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Re: Waiting for a giant bird and dino physiology revisted

On Sun, 13 May 2001, David Marjanovic wrote:

>         Of course, the size of flying predators is limited (though we still
> don't know where the limit is... *Argentavis*...), so the selection pressure
> for larger size in herbivores so that they are safe from predation stops
> somewhere, I'd say.

Interesting to think of how that works for moa.  This eagle must depend
upon finding the moa in relatively open country.  And, juveniles below
some size would be immune from predation, I would think.  Where
terrestrial predation is significant (i.e., by mammals and reptiles),
these reproductive safe harbors would not exist.  In such a
situation--for a layer of eggs who cannot move them from place to place
but must leave them prone for months--its hard to imagine any benefit from
increasing size.  I mean a monster moa sitting in the middle of the
Serengeti would be torn apart by marauders of many stripes.  Ostriches are
small enough to hide, fast enough to outpace most carnivores.  I would
argue that concealment at the nest is critical for this body plan, and
that selection _for_ large size, and selection _against_ large size is at
equilibrium in the ostrich: further increase in size would
both slow it down and increase its visibility at the nest; a decrease in
size would also slow it down (I'm guessing).

Anyway, these particular selection forces didn't apply to the moa.