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Re: Sauroparental care



On Tue, Dec 23, 2003 at 01:21:13PM -0500, John Bois scripsit:
> On Tue, 23 Dec 2003, Graydon wrote:
> > On Tue, Dec 23, 2003 at 08:46:16AM -0500, John Bois scripsit:
> > > On Mon, 22 Dec 2003, Graydon wrote:
> > > But the bracket is important.  How do you rate such brackets?  I
> > > imagine the bracket is strong enough for you to agree that
> > > sauropods had red blood without any direct evidence.
> >
> > That's not the bracket.  That's 'common descent from an ammonite
> > ancestor'.
> 
> I thought the bracket in this case was, then, ammonites and birds.

Birds *are* ammonites.  Haemoglobin as the means of oxygen transport is,
like the tetrapod skeleton, ancestral.

A bracket is a way of saying something about likelihood when you don't
have any evidence.  In the case of 'red blood', we do have evidence from
the genes that code for the stuff; they're pretty much the same across
everything that bleeds red.  That means it evolved once and got copied
into descendants.  Supposing that sauropods evolved a novel form of
oxygen transport then violates parsimony.

> > Talking about the phylogenetic bracket to set a rough, doubtful, range
> > on probable behaviours is a very different thing that looking at common
> > descent for basics of biochemical composition, and much less confident.
> 
> Common descent is what makes any bracket relevant.  And, at some level, at
> some future time, hypotheses regarding the biochemical nature of parental
> investment may be testabl (e.g., shared parental investment genes in both
> birds and crocs).

That's still not evidence they made it into sauropods (or
ornithsicians).  They'd have to be essentially the *same* genes, which
is monumentally unlikely given that the range of parental care
behaviours is large.

> > > Parental care isn't as strong of course--but it _is_ strong.
> >
> > It's a very tenuous inference.
> 
> How is it tenuous if within the bracket it is almost universal?

Because you're ignoring size effects; "parental care" isn't an
independent variable.  Smaller creatures make relatively larger
reproductive investments.  So *all* modern birds are bad examples of the
choice space for sauropods.

Crocs are similarly very small by comparison, and don't have parental
care; they have a variable degree of nest defense, and some
intermittent care of hatchlings.

> > Parental care has a cost.  This cost is not obviously, or always,
> > lower than the cost of producing lots and lots of offspring.
> 
> This is what we are discussing--but how do you respond to the fact of
> its universality.

By pointing out that it *isn't* universal?

[snip]
> > It works for turtles in an aquatic environment with a greater
> > proportion of predators than a terrestrial ecosystem; why wouldn't
> > it work for sauropods?
> 
> Then why do they scramble for the water?  Predation must be
> lower--locally, at least.

They scramble for the water because that's where they can move and eat,
I suspect.

Also note that current sea turtle populations are very low, compared to
historical numbers; this is because people eat them.  This makes the net
effect of predation pressure worse.

> > All you have to do is to assume that the number of predators is a
> > little too low to always get a perfect kill off.  That's very easy
> > if the egg laying is seasonal, because the predators have to get
> > through the lean season, and can't carry maximal numbers to the next
> > hatching.
> 
> On the contrary, a big crop of unprotected growing sauropods could
> support a few predators w for a number of yewars.

And it probably did, but that's not the point I was trying to make.

You have an episodic food source.  It can carry the number of predators
that can survive through the *dip*, not the peak.  (Consider lemmings
and the arctic weasel population.  It's the dip, not the peak, that
constrains the weasel population.)

If the peak is sufficiently large compared to the dip that the predator
supply is flatly incapable of eating all of the peak population of
sauropod hatchlings, the R strategy will work fine.

> > > Firstly, "R-strategy" is only a relative term.  To a crab turtles look
> > > like K strategists.  Secondly, as argued above, if turtles stayed on
> > > land they would not survive.
> >
> > They get eaten in the ocean, too.
> 
> They scramble to get in the ocean.  This is a survival tactic, right?  I
> don't imagine such a trait could survive if the rush to the ocean didn't
> pay reproductive dividends.

It's a tactic to get where they can move and eat; they're sea turtles,
they have flippers.  They would rush to the ocean without the gulls.

> > The available fossil sauropod clutches have very large numbers of
> > eggs in them.  That's not what a trending-K strategist egg layer is
> > observed to do, that's what a trending-R strategist egg layer is
> > observed to do.
> 
> 20 eggs is R strategy only in a relative sense.  One can't
> overestimate the cost of courting, mating, traveling to nest sites,
> etc.

Sure one can.  Those are all real costs, but they're not necessarily
high or even high-risk.

> At the very least, you can't make a statement that for sauropods eggs
> were cheap, plentiful, and not limiting.  I mean, you can make
> it...but it doesn't have evidenciary support.

Sure it does.  Mass.  A clutch of eggs that represents 50% of the adult
weight of the bird that laid them is a huge investment.  A clutch of
eggs that represents 0.5% of the weight of the sauropod that laid them
is not a major investment.

> > Elephants are compulsory K strategists because they are mammals, for
> > one, and for two, they aren't wildly successful, reproductively;
> > they have narrow reproductive margins and deal very badly with the
> > introduction of novel predators, as evidenced by North America's
> > (and the rest of the world's) lack of mammoth.
> 
> The novel predator is man?

Yup.

Elephants don't have much reproductive elasticity; one female elephant
has a maximum lifetime number of offspring around ten.

Postulating 20 eggs per clutch, 1 clutch a year, and the same 40 year
reproductive life, the female sauropod has 800 potential offpsring,
probably for a smaller proportionate energy investment.

There is no particular reason to suppose that the sauropod clutch was
that small, or that egg-laying was that infrequent.

-- 
oak@uniserve.com | Uton we hycgan    hwaer we ham agen,
                 | ond thonne gedhencan    he we thider cumen.
                 |   -- The Seafarer, ll. 117-118.