The question is really why they have continued to lay eggs, rather than
evolving the ability to bear live young. Sitting on a nest is very risky
behaviour and bearing live young could well be safer and be evolutionary
advantageous.
My answer would be that the first step to this would be to hatch the egg in
the body before laying. The high oxygen requirement of a bird precluded this
as the young would suffocate either in the egg, or before birth. Mammals got
over this hurdle by being marsupials first where the young was born as a
very unadvanced embryos. I may be quite wrong but that's by guess.
My Google said the sea snakes lay eggs in sand.
Tim Murphy
-----Original Message-----
From:
Behalf Of Philip Veerman
Sent: Wednesday, 14 June 2006 2:55 PM
To: harry clarke;
Subject: Why don't birds get pregnant?
Well, what a dumb question. Because they lay eggs. One could suggest the
period of production of eggs is like short pregnancy. For most birds, the
demands of flight would be significant force against retention of eggs in
the body for any more than the minimal time. However quite possibly in many
millions of years, some birds such as descendants of modern penguins may
evolve into a viviparous lifestyle. Just like whales evolved from animals
that probably originally had a breeding system similar to what seals do
today. I wonder what sea-snakes do.
Philip
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