Cystinuria is NOT a genetic shift allowing tolerance of low protein
foods.  Rather, it is a defect in the amino acid transport mechanism
of all cells (especially in the kidney) that creates difficulty in
transporting the COLA amino acids across cell membranes.  Normally,
cystine is filtered from the blood in the tubules of the kidney, where it
becomes part of the constituents of urine.  As the urine is transported
towards the ureter, water and various molecules are resorbed back into
the body.  When a ferret has cystinuria, cystine and the other COLA
amino acids are not efficiently transported, and remain in the urine.  IF
the pH of the urine is acidic, if the concentrations of other chemicals
in the urine are high, and if the levels of cystine are high, a
precipitation cascade ensues that causes the formation of cystine stones.
The problem exists regardless of the protein load of the diet, which
means you cannot use the excuse of cystinuria to proclaim there is such
a thing as too much protein for ferrets.  In and of itself, cystinuria
would be a benign disease except for the formation of cystine stones.
 
This is NOT an adaptation to a particular diet, and since the
precipitation cascade can occur regardless of consumed protein loads, it
cannot be effectively masked; SOME number of ferrets will always display
the problem if they have the disease.
 
The idea that ferrets, being domesticated, would have different
nutritional requirements has been presented many times, usually by pet
food manufacturers justifying a high carbohydrate diet.  This hypothesis
is the cornerstone for all subsequent dietary hypotheses stacked on top
of it.  If it is falsified, all their other dietary hypotheses fail.  So
the question is, could domestication have changed the basic digestive
physiology of the ferret, driving it towards a lower protein, higher
carbohydrate diet?
 
If this single hypothesis is falsified, then pet food maker's entire
argument crashes to the ground.  In response, I point out that in
three published reports (and several under review), introgression
(hybridization) between the domesticated ferret and both forms of polecat
was seen to be so extensive that a progenitor could not be determined.
On this basis alone, whatever changes that "could have" been introduced
into the ferret would have been negated by hybridizing to polecats.
That is, even assuming such a change to basic digestive physiology could
have taken place.  ANY person suggesting such a change has the burden
of proof, since even tiny changes to basic physiology generally have
horrendous negative results.  Making such a claim is extremely
disingenuous, because it grants credence to a hypothetical change having
little or no actual evidentiary support.  My response is, prove it or
shut up.
 
When I responded this morning to the person who sent me the post, I give
a good number of reasons why such a physiological shift could have not
taken place.  These objections ranged from pointing out that ferrets bred
to drink milk would surely be more lactose tolerant than they are, to
the observation that as many (or more) historic writers rejected feeding
ferrets milk as those advocating the practice.  I was about to offer a
similar argument here, when it dawned on me that people were confusing
"adaptation" with "selection." It is an easy one to make, and it has
fooled the brightest naturalists among us.
 
Ferrets are extremely adaptable animals, capable of consuming many
different diets in order to survive.  Long time readers on this list
might remember the ferret that was found living off beer, or the one that
stayed alive by eating donuts.  How many times has someone from a shelter
mentioned the rescue of a ferret that had been given cheap, low-quality
cat food for several years, or ferrets only getting cereals and fruits?
Ferrets can reproduce while in a state of malnutrition, and can survive
for long periods of time near starvation.  These are not caused by
genetic changes, but by adaptation.
 
Ferrets have adapted to a kibble diet; of that there can be no doubt.
However, adaptation is not evidence of selection, human or natural.
The digestive system is extremely plastic, adapting itself to local
conditions in order to give the best chance to survive and reproduce to
the body in which it inhabits.  Ferrets will get sick, usually with the
runs and an upset stomach if their diet is rapidly changed.  This is
because it takes a while for the GI tract to readapt to new conditions.
Again, if ferrets "evolved" during domestication to a different diet,
then the burden of proof lies with those suggesting the change.  They
have to prove it, which includes coming up with an explanation of why all
the hybridization with polecats introduced wild genes into every single
system EXCEPT the digestive one.
 
I've talked at great length on cystine stones and I am starting to feel
like I am on a carousel.  It is obvious that regardless of the facts,
some people are going to remain addicted to their carbohydrated ways,
regardless of the scientific facts, and there is nothing that I can do
about that.  As far as I am concerned, this is a horse that has been
beaten into a bloody mess, and I have better things to do, like play
with my ferrets or teach this one really great kid the lost secrets of
darkroom work.  Unless something new can be added to this discussion, I
think I'll let the rest of the FML pursue this White Whale while I tread
water, looking for the casket.
 
Bob C  [log in to unmask]
 
"Little Brothers [weasels and ferrets] were sent by the Great Spirit to
keep watch of what we do.  They rush to whisper our misdeeds into the ear
of the Great Spirit."
--Northern Plains folklore, recorded 1827.
[Posted in FML issue 4789]