
the toe of the slope).
 
So, if you do as I have and chart the onset of specific diseases in
ferrets, such as adrenal and pancreatic disease (but not limited to those),
you will find many of these diseases fall on the descending portion of the
ferret's physiological curve.  In other words, they are not diseases which
would typically be found in the wild, because the ferret (or polecat) would
not normally live long enough for the disease to present itself.  They are
typically only seen in older animals kept in captivity.  For example, plot
the onset of adrenal disease and you will find VERY FEW examples prior to
the ferret's physiological peak, and as you descend the slope, adrenal
disease becomes more and more frequent.  Now ready to get REALLY confused?
Physiological condition falls on a bell curve, but diseases of age are
exponential by nature; that is, they increase over time.  You want to know
how I finally figured out adrenal disease was not related to Marshall Farms
breeding practices?  Because they form an exponential curve, regardless of
breeder.  And you want to know how I figured they require an environmental
trigger?  Because adrenal disease is predominately a North American
phenomenon.  It is just like smoking and lung cancer; you need both the
genetic predisposition as well as the environmental trigger.  Mortality and
disease morbidity curves do not lie.  Chart it out yourself.  Do the math.
 
In my next post I will cover environmental risks, how they relate to
disease and how to control them.
 
Bob C and 16 Mo' Geriatric Goofballs
[Posted in FML issue 2970]