Re: gigantism as liability

[Michael Habib <mhabib5@jhmi.edu>]

john bois wrote:
> > This in an interesting hypothesis, but I remain skeptical in that 1)  
> > many
> > dinosaurs that also used an r-type strategy were much smaller...
>
> a) Can't we safely say that most dinosaurs were bigger than most 
> mammals
> today, so they were less likely to be able to hide their nests, ergo, 
> the
> need for nest defence?

No, I don't think that is a safe set of assumptions at this time.  The 
first statement seems to be about correct, if you exclude birds: the 
average body size of non-avian dinosaurs exceeds that of terrestrial 
mammals.  Largely because there are a lot of tiny mammals.  However, we 
don't have good information on the thresholds of nest defense and 
hiding, at the moment.  For one thing, large animals can have rather 
modest sized nests.

> b) I'm not arguing that this strategy is only for big animals. Rather, 
> that
> really big animals that can't hide nests need _both_ r-type strategy 
> and
> nest defence.

They might, but you need to bolster your argument a bit more.  I'm not 
saying you're wrong, mind you, but just being big doesn't immediately 
indicate active nest defense per se (especially if the nests and eggs 
are proportionately small).

> As long as hypotheses are being thrown around in the article 
> discussed...nest defence is universal
> against defensible predators.

I don't think this accurate as stated above; perhaps I misunderstood 
what you mean.  Many egg-laying species leave their eggs, even some of 
those that could conceivably engage in nest defense.  The cost/benefit 
ratio for active defense is apparently somewhat complicated.

> > ...as well as evidence that giant size would improve nest protection
> > ability.
>
> But the very same hypothesis is always invoked (and in the article) 
> regarding adult vs. adult
> predation!

Yes, but that's because size as a defense against large predators is 
better understood and supported.  It isn't clear if being super huge 
helps much in nest protection, because many (if not most) nest 
predators are rather small.  Being 40 tons might be important in 
defense against groups of 2-ton allosaurs.  But that same 40 ton bulk 
might be overkill for fending off nest predators.  Again, maybe you're 
right on the money - but the data aren't there to say one way or the 
other yet, so far as I know.  Sounds like the making of a project 
and/or grant to me :)

> So interesting...I wonder if we still posses silenced shell-making 
> genes?

Great question - wouldn't that be wild?

> When one considers the extent to which _all_ extant animals go to 
> protect
> their reproductive investment, and overlay that imperative on the need 
> to
> defend nests, I think this presents an opportunity to envisage some 
> very
> different ecological dynamics.

Envisage - absolutely.  But our imagination may be getting ahead of the 
data in this particular case.  Intuitively, I like your hypothesis that 
live bearing and egg laying effect things like body size selection.  
I'm logically skeptical, though, simply because the required 
information is still a bit limited on the matter.

> Independent of numbers, it is reasonable to argue that these different 
> dynamics conferred different value on various body plans.

It's a very reasonable hypothesis - but I'm not willing to conclude one 
way or the other without some of the critical numbers in hand for 
extant systems.  Should be reasonably testable, though.

Cheers,

--Mike H.


Michael Habib, M.S.
PhD. Candidate
Center for Functional Anatomy and Evolution
Johns Hopkins School of Medicine
1830 E. Monument Street
Baltimore, MD 21205
(443) 280 0181
habib@jhmi.edu