Here are some comments and a reference on the lifespan post.
 
LIFE SPAN.  Someone asked me if there was some sort of equation to predict
life span.  One current equation is:
 
Lifespan = 11.8(body size in Kg)
 
So ferrets, which average about .68 kG (1.5 lbs) for both males and females
would be expected from the equation to live:
 
Lifespan = 11.8 (.68) = 8.024 years.  Understand, this is the *median* or
average lifespan, and depending on the species, the distribution of deaths
will follow a normal distribution curve (bell curve).  In other words, when
you add up all the age-at-deaths you can find for ferrets, they should be
close to 8 years for males and females combined, but will include many that
are much sooner or later than the expected value.
 
This equation is for mammals in captivity; wild animals can be expected to
average 1/3 to 1/2 of the predicted value.  Brilliant readers who plug in
human weights will discover we live about 4 times longer than expected, and
the reason is still controversial.  One idea is that in most mammals, the
brain size is roughly correlated to body size, but in humans the brain size
is markedly higher, but that doesn't actually offer an explaination, nor
does it explain marsupial lifespans, who follow the body mass rule.  Its
still a mystery and is most likely related to the prolonged juvenilization
of humans.
 
One person wrote to say my final joke was in bad taste (What is a few years
for people is a lifespan for ferrets).  It was meant to be a pun, referring
to "psychological time," which addresses the perception of time by
individual species.  This is a very complex issue, one which I feel has a
lot of arguments left unproven, but it basically says those mammals that
have shorter lifespans will PERCIEVE their alloted time similarly to those
with longer life spans.  In other words, a ferret may only live 8 years, but
it will percieve its lifespan as roughly equal to that of a dog or a horse,
even though the actual time is shorter.  This is based on physiological
efects of rapid metabolism, heartrates and respiration rates, and how the
brain percieves time.
 
Think of how the human eye sees things.  The human brain visually sees time
is slices of about 1/30 sec.  each, which is why a movie running at 60
frames per second is seen as unbroken movement.  Some studies have shown
smaller mammals would see the movies as a series of still frames, because
their brains see time in shorter segments.  (This is one explaination why
smaller mammals can react to movement much faster than humans can) The
argument is those brains percieve time in tiny segments, which, when
normalized to the human perspective, would add up to a human lifespan (or
what it should be without us being such a nasty exception).  So
psychological time basically says the perception of time is relative to the
species, but is correlated to metabolic and weight relationships.
 
There are many good references on this subject, but I recommend for basic
understanding:
 
Knut Schmidt-Nielsen  1984  "Scaling: Why is Animal Size So Important?"
Cambridge University Press.
 
Bob C and 20 MO Furbutz
[Posted in FML issue 2162]