r norman  wrote in
news:cnftq6$1a8d$1{at}darwin.ediacara.org: 

> On Wed, 17 Nov 2004 03:14:40 +0000 (UTC), William Morse
>  wrote:
> 
>>r norman  wrote in
>>news:cnaq3q$2l46$1{at}darwin.ediacara.org: 
>>
>>> On Mon, 15 Nov 2004 03:57:09 +0000 (UTC), "Number 6"
>>>  wrote:
>>> 
>>>>Size isn't everything so we hear. The dinosaurs were pretty big and
>>>>now we have discovered small human creatures. So given the size of a
>>>>planet there must be a physical maximum size for its mammals. For
>>>>instance, for a given force due to gravity the heart would need to
>>>>be big enough to circulate the blood.Same for trees - how high can
>>>>they get? The fluids need to be pumped up to the leaves.
>>>>Therefore how small can we get and how big can we get and if the
>>>>planet had less could the creatures be bigger? (conversely for a
>>>>large planet). 


>> Higher gravity would seem to tend to reduce the
>>size of land animals (other things being equal), but I don't know
>>whether the increased gravity would significantly affect aquatic
>>species. 

 
> Supporting the body against gravity is a major factor in limiting size
> of terrestrial animals.  Eliminating metabolically produced heat is
> another. Still there are others mostly related to the surface area to
> volume ratio: producing adequate respiratory, digestive, and excretory
> exchange surfaces.  Aquatic organisms tend not to have the first two
> problems.  Buoyancy mostly solves the first and water is such a good
> heat conductor with such a high thermal capacity that the second is
> mostly solved in reverse:  it is hard for small aquatic animals to
> regulate their temperature independently of their environment. That
> leaves mainly the exchange surface  problem which is distinctly less
> important.  As a result, many aquatic creatures are very large
> compared with their terrestrial counterparts.
 
> Still, there are ecological concerns relating to just how much food is
> available to a single organism.  Large animals require large
> territories for foraging, grazing, or predation.  That limits the
> population density.  At some point, the population size becomes too
> small to support successful and sustained reproduction.


Then in answer to "Number 6"'s  original question, land organisms would 
be expected to be smaller on larger planets and larger on smaller 
planets, but aquatic organisms shouldn't change much in size (again with 
other things being equal, which they probably wouldn't be). Now we just 
have to wait a couple of years for those expeditionary space flights to 
return results from a few dozen habitable planets :-)

Yours,

Bill Morse
---
þ RIMEGate(tm)/RGXPost V1.14 at BBSWORLD * Info{at}bbsworld.com

---
 * RIMEGate(tm)V10.2áÿ* RelayNet(tm) NNTP Gateway * MoonDog BBS
 * RgateImp.MoonDog.BBS at 11/18/04 7:39:03 AM