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Re: Egg sizes




On Fri, 7 Nov 1997, Dann Pigdon wrote:

>       Most of the largest dino eggs seem to be spherical, probably
> to improve their structural strength. The elongated shape of many
> smaller eggs may also have to do with the narrow pubic opening of
> the species that layed them.

Also, if eggs are supported in sand they don't need the structural support
required by an open-air egg which is sat on, rolled, bumped, and whatever
else.

> There is also predation to think about.
> Many mammals can either run soon after birth, or can be carried by
> the parents, or are hidden away in hard to get places (tree top nests,
> burrows). I can't see a sauropod nesting in trees or down burrows, 
> or carrying small fragile eggs, and eggs are notoriously bad runners :)

Dinosaurs eggs probably lacked chalazae--the protein support which keeps
embryo upright in Aves--and would have been crushed by egg contents if
moved.

> I think it would take just as much parental
> care to look after a large clutch of eggs as it would for a single
> egg, especially so once they hatched.

This is true.  Parental investment seems to have little to do with the
relative size of offspring vs. parent, or number of offspring.  The
critical thing to consider is whether or not the parent's influence
affects fitness.  In Aves, rapid metabolism and fast growth rates are
aided by brooding (the embryo, by the way, is ectothermic).  In effect it
can gain a more complex physiology in less time due to this parental
intervention.  Mammals approach this differently, but the effect is the
same, i.e, offspring are well prepared to meet the world.  Non-guarded
reptiles are also well prepared but their body plans are not competitive
with mammals and birds in those niches.   
        Could it be that the big play made by dinosaurs for their niche
was enabled by parental intervention, intervention that influenced
ontogeny.  Certainly, for Aves and Mammalia keeping embryonic
environmental parameters at optima is crucial.  The question is: did nest
guarding in dinosaurs serve only to ward off predators, or was there some
maintenance of these optima.  And if so, how?  Such activities as
ventilating mounds, shading sand, warming sand, or even active brooding of
partially exposed eggs (as likely in Troodon) may have been the key to the
brave new Dinosauria's success.