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Re: Meter-wide "dinosaur" eggs reportedly found in Chechnya
Dr Ronald Orenstein <firstname.lastname@example.org> wrote:
> Would these considerations apply only to hard-shelled eggs that had to fit in
> the oviduct and pass through the pelvic girdle? Would a flexible-
> shelled, elongate egg (say) have been bound by different size constraints
> (though a soft shell might tear more easily on a heavier egg)?
Yes, at some point an egg simply becomes too big to be held inside the
body, and laid. The largest egg relative to the mother's body size is
not found in the largest flightless birds like _Aepyornis_ or
_Dromornis_, but in the smallest living ratites, the kiwis (_Apteryx_
spp.). Kiwis have the largest eggs relative to body size of any bird
- I've heard a figure of 25% of the mother's body weight bandied out.
The amount of yolk per kiwi egg is about the same as that of an emu
egg. This is according to Calder, W.A. (1979; "The Kiwi and Egg
Design: Evolution as a Package Deal" BioScience 29: 461-467), who has
a few ideas on why kiwi eggs are so big.
The shell of an egg has to be thick enough to resist external stresses
- not just when being deposited into the nest, but subsequent
incubation by the bird (i.e., being sat on), as well as being jostled
inside and against the nest. But the shell has to be thin enough that
the chick can break out. Kiwi eggs have very thin shells. It is
thought that the kiwi's underground burrow helps cushion the eggs,
allowing the eggs to have thinner shells (relative to the volume of
the egg). Plus, adult kiwis aren't that big - allowing underground
burrows and nests to be part of their ecology in the first place, and
meaning that the adult doesn't exert too much pressure on the egg when
he/she incubates it.
The egg inside the mother kiwi is so large that it does take up a
great deal of internal space in the period just before laying.
Apparently the egg expands at the expense of the stomach, preventing
eating in the period prior to laying. I don't know if the sheer mass
of a herbivorous bird the size of _Aepyornis_, _Dromornis_ or
_Dinornis_ would have allowed the mother to break off from feeding for
a few days or more.