The following reference is for those interested in reading about the
difference in polecat-ferret brains.  I've included an abstract of the
article for those of you who do not wish to invest the time to get the
article.
 
Its a pretty good paper, worth reading if you are interested in the
differences between wild and domesticated forms.  This article has two basic
goals; to document the changes in brains because of domestication, and to
see if those changes can be time-indexed.  In the former, the authors did a
good job of documentation.  In the later, they concluded the difference in
brain sizes were a result of time-of-domestication.
 
While I agree time might be one cause, it does not follow that it is the
*only* cause; mink breeders may not care about behavior and associated
changes--they may only be interested in fur or other factors, like color, in
the domestication of mink.  The behavioral qualities of ferrets are quite
important, and it is quite likely ferret breeders selected animals of a more
human-friendly bent.  These traits could have more of an impact on the final
size of brains than the selections for fur.  I think it is entirely possible
to breed ranch mink with brains 1/4th the size of wild mink well within my
lifetime, should the desire exist.  I personally find the idea that smaller
brainweights are correlated to time of domestication to be interesting,
perhaps even factual, but not proven.
 
There is a correlation between memory, problem solving and behavioral
attributes with brain size and complexity.  If you make the assumption
complexity remains equal between wild and domestic forms of the same species
(which is argued for in this paper), then a lowered brain size should be
correlated to any lowering of what is percieved to be intelligence.  Since
tests by Poole, Lode and others have demonstrated behavioral changes and
lowered abilities for problem solving and memory in smaller-brained domestic
forms, then you can safely conclude brainsize is a factor in those changes.
 
I have to say up front that a couple of studies have shown there are
physiological changes in the brain structure of wild vs domesticated forms,
including that of the ferret.  So Roehrs' argument that brain structure is
basically a constant must be flawed to a degree.  What is most likely is
that domestication actually affects *both* brain size AND structural
complexity, so lowered intelligence and behavioral changes are due to a
combination of factors rather than either separately.  Still, it is a good
paper to demonstrate domestication leads to smaller brains, lowered
intelligence and behavioral changes.
 
M.  Roehrs 1986 Effects of domestication on brains of Mustelidae.
"Zeitschrift Fur Zoologische Systematik und Evolutionsforschung"
24(3):231-239.
 
Abstract:
"In wild minks _Mustela_vison_, farm minks _Mustela_vison_ f. _domestica_,
in polecats _Mustela_putorius_ and ferrets _Mustela_putorius_ f. _furo_
subadult animals possess heavier brains than adults, the difference may be
more than 15%.  In wild and farm minks and in ferrets exists a sex
dimorphism in brainsize, females have smaller brains than males.  The
intraspecific slope of the allometric line brainweight-bodyweight in
Mustelidae is apprx.  0.25.  The comparison of the brainweights of the adult
wild and domesticated animals leads to the following results: farm minks
have 5.3% less brainweight than wild minks; ferrets have 28.6% less
brainweight than polecats.  Minks and polecats have the same values for
cephalisation and neocorticalisation.  Therefore the difference in decrease
of brainweight between the two species is due to the different times of
domestication; farm minks are domesticated since 100 years, ferrets since
more than 2000 years."
 
Bob C and 20 Mo Cranial Lightweights
[Posted in FML issue 2173]