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Re: Evolutionary Scales-Systems Formation

On Wed, 19 Mar 1997, Michael wrote:

> Matt wrote concerning hypothal-pit-ovar/test axis
> > 
> > Thanks.  The "Critical Fat" hypothesis of Rose Frisch did (and still does)
> > have a following, but my research suggests that this is merely a
> > modulator, and the real action occurs with skeletal development.
> Please, explain.  Is it related to growth plate activity?

Uhmm, yes, but that's not the only factor involved (although it seems to
be the most important). But I don't want to get TOO detailed until I
defend my dissertation, know what I mean?  But yes, skeletal development
appears to be key.

When you think about it from an evolutionary standpoint, for vertebrates,
the skeleton is what gives them height and size, very important when you
are competing for mates.  Don't want to be too big (makes you a better
target for predators and/or makes you metabollicaly more costly to
maintain) or to small (makes you less physically competitive for mating
duels).  So, it makes sense that skeletal development should in some way
dictate when the individual is ready for puberty (barring pathologies such
as dwarfism, etc.)
> Does anybody know reptilian or bird puberty events?  Also why does 
> thyroidectomy not affect reptilian metabolism much (if what I read is 
> right)?

I am not up to snuff on reptiles or birds, but I can tell you all about
mammals (my intro contains a review of all mammalia - where there is data
available to review).  My guess is that because the bulk of all mammals
are primarily limited as far as the timing of the onset of puberty is
concerned by the continuing postnatal development of the GnRH neurons in
the brain and their connections to the specialized portal vasculature that
communicates with the pituitary, that this developmental process is a
primitive retention shared by most other vertebrates.  

Now there are certain factors that are superimposed on this basic
developmental phenomenon, such as seasonality and nutrition (don't want to
become sexually mature in "bad times"), but the rate limiting step is
still the development of the functionally intact
hypothalamo-pituitary-gonadal axis.  All except for primates.

As far as reptiles and thyroid hormone, I can't profess to know too much,
but I would say that I am sure that there is some link there to

See Ya,



                     Member of The Paleo Ring


Matt Fraser                               "The Real World is very 
                                            complex and chaotic,
mattf+@pitt.edu                         so that, in order to survive,
                                   humans find it necessary to construct   
                                           an illusion of reality."
                                               Author Unknown