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Re: Big = Old = Advanced?
On Fri, 29 Aug 1997, Ronald Orenstein wrote:
> And remember also that for many birds predation may well be higher after
> hatching than before (there are a lot more baby bird-eating snakes than
> egg-eaters, for example) and we do not know how altricial baby dinosaurs
> We also do not know how long it took the eggs to hatch (do we?) -
> the longer it takes the more vulnerable they are, especially if the young
> could scurry off to hide in the brush shortly after hatching.
I think we can make informed guesses from extant species. Emus, and
ostriches would seem to be germane at something like 7 weeks. Bigger
species would take longer of course. And then there is the laying period
which also can take time. I wonder, though, about your second comment.
Yes, the nest may well have been a place to avoid and so there may
have been a premium on precociality. But appropos of your earlier comment
about high hatchling mortality, Horner goes even farther saying that it is
practically suicidal for hatchlings to be untended. If this is true, I
have not too much of an idea why turtles (as untended as they are) don't
seem to have a problem with this. Also, it's still very much an open
question as to whether cover was safer. After all, it is widely believed
(from only negative evisdence, however) that _adult_ small dinosaurs were
outcompeted. Wherever they were hiding may have ceased being a haven at
some point due to the evolution of some creatures.
> Remember that we don't know what their "worst predators" were - only that
> there were some potential candidates. We certainly don't know which were
> the most frequent egg-eaters as opposed to hunters of hatchlings, etc. For
> all we know there could have been something about the siting of Maiasaur
> colonies that related to mortality factors we have yet to guess.
> Actually male megapodes do stay with the nest mound until hatching. They
> do not incubate, though.
W.P. Coombs Jr. in _Modern Analogues for dinosaur nesting and parental
behavior_ Geological Society of America, Special Paper 238 1989, says that
there is a hole-nesting megapode in which both parents are completely
absent--you may well have more recent information.
> Actually many birds hatch at a much less developed stage than any turtle.
> I know of no altricial turtle (or reptile, for that matter) - that is, none
> that is incapable of feeding itself at birth. Of course many birds,
> including all songbirds, are altricial. What I think you mean is that
> there is less need for imprinting or other forms of early-stage learning in
> turtles than in birds; we know that songbirds in the nest learn aspects of
> song, navigation etc before fledging.
No, what I meant (but didn't express) was that it takes more to make
a dinosaur than it does to make a turtle. More complex motor programs and
behavioral programs require more neuronal wiring. And, if the brain i9s
the pace maker in embryological development--Brian Bertrand and Leigh Van
Valen believe that it is--dinosaur babies needed to wait until these
complex connections were made befor they could be safe outside the egg.
All of this (if true) makes dinosaur eggs more expensive.
> I would say that most dinosaurs I have heard of lay remarkably small eggs
> in comparison to their body size. In birds, the smallest egg in relation
> to size belongs to the largest living bird, the ostrich, which may tell you
> something, but I'm not sure what.
It could tell us that bigger eggs are relatively more expensive to make.
> Anyway, the amount of investment a
> maiasaur put into a single egg may have been a lot less for its size and
> metabolism than with modern birds.
Investment in eggs consists of many things, not just their size. An
accurate accounting of this expense must include, for example, defence of
nest (if required) and this must include all the food to bring the animal
up to size, mating and courtship expenses, expense of territorial defence,
expense and so on. I know you know all this but I sayh it to emphasize
the likelihood that even big dinosaurs may not have had much in the way of
discretionary spending--that clutch sizes represented optimal expenditure.