Question: What is the process of animals elvolving from laying eggs to viviparity?

  1. That’s the puzzle I’m trying to solve. I don’t have the answer just yet.

    The main difference between embryos in live-bearing and egg-laying lizards is the eggshell. The eggshell around the embryo is very thick in lizards that lay eggs. On the other hand, the eggshell is very very thin or nonexistant in lizards that give birth to live young. So we predicted that the eggshell became thinner over time during the evolution of from egg-laying to live birth.

    To test this, we looked at a strange type of lizard called the three-toed skink. In Sydney, this lizard lays eggs, but near Armidale, it gives birth to live young. This means that viviparity evolved not long ago in this species (thousands instead of millions of years ago).

    We measured the thickness of the eggshell around the embryos of both Sydney and Armidale skinks and guess what we found? The Sydney skinks that laid eggs had a much thicker eggshell than the live-bearing Armidale skinks! This matched our prediction that the eggshell became thinner during the evolution of viviparity.

    Thinning of the eggshell is only part of the process – I’ll get back to you when we’ve figured out the rest! 🙂


  2. Nice use of the technical term (viviparity), you had me thrown for a moment there.

    I have no idea how the transition happened but I can see the advantages and disadvantages of each parenting style.

    The options are:

    – Lay an egg and leave: advantages are that the animal can get on with it’s life with minimal disruption or parenting responsibilities as offspring are self-sufficient upon hatching. Disadvantages are that the eggs are unprotected and you’d never know if any of them lived.

    -Lay an egg and sit on it: advantages are that you can protect the egg somewhat, but you don’t have to take it with you wherever you go, like out hunting, or to the shops. Disadvantages are that you cannot easily move the gg if you get sick of the scenery or feel that it is in danger.

    – Baby inside the belly: advantage is that you can protect it at all times and easily carry it around with you. Disadvantages are that it inhibits movement and might be a bit of a pain to carry around all the time.

    At some point there must have been a mutation which meant that the fertilised egg was not laid but stayed inside the body. If this was an advantage then the mutation would stick around.


  3. Yes, nice word. One for Bridget, maybe. Come back Bridget!



  1. I have zero idea on this one.