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Re: gigantism as liability
On Mon, Oct 27, 2008 at 11:02:17AM -0700, Erik Boehm scripsit:
> I wouldn't be surprised if one were to take certain mammalian
> lineages, and somehow put them back in the triassic, that those
> lineages would evolve towards gigantism.
Sauropods laid a *lot* of eggs. They were R strategists; if, on
average, two eggs survive to successfully reproduce out of all those
laid in a lifetime, that's enough.
Postulate a 20 ton sauropod; 40 eggs twice a year for 50 years is 4,000
2 kg eggs x 4,000 is 8,000 kg.
A 20 tonne mammal mother will invest about a tonne of mass per
offspring. Eight of them is the same reproductive investment by mass as
the sauropod; it would require 25% reproductive success rates to get the
same result, high but not implausibly so for something so huge.
Where it's not equivalent is energetically. The advantage to gigantism
-- why all the really big land mammals are hind-gut fermenters -- is
that you can do well eating lots and lots of poor-quality fodder. If
you have to gestate a 1 tonne mass-investement baby, you are probably
food-limited for something like three years; any drought, dearth, or
even late spring and you have a problem.
The sauropod does not have a problem that problem; it either skips a
clutch or delays it, and a single clutch is a 40 x 2kg = 80kg = 0.4%
mass investment, instead of about 5%.
Also, no care investment, no gut mass displacement, and no rate issue;
that three year lead time per single offspring plus the requirement for
care means that you need these big multi-year slices of decent
conditions to reproduce successfully.
So I strongly suspect that the 10 tons or so elephants get up to is the
long term limit for mammal just on reproductive issues. Any larger, and
gestation takes too long; the time slice of good conditions doesn't
happen often enough.