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Re: Egg sizes

I think it turns out that the limitation on egg size is structural rather
than relating to gas exchange.  I was interested by Dan's reference to large
bird eggs, so I visited a web site sponsored by the Emu Growers Association,
or something with a similar name.  (If anyone cares, I'll try to find it
again.)  They have a rather long section on succesful incubation of emu
eggs.  It seems that respiration in these large eggs is active.  The eggs
are incubated in warm temperatures at low humidity.  The egg loses water by
evaporation at a fairly healthy clip due to the difference in water vapor
partial pressures, which "actively" draws air into the shell at a rate
faster than diffusion could account for.  I suppose this also keeps
appropriate parts of the embryo dryer than one would expect, thus probably
promoting gas exchange.  (I'd also suspect that at early stages the bird
embryo is a wee thing and, until well into gastrulation, sits on top of the
albumin just under the shell.  At that point, it doesn't matter whether the
egg is large or small.)

If this mechanism also worked for dinosaurs, it raises some valuable
behavioral hints.  According to the emu-growers, the eggs must never be
allowed to become dirty or wet, as this would block gas exchange.  So the
dinosaur either has to bury it or take care of it.  If its buried, gas
exchange can't be as effective, so parental care is the more efficient
strategy, all other things being equal.  The critical part played by
humidity suggests that egg-laying was seasonal and/or involved a migration
to special dry and easily defended places.

I'll bet there's a lot more useful fodder for paleontology in that general
vein.  Anyone out there an emu cowboy?

  --Toby White