The URL of this Ferret Health List message is:
http://groups.yahoo.com/group/Ferret-Health-list/message/13210
 
Q: "Your [comments] that ferrets are only designed to live about five
    years was interesting?  Why do they live so long, then?"
 
A: To throw off the life-tables of future zooarchaeologists, damn their
   hides!
 
Contrary to popular belief, domestication does NOT necessarily extend an
animal's life--at least not directly.  For example, wolves live longer in
captivity than most domesticated dogs, especially those bred to be large
or massive.  Generally speaking, domestication either shortens the life
span of the domesticate, or, it only has a relatively minor effect in
lengthening it.  One of the major reasons domestication causes a
shortening of the life span is because it is a process that requires a
significant amount of inbreeding, which has the unfortunate tendency of
causing an increase in genetic ailments.  This is minimized in animals
like cats and ferrets, where the primary focus of domestication has been
towards changing behavior, rather than body shape or size.  As a general
rule of thumb, animals that have been bred to be significantly larger or
smaller than the progenitor suffer shorter life spans.
 
Ferrets have not been bred to be larger (or smaller) than the wild
polecat, so you do not see the natural lifespan shortened as much as in
dogs (or some cats).  In general, ferrets live to be about the same age
as polecats in the same circumstances.  In other words, ferrets living
in a feral state, as in New Zealand, tend to have a lifespan that
approximates that of polecats in general.  Conversely, polecats living
in a captive environment (that does not subject the animal to extended
stress) tend to have life spans similar to domesticated ferrets.
 
How does this work?  Life spans are intimately correlated to the metabolic
rate of the animal, complicated by such factors as the life strategy
(where does it land on the r-k breeding strategy continuum?), body mass,
body shape, and specific physiological adaptations.  If you plug all these
factors into a complicated logarithm and compare the result to accepted
longevity charts, you can extrapolate a rough idea of how long individuals
of a particular species may live.  Or, you can ignore all that and just
take the animal's pulse.  There is an old saying that all mammals have
the same basic number of beats built into their heart, and those with
high pulse rates live shorter lives than those with slower pulse rates.
While simplistic, the generalization is generally accurate, at least in a
relative way.  If you plot the body mass of the animal against the average
resting heartbeat, you gain a rough estimate of expected longevity that
approximates the more complicated logarithm.
 
There are some minor exceptions to the rule; ferrets are sexually
dimorphic, with males twice or thrice the size of females, yet there is
little difference in expected longevity.  Ferrets are long and slender,
making their energy requirements (and metabolism) higher than other
animals of corresponding size.  This means they live slightly shorter
lives than other animals about the same size.  Put together, established
logarithms place the polecat (and ferret) lifespan at about 4 to 5 years.
Here is trick number two; you can ignore the logarithm if you just chart
when diseases of old age become predominant.  The diseases of old age
include non-juvenile cancers, heart, kidney and liver disease, circulatory
problems, and deterioration of tactile and sensory systems.
 
These diseases, regardless of the proximal cause, are designed to KILL the
animal.  I know it sounds cruel, but without old individuals dying to make
room for the young, a species would NEVER evolve, and THAT is a species
death sentence.  Consider this; in all species, most offspring die from
predation or starvation.  Survivors are experts at obtaining food, finding
shelter, and evading predators.  With age, they gain so much knowledge and
experience that they hold a tremendous advantage over the newly dispersing
offspring.  That advantage translates as an offspring death sentence.  If
the old individuals didn't die, the young would never survive to have
offspring of their own.  An elegant solution is to limit the time any
particular animal can dominate their piece of landscape by limiting the
amount of time they survive.
 
A general rule of thumb suggests animals live long enough to have enough
babies to replace the parents and raise them until they can disperse and
become adults.  For humans this is about 35-40 years, which was, for most
of human history, the average lifespan.  In ferrets, because juvenile
mortality is so high (the average age of death of wild polecats and feral
ferrets is about 11 months, meaning they die within 5 months of
dispersal), the average lifespan is about 3-4 years.  Any polecat or
ferret living longer than that is a grizzled old veteran, and is only
rarely found.
 
The diseases of old age start shortly after.  It hardly seems fair--to
be good enough to solve all the problems of life, only to find yourself
dying from within.  But, that is the only way to insure the survival of
the species.  So, sometime after the time an individual can replace
themselves, they start growing cancers, having heart problems, and
generally deteriorate.  But, not all die at the exact same time.
Variation in the timing exists, so some live a couple of years, a few
other a little more, and a very few a long time after.  Everyone has
heard of people living to 120 years, but it is a very rare event, much
like a ferret living to 14.  On every bell curve, there is always someone
who defines the very top end.
 
Domestication appears to double the lifespan of animals (and humans), but
the appearance is illusionary.  What is really happening is that predation
is removed, diseases are fought, and nutrients are made permanently
available.  With these constraints removed, animals (and humans) start
living to their genetic maximum, and extraordinary life spans are
realized.  In other words, they live PAST the age they were DESIGNED to
survive.
 
Intuitively, you should realize the truth of this process.  Consider how
much care elderly animals require.  Chrys, who is 7, lives in a hospital
cage, is completely blind, and is exhausted walking from one side of the
room to the other.  I feed him twice a day, clean him if he makes a
mistake, and weigh him daily to make sure he is not dehydrated or losing
weight.  If Chrys lived in the wild, he would have died long ago.  It
hasn't been domestication that has given Chrys his long life, but my care.
The same is true of me; I have a rare autoimmunity disorder I have only
survived because of modern medicine, drugs, a lot of surgery, and one
hell of a nasty bone-marrow transplant--the equivalent of the care I give
Chrys.  Most of the people on this list who are older than 40 can probably
say the same thing.  They may have had a serious trauma, maybe needed a
c-section, had a serious infection, or even some sort of genetic disorder
that, without medical treatment, would have been fatal.  Our lives are
not longer because we have evolved the capacity to live longer, but
because of our state of nutrition, low risk of predation, and high quality
of medical care.
 
The test is comparing current ferret life spans (6-8 years) to those of
historic ferrets prior to the post-WWII "explosion" in veterinarians.
Prior to 1940, the average lifespan of ferrets was about 5 years.  Prior
to 1920, that age was only a few seasons, maybe 3 or 4 years, about the
same as found in the wild.  While some ferrets always lived a very long
time, most of the difference in ferret life spans is due to care, not
specifically domestication.
 
The bottom line is that ferrets live as long as they do because WE care
for them.  Domestication has not extended ferret life spans; rather, it
has brought ferrets into our lives and WE have cared for them until long
after the age they were originally designed to live.  You can now thank
your local veterinarian.  And yourselves.  Good Job!
 
Bob C
[Posted in FML issue 3792]