|MadSci Network: General Biology|
Your question is an interesting one, and sent me to a wonderful book on snakes: Snakes, the Evolution of Mystery in Nature by Harry W. Greene. If you have an interest in snake biology, this is a book you’d really enjoy. The following information came from this book.
You are correct that most snakes are oviparous, which means that they lay shelled eggs. This is considered the primitive, or ancestral, condition for the amniote vertebrates (birds, reptiles, mammals) and may have been secondarily re-evolved from a live-bearing ancestor in a few snake groups. There are many extant live-bearing snakes. However, it is difficult to distinguish between ovoviviparity (females nourish embryos with yolk that is deposited before ovulation) and true viviparity (females actively nourish offspring through placental connections in their oviducts) because little is known about the tissue connections between mother and offspring. Thus, the term "viviparous" refers to snakes that give live birth instead of laying shelled eggs.
For the truly viviparous snakes, there are placenta-like connections between a female and her developing offspring — for example, common garter snakes (Thamnophis sirtalis) are known to provide a lot of nutrition to their embryos and dispose of embryonic wastes. Greene lists the following groups as viviparous: Asian pipesnakes, Red Pipesnakes ( Anilius scytale), shield-tailed snakes, boas, dwarf boas, and seasnakes.
I hope this answers your question!
Allison J. Gong
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