Re: Nest predation

[John Bois <jbois@umd5.umd.edu>]

Chris Brochu wrote:

>...(behavioral characters) aren't necessarily more
> likely to converge...

True.  But I argue that this depends entirely on the specific behavior
and that nest parasitism _is_ more likely to converge (see below).

> The percentage of bird species that parasitize is very, very
> small.  Perhaps more behavioral variation would arise if crocs were more
> diverse - we'll never know.

Good point.  This would also affect its appearance among non-avian
dinosaurs.  Like a sort of balanced polymorphism, this sort of parasitism
might only be possible if there is a relatively high diversity of hosts.

> At what node do you draw the line and say "It's more likely from here
> on up."?  As far as I know, no living paleognath parasitizes - one could
> make a case that parasitism is restricted not to birds, but more
> specifically to neognaths.  Moreover, IIRC parasitic taxa are not found
> among galliforms, loons, or waterfowl, the lineages thought to be toward
> the root of Neognathae, and so parasitism may be more restricted still
> within Neognathae.

The case I am trying to make is that this particular behavior, while
heritable, may not be appropriate for cladistical analysis; that a
species' immediate evolutionary ecology may be a better predictor for nest
parasitism, and that its appearance may well be somewhat independent of
phylogenetic relationships. Rather, I should say that such relationships 
might be ephemeral. In the same way that water acts as a kind of selective
factory for making fish-shaped vertebrates, heavy parental investment may
be a crucial prerequisite for the evolution of nest parasitism.  Your
argument above re the low frequency of parasitism in birds is a good one 
in favor of potential croc nest parasitism.  But it also makes my point
about the inapplicability of cladistics.  In contrast to say, bower-bird
nesting structures, nest parasitism doesn't (I'm guessing) have a 
long pedigree within clades.  (But I don't want to be misconstrued about
cladistics.  It is a valuable and illuminating tool.)

> >Likely rapid growth rates of dinos in vegetation mounds
> >needed ventilation ala megapode "taking the temperature" (heat may be the
> >stimulus for that bird's behavior.  But carbon dioxide concentration is
> >directly related to heat in these nests).  Some dino nests are known to be
> >shallow (Troodon, Oviraptor, and Maiasura).
 
> So are some croc nests - not all of them build mounds of vegetation.

Right.  And a very telling point in favor of my idea that fast embyronic
growth was a prime adaptive advantage made possible by increased parental
investment, is the following paraphrased from Seymour and Ackerman: the
Nile croc (ovipositing in sand) has a clutch size of about 13 kg. whereas
the similarly-sized _C. porosus_ makes a vegetation mound but can only
manage 5.6 kg.  S & A believe this is a clutch size suppressed by lack of
oxygen.  And if _clutch size_ is restricted by limited oxygen,
so would embryonic growth rate.  Compare _C. porosus_ with mound building
megapodes which ventilate, i.e., they, like nearly all other avian
dinosaurs, employ strategies selected to optimize embryonic growth rates.

Anyway, I agree with you that nest parasitism would likely be rare in
non-avian dinosaurs.  And I appreciate your patience.
John Bois.