Re: gigantism as liability

[john bois <jbois@verizon.net>]

> Well, (r-type strategy) sure is common among vertebrates, so the simple
> explanation is that the strategy works.

Yes...nature selects strategies that work...therefore all strategies that
work are selected for, valid, good, etc. Beyond the tautology, as you have
argued, circumstances are everything...my complaint is that we don't 
consider the circumstances deeply enough in this case.

> >  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 in an interesting hypothesis, but I remain skeptical in that 1)  many
> dinosaurs that also used an r-type strategy were much smaller...

a) Can't we safely say that most dinosaurs were bigger than most mammals
today, so they were less likely to be able to hide their nests, ergo, the
need for nest defence?
b) I'm not arguing that this strategy is only for big animals. Rather, that
really big animals that can't hide nests need _both_ r-type strategy and
nest defence.

> To be more convincing, your model would require stronger evidence of
> intense nest protection by sauropod parents...

As long as hypotheses are being thrown around in the article 
discussed...nest defence is universal
against defensible predators. And animals are careful about who they will
engage--I forget the bird who defends against other birds but will abandon
its nest if threatened by a hedgehog.

> ...as well as evidence that giant size would improve nest protection
> ability.

But the very same hypothesis is always invoked (and in the article) 
regarding adult vs. adult
predation!

> I think we also need more data about the impacts of high- care,
> high-investment viviparity in living mammals.  The strategy clearly has
> distinct advantages, but there are substantial costs as well.  The
> optimum conditions and size ranges for the operation of  such strategies
> is, as far as I know, poorly typified (and is further  confounded by
> phylogenetic history - there are simply no secondarily oviparous
> placentals to use as comparisons).

So interesting...I wonder if we still posses silenced shell-making genes?

>  It seems intuitive that  live-bearing and egg-laying should have radical
> effects on overall  ecology, but I'm not sure if there are data to support
> this intuitive  sense of things, or not.  It might be, for example, that
> the net  survival of offspring would have been roughly the same for a
> giant  mammal and giant dinosaur of the same mass, for example, despite
> the  radical differences in life history.  We don't actually have a strong
> reason to believe the ultimate numbers would differ wildly - this  could
> be a case of different solutions, similar effect.

When one considers the extent to which _all_ extant animals go to protect
their reproductive investment, and overlay that imperative on the need to
defend nests, I think this presents an opportunity to envisage some very
different ecological dynamics. Independent of numbers, it is reasonable to 
argue that these different dynamics conferred different value on various 
body plans.