Text 809, 43 rader
Skriven 2004-11-15 16:55:00 av Reason (1:278/230)
Ärende: Re: What determines size?
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"Number 6" <No6@distant.island.nz> wrote in message
news:cn99el$26nq$1@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.
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