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Re: Obligatory parental nest attendance.
On Mon, 17 Nov 1997, Dann Pigdon wrote:
> This is precisely what megapodes (like the Australian brush turkey)
> do. The male of most species tend the nest mound carefully, and a
> temperature receptor in their beak helps them to decide whether the
> eggs need to be covered more or uncovered. However this seems to have
> more to do with temperature control than it does oxygen supply.
But Seymour and Ackerman (1980 Amer. Zool.20 437-447) believe that without
this behavioral intervention by the parent that the embryos wou;ld
suffocate. As you say, temp. is the stimulus for this intervention, but,
apparently, PO2 and PCO2 pressures correlate pretty closely with temp. (it
seems the mound itself is doing most of the breathing!).
> Apparently the Madagascan elephant birds incubated their eggs by
> burying them in sand, and those eggs were huge.
> Remember that plant roots need oxygen too,
But different plants have different tolerances. Grass is tolerant of low
O2. Beets (I think) are not.
> ...so I suspect that in all but the most hard
> packed soils that there is plenty of oxygen to go around.
An important issue, though, is drainage. After a rain the O2 goes way
down in soils with poor drainage. Turtles may choose sand with low
porosity (i.e., less ability to conduct O2) as long as it will improve
drainage after a heavy rain. Eggs can both drown or be oxygen starved by
water taking up air spaces.
> eggs are usually buried quite deep - I've seen people excavating them
> have to plunge their entire arm up to the shoulder down into
> the hole.
Ackerman proposed "that the requirment for optimal gas tensions in the
nest might limit clutch size in marine turtles and account for i) the
remarkable selectivity of the females for certain nesting beaches with
suitable gas transport properties, ii) the similarity of clutch mass
between species of very different body weights..". S and A says there is
a ceiling of about 800ml O2 /hr. and that this is a limit on clutch mass.
Now if, as we believe, that some dinosaurs had embryonic growth rates more
akin to avian rates, this would mean that they would suck it up faster.
If this were true they would have to have a smaller clutch mass (which,
according to Seymour they didn't) or have improved access to O2. One way
to do that is to oviposit in a more shallow bed.