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Re: Sauroparental care



On Tue, Dec 23, 2003 at 08:46:16AM -0500, John Bois scripsit:
> On Mon, 22 Dec 2003, Graydon wrote:
> > > I agree that at the present state of evidence we can only make
> > > inferences about parental care.  I must disagree about the
> > > _strength_ of the inference.  In my view parental care is a strong
> > > inference--so strong that we can assume it happened.
> >
> > In sauropods?
> 
> Maybe not.  But generally.  But even with sauropods what alternatives is
> anyone suggesting?

Straight R strategy, lay'em and leave'em.

> > I don't think we've got anything more than the phylogenetic bracket and
> > the existence of eggs in clutches, which is a reason not to dismiss the
> > idea out of hand, but which is also not any kind of positive evidence.
> 
> 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'.

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.

> Parental care isn't as strong of course--but it _is_ strong.

It's a very tenuous inference.

> If you see parental care in the larger phylogenetic context as an
> important watershed: as part of the evolutionary canon.  Having
> derived such an important and time tested and ubiquitous ability, why
> would a species _not_ employ it? 

Because they didn't need it.

Parental care has a cost.  This cost is not obviously, or always, lower
than the cost of producing lots and lots of offspring.

> Why would a species give it up?  Certainly there are
> exceptions--megapodes are presumably secondarily non parental
> (right?)--so what is the argument for sauropods as an exception?
> Which is the "little green men" argument: parental or non-parental?

A species would give up parental care if, as a matter of statistical
averages, more offspring got into the future where they themselves
survived to breed from more R-strategist parents than more K-strategist
parents -- if the survival chances are even very marginally higher for
producing 80 eggs and walking away instead of 50, and stepping on some
of the offspring, say.

> > > assume it happened?  I'm trying to think of an analogy.  Parental
> > > care--i.e., increasing investment in egg and therefore baby (once an
> > > organism has invested considerable time and effort in an egg it pays
> > > to maintain investment in baby!) seems to have increased over
> > > evolutionary time.
> >
> > News to all those frogs, that.
> 
> I'm missing the point--frogs have little investment in either eggs or
> baby, right?  They are beyond the bracket.

The bracket is an *inference*, it's not *evidence*.  It's permission to
make the supposition, not evidence of the behaviour.

> > That hypothetical clutch is 100 kg out of 20,000 kg; half a percent
> > of body mass for fifty chances to successfully reproduce.  Lay eggs
> > at that rate once per wet season over a fifty year reproductive life
> > and you have 2,500 chances.
> 
> I think this is not borne out.  It seems to argue that sauropods could
> never experience reproductive failure, that they had landed themselves
> the perfect strategy.

Essentially unchanged bauplan for a hundred and twenty-odd million years
is a pretty solid argument that whatever strategies they had were good
ones.

> Predators know better.  Making more unprotected babies is simply
> setting new places at the predator table.  The offspring must have
> predator-resistant strategies.  The idea that they could be like mass
> hatching turtles is unlikely. 

Why?

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?

Almost everything that is born, dies. Stable populations *average* one
offspring per parent itself surviving to breed.

> Surely predators may be locally or temporarily swamped--but these
> babies cannot resort to any refugium remote from regular "lawn mower"
> predators--or, if they could, where is it?  What hypothesis could one
> derive to test this?

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.

> > That looks like sauropods could be as completely R-strategist as sea
> > turtles.
> 
> 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.

> So really, if one is to make a non-parental arguement, one has to
> explain how?  I mean, I'm happy to say "we just don't know for sure".
> But I'm not willing to accept nonparental as a default
> hypothesis--either hypothesis needs testing/support.

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.

> > Small creatures have to be in a hurry; they have short lifespans and a
> > high risk of predation.  Gibberingly enormous creatures that live a long
> > time don't have to be in a hurry to reproduce, and can successfully take
> > a statistical approach.
> 
> So how do you explain the wild success of elephants?

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.

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