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Re: Vegavis gen. nov. - new anseriform in today's Nature



On Mon, Jan 31, 2005 at 08:21:08PM -0500, John Bois scripsit:
> On Sun, 30 Jan 2005, Graydon wrote:
> > Ostriches are not more of an R-strategist than a hadrosaur --
> > ostriches definitely do care of the young, where that's only
> > inferred for one hadrosaur...
> 
> And no sauropods.

Absence of evidence is not evidence of absence.

I have no difficulty imagining a whip-tail sauropod keeping an eye on
its nest, but we're not going to find many fossils of small would-be
nest robbers blown into fragments by apatosaurian tail strikes.

There are also some sauropods-in-company trackways that appear to
indicate herding behaviour, with the smaller animals in the middle and
the largest to the front of the direction of travel.

[snip]
> > As a general rule, clutch size ties tightly to mortality and
> > resources; those big hadrosaur clutches indicate some mix of lots of
> > mortality and plentiful food, or at least a mortality curve that
> > stays under the food supply as a reliable matter.
> 
> Right.  There is nothing magical or superior about large clutch size
> or r strategy _per se_.  But they do indicated high mortality--and
> high predator pressure! 

And you can test your hypothesis by looking at clutch sizes over the
Cretaceous, to see if there's an identifiable trend.

> This is the conclusion also of TE Martin's study of North vs. South
> hemisphere birds (greater clutch sizes in comparable n vs. s clades
> reflects greater pred. pressure in Nth--from continental predators).
> The idea that often gets a free pass is that an endless supply of
> babies as food somehow satisfies the predators allowing the parents to
> keep a couple for their genetic endowment.

I think you've erected that as a strawman.

Have you ever looked at the *degree* of R strategy in some fish or some
frogs?  (We can leave pine trees out of this.)

It's not perfect, but sheer statistics starts to weigh in once an R
strategy is extreme enough.

If -- as does not seem implausible -- a sauropod takes 30 years to reach
full adult size, and then spends another 30 busy reproducing, and lays
only one clutch of 30 eggs a year (though if they can grow that fast
they can lay more), you're looking at 900 eggs; that's probably not
enough.

But if it's 20 years, 50 years, and 200 eggs a year -- still a tiny
fraction of maternal mass, smaller than pretty much all extant birds --
you're looking at ten _thousand_ eggs, and pretty good odds two of them
make it to stable reproductive adulthood, on average.

Which is all it really takes, and being a huge R strategist is quite
cheap, energetically.

Mammals are compulsory K strategists, and I think this messes with our
heads to a certain degree when thinking about these questions, but
treating offspring as Darwinian ammunition *does* work.

> In the case of ostriches, they exist thanks to the fact that are very
> effective at hiding--because they are practically immune to all
> population deepression as adults.  But this doesn't mean they will
> keep on keeping on until the next rock falls from the sky!

Sixty million years of mammalian predator ecologies, and at least ten
million of dinosaurian predator ecologies.  That's as well or better
than pretty much anything else that's fully terrestrial is doing.

> Which leads me to ask if it is true that sauropod diversity declined?
> And why do we think they declined?

People used to think it declined because they weren't finding sauropod
fossils from the Cretaceous.

Since people have subsequently found quite a number of Cretaceous
sauropods, it's far from clear that their diversity declined.

Answering the question involves proposing a measure of diversity,
though, and that's one of those tricky things with living populations
that gets much more tricky with fossil ones.