"Reason"  wrote in message
news:...
> "Number 6"  wrote in message
> news:cn99el$26nq$1{at}darwin.ediacara.org...
> > 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).
> 
> 
> Good questions all, here are a few responses.
> 
> Large size in animals requires many special adaptations.  Blood circulation
> becomes much more critical and difficult with size.  Heat radiation out of
> the core, nutrient and oxygen distribution require many special adaptations.
> Longer distances slow nerve responses.  It's a bit analogous to designing a
> skyscraper.  Architects often say that as a building doubles in size, the
> engineering required to make it function goes up by 10 times.
> 
> Large trees do not "pump" water to the leaves.  It's all drawn up by
> capillary action and evapo-transpiration.  Even root pressure is
> insignificant to the process. Redwoods have invaginations, special
> adaptations in the xylem tubes (veins) which greatly increase the capillary
> power to lift water to over a hundred meters above ground.
> 
> Gigantism in organisms has arrived slowly, as a result of many necessary
> adaptations.  When food is plentiful, the advantages of size are obvious;
> better ability to compete for food, mates and territory and less danger from
> predators.

The ultimate restriction is the hieght of the atmosphere, and the
effect of cold or vaccuum in the part of the organism above it.

~Iain
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