Q: "I read [feral] ferrets live less than a year in the wild.  How do they
reproduce?"
 
A: Sexually, and with wild passion.
 
In nature, regardless of the number of offspring produced, all any one
parent tries to do is create a "replacement" for itself.  In other words,
as a female feral ferret, if you pump out 20 offspring during your
lifespan, you only really expect ONE offspring to survive and successfully
reproduce (the other 19 will not successfully reproduce, will starve or
become lunch, sad to say.  Life is hard.).  Now, in reality, nothing is
this simple; some females will have zero offspring surviving to successful
reproduction and others will have several.  This is the basic biology
behind natural selective processes; all animals produce more offspring than
will survive and only those best adapted to their world will survive to
reproduce.  Since the goals of any species are "eat and drink, survive, and
reproduce," you don't have to die to be selected against, you just have to
fail to reproduce.  Some computer geeks should take note.
 
So why is the "offical" lifespan of a feral ferret in New Zealand under a
year of age while the breeding age of ferrets is over a year of age?  If
eveyone dies before the breeding age, how can a species maintain itself?
Because the "offical" lifespan is actually an average that is heavily
skewed towards the young side.  Confused?  Ok, suppose you have 20 ferrets
with death ages (in years) as 4, 3, 2, 1, 1, .5, .5, .5, .5, .4, .4, .4,
.3, .3, .3, .2, .2, .2, .1, and .1.  The average age at death is 0.795
years, or roughly 9.5 months.
 
Obviously, the demographics of the situation show something different than
the average age at death shows.  Now, without knowing the ACTUAL size of
the litter, I would know from the demographics that litters would average 4
neonates surviving at least 0.1 years, or about 5.2 weeks of age (The
numbers do not tell me the actual number of births because the first
reported death is at 5.2 weeks of age; there could have been spontaneous
abortions, stillbirths or neonatal deaths before 5.2 weeks).  What this
means is, about 3/4ths of all feral ferrets that survive the first 5.2
weeks of life will not *LIVE* long enough to reproduce.  And that is just
when considering a single litter.  In truth, on average in a large
population, only a single offspring from all litters will successfully
reproduce, far less than the 3/4ths shown here, because even though they
may live long enough to make babies, they are unsuccessful in doing so.
 
Now, I've made up these numbers in the above example to show how
demographics work, but they are not far from the truth.  In real life, wild
polecats and feral ferrets have about 6 offspring per litter (ranging from
4 to 8 neonates).  Domesticated ferrets can have far more, up to a dozen,
because of human selection, increased nutritional levels and protected
environment.  In the wild, the average age at death of polecats and ferrets
is about 9 months of age.  So, these numbers are not too far from those I
made up for the example, with my invented numbers being a bit more
optomistic than reality.  The point is, because of the large number of
offspring, even though a large number will die before reproduction
(reducing the average age at death), enough will survive to maintain
population levels.
 
The study of reproductive potiential is called "fecundity." Since we can
safely say the goal of reproduction in a stable population (in the long
term, stable populations rarely exist in the wild.  They usually start
small, grow large, then become extinct, graphed as the infamous "battleship
curve"), we can infer only a single offspring will survive to reproduce.
Fecundity is a female's lifetime reproductive potiential--males are not
considered.  So, if 95% of female feral ferrets reproduce for 4 years,
having a single litter containing 8 kits, the fecundity would be
4 x 1 x 8 = 32.  That is, 95% of female feral ferrets could potientually
produce 32 kits in their lifespan.
 
What this really means is that ferrets make one baby to survive and
reproduce and 31 others as carnivore targets, to starve, or to live but not
sucessful reproduce.  Is this a high number?  Well, consider modern humans.
About 95% of woman can start reproducing at 12 and continue until 45 (33
years), and can have a baby every 9 months (or 1.34 "litters" a year); so
human fecundity would be 33 x 1.34 x 1 = 44.  What this actually means is
that at one time in our evolutionary history, humans had a very high death
rate and we have never lost that reproductive potiental; we control it
using cultural--not biological--controls.  Don't give me any poo about
breast feeding slowing down reproduction.  Fecundity is a potiential; all
it means is 95% of human females can potientially make 44 babies (one every
9 months for 33 years).  Talk about your stretch marks.  That would stretch
your round ligaments to your knees.  Don't stand up too fast....
 
The point about fecundity and demographics is it gives the observer a clue
about the animal's responses to external limiting factors, like predation,
starvation, disease, etc.  It explains why polecats can have such a high
death rate yet still remain capable of maintaining populations.  It can
also explain why some populations, once reduced past a minimum number, have
a very difficult time replacing their populations.  Animals, like the
polecat group, have a fairly low fecundity, which means a slow replacement
of populations.  (Some rabbits have fecundities of 200 or more, and some
mice have them reaching 400.  Insects can have fecundities in the 10,000s,
while some sealife can reach the millions).  Since the maximum number of
reproducing offspring per year is so low, it can take decades for
population numbers to return to normal.  This is what happened to the
black-footed ferret.  Starting with a low fecundity, external factors
(humans) increased the death rate beyond the little guy's ability to
maintain reproductive populations, and the population dwindled quickly.
 
Bob C and 19 MO' Fecundless Ferts
[Posted in FML issue 2874]