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Re: gigantism as liability

I will stipulate to these arguments and all the other excellent points made in posts and the original article.
However, I want to make a distinction between a taxon's ability to respond to selection pressures, and the pressures themselves. Both are interesting topics for discussion. I admit that the ability to process bulk food of poor quality could be seen as either a selective pressure itself or an accomodation to other selective pressures. And we really don't know which it was or if it was both. The other main selective pressure invoked is predation, i.e., dinosaurs grew big to avoid predators. But, unless sauropods vigorously protected their offspring, those dinosaur babies faced a gauntlet of fresh predators at each size class--growing from chicken size through adult! This should rather be seen as a predator opporunity rather than a predator-resistant strategy. I question the value of r selection alone as an effective means of predator defence (although this argument is often made without reference to parental protection), unless we're talking oysters and oak trees. And, while terrestrial mammals face body plan restrictions against such "impressive" sizes, they are capable of attaining larger size than the current crop of taxa. So it might be argued that mammals have fewer advantages to being large than sauropods, that selective pressures for large size are not as great for them. If not, why not? Because sauropods defended their nests; at this time of the year they likely faced the entire decimation of reproductive output unless they were large enough to engage in its active defence. This was almost certainly an additional selective force operating on them and absent from mammalian reproductive effort (i.e., mammals can run and protect their offspring without the need to stand and fight). And yet this factor is yet to appear in otherwise serious discussion on the issue of why sauropods were so big.
So I here humbly reassert its importance.



----- Original Message ----- From: "Graydon" <oak@uniserve.com>
To: <dinosaur@usc.edu>
Sent: Monday, October 27, 2008 4:21 PM
Subject: 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.


Doesn't follow.

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
eggs.

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.

-- Graydon