On Wed, Jun 14, 2006 at 04:21:23PM +1000, brian fleming wrote:
> Many modern reptiles, such as the Bluetongue Lizard, and many snakes,
> give birth to live young - technically they are ovoviviparous
> because they retain the eggs inside and the young for some time after they
> 'hatch,
> while the young are still being nourished by the yolk-sac attached to the
> stomach.
Its not really useful to distinguish ovoviviparity from viparity in
reptiles because there is a spectrum of reproductive modes. Placental
nutrient transfer to the embryo has been observed in many species
including the Red-bellied Black Snakes. In some lizard species the embryo
obtains most of its nutrients this way. The australian skink Lerista
bougainvillii species has both viviparous and ovoparous populations.
> I believe that birds lay eggs rather than having an internal pregnancy,
> because
> 1. they are descandants of dinosaurs which laid eggs and
> 2. like so much else in birds, it is a necessity enforced by flight.
The story must be more complex as viparity has arisen many times and
in various forms in reptiles, there are viparous flying vertebrates and
there are a number of groups of flightless birds.
Andrew
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