Re: nest predation

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

On Mon, 1 Jun 1998, chris brochu wrote:

> 
> >> This means that one cannot actually
> >> optimize nest parasitism on a tree such that nonavian theropods would be
> >> predicted to show nest parasitism.  Still, it could happen.

I responded:

> >I would argue the opposite.  Nest parasitism--trusting your eggs to other
> >parents--would be expected to be more common among taxa which practice
> >active parental investment in the raising of their offspring.  Is that
> >non-avian dinosaurs?  At least some, if not most, if not all, right?
 
> Living crocs take active care of their young and do not parasitize nests,
> but that's another matter.

> Actually, you've missed my point entirely.  I was speaking of phylogenetic
> optimization.  An example:

Yes. And thank you for taking the time to explain it.  I am familiar with
the extant phylogenetic bracketing method.  I am not familiar with 
the language of cladistics, and so "optimize nest parasitism on a
tree" threw me.

If I understand it correctly, extant phylogenetic bracketing can be of
little use here.  First, behaviors (being more plastic) are more likely to
show convergence than morphologies.  Second, the specific behavior (nest
parasitism) is, I believe, more likely to be found only on the dinosaur
branch after any shared ancestor of crocodiles.

Here are my reasons, hypothetical though they are.
If bringing up baby is cheap you might as well do it yourself.  However,
if it is expensive, why not get someone else to do it and put your energy
into additional reproductive effort!  I think this might be the reason why
you see it in birds but not crocs.  Crocodiles do not manipulate
microenvironmental optima as much as birds do.  As such bringing up croc
baby requires less investment than bringing up bird baby.
If there is a continuum between nest attendance in crocs, and nest
attendance in birds--that is, between nest attendance which involves no
environmental manipulation and total manipulation, non-avian dinosaurs
would be more to the bird side.  Evidence for this is scarce but
includes: Likely rapid growth rates in dinos required greater access to
surface oxygen (and subsequent protection from wider optima swings near
the surface).  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).
Also, if altriciality were more common in non-avian dinosaurs than crocs
this would be a greater selective incentive to parasitize.  
An argument against dino parasitism is the logistics of even shallow
burial, ie, it's easier to plop your egg into an open nest.