RE: Endothermy speculation

[Toby White <augwhite@neosoft.com>]

The selection for a relatively high temperature may well be the product of a 
number of things.  Most notably, biochemical processes speed up -- roughly by 
2x for 10 deg. C.-- at higher temperature.   In terms of an energy budget, this 
means that the organism's return on its investment in enzymes (durable goods) 
and fixed overhead roughly doubles with a 10 deg. increase in operating 
temperature, a very strong selective pressure.    Fortunately, there are also 
limitations on this trend.  Its hard to maintain protein tertiary structure or 
membrane stability at really high temperature.  The practical limit, without 
really specialized thermophillic adaptations, is probably not far from where 
endotherms usually operate: in the 40 deg. range.

  --Toby White

On Wednesday, July 14, 1999 7:07 AM, gmbra@cygnus.uwa.edu.au 
[SMTP:gmbra@cygnus.uwa.edu.au] wrote:
> Hello, all
> Bit of speculation that occured to me as I was going back over some posts
> on endothermy.
> Given that body temp needs to be maintained relatively constant at least in
> part because critical enzymes in metabolic pathways have a fairly narrow
> operating range, I wondered why this should be so. The ideal situation in
> theory would seem to be metabolic systems that work most efficiently at
> ambient temperature, as this removes any need for drastic temp regulation.
> For todays humans, that max efficiency temp is around 37 C, considerably
> higher than ambient temp even in the tropics. The basic metabolic processes
> that necessitate such a temp go back a long way, evolutionarily speaking.
> My first question then is - did they first evolve to function most
> efficiently at the then ambient temp? If so, was the temp at that time
> (whenever it was) as high as 37 C? If it was, then the problems of temp
> regulation for endotherms stems from a subsequent decrease in ambient temp
> which has not been compensated for by adaptation. If the temp was not that
> high, then did the metabolic processes originally function maximally at a
> much lower temp (say 25 -30 C)? If this was the case, then for reasons
> unknown (?) these processes have since changed their temp maxima, and the
> question arises - when and in what fashion did the change occur? The
> question for dinosaurs becomes something like - did they exist in a
> situation where their metabolic processes did function best at the then
> ambient temp? THis would presumably have a large impact on the amount of
> energy (and hence food intake/oxygen intake) required for them to maintain
> homeothermy.
> Does anyone have any data pertinent to this speculation, or indeed, any
> comments at all (polite, of course - remember the list rules!)
> Graeme Worth