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Re: the biggest predators



> Date: Wed, 17 Jan 2001 17:09:12 +0100
> From: William.BONNET@tcc.thomson-csf.com
> 
>       > So what's to stop marine animals, whose body density is
>       > pretty much identical to that of their environment, for
>       > growing forever?  Why do whales top out at a puny two
>       > hundred-odd tonnes?  Where are _really big_ whales?
> 
> So I think the amount of food an organism is able to eat during a
> given lapse of time can be a factor of size limitation.

At first glance, I thought this reasoning was nonsense, since I'd
assumed that an animal's ability to gather food varied with the
animal's size.

But on further reflection, your argument makes sense for filter
feeders like the large whales, in which food is gathered in accordance
with (roughly) the frontal area of the mouth =~ the cross-sectional
area of the animal.  So at second glance, we'd think that there's a
limit on the _length_ of filter-feeders (i.e. mass dividied by
cross-sectional area.)

At a third glace, the cross-sectional area of a filter-feeder can't
keep growing forever either, because the wider the animal gets in
proportion to its length, the less aerodynamic it gets.  So all in
all, it's quite easy to believe that blue whales are at or close to a
fundamental limit (although I'm _always_ wary of fundamental limits,
since people are constantly discoving new animals that go right the
hell ahead and break them :-)

At a fourth glance, I wonder how food-gathering ability varies with
size in active predators such as sperm whales.  Maybe its length^(2/3)
as with filter-feeders, but it's not obvious to me that it must be so.

 _/|_    _______________________________________________________________
/o ) \/  Mike Taylor -- <mirk@mail.org> -- http://www.miketaylor.org.uk/
)_v__/\  "If it breaks, you get to keep the pieces" -- Paul Gortmaker.