• Question: when some animals have babies why are they born in a shell or egg? how is that shell actually produced and why aren't human babies born in one too? :)

    Asked by to Daren, Lynne, Phillip, Simon on 17 Jun 2014. This question was also asked by .
    • Photo: anon

      anon answered on 17 Jun 2014:

      Egg shells are made of calcium carbonate, which is the same mineral in chalk or the White Cliffs of Dover! They’re produced from the minerals that the mothers eat, a bit like how bones grow.
      I’m not really sure why some animals do and others don’t lay eggs. I think it’s an evolutionary thing – some animals like birds found it easier to fly having laid an egg rather than carrying around their babies, whereas other animals have a better chance of surviving if they grow and develop for longer in the womb.

    • Photo: Daren Fearon

      Daren Fearon answered on 17 Jun 2014:

      Most birds, reptiles, amphibians and insects lay eggs. Oh and dinosaurs! These eggs differ quite a bit depending on the species – some are hard eggs that are fertilized before being laid, others are soft and fertilized after.

      Humans are mammals and with the exception of platypuses and echidnas, mammals give birth to live young.

      This is likely because of differences in how these species evolved. Amphibians, reptiles and birds are much older than mammals and would have evolved under different environmental pressures which may have led to them them laying eggs.

      Mammals came a bit later than these other species and likely had different environmental pressures that influenced their evolution. Carrying the baby inside it’s mother offers lots of benefits such as increased safety and its easier to share nutrients with the baby. I am not sure why echidnas and platypuses lay eggs but they do retain the egg inside them to help transfer nutrients to the egg.

    • Photo: Lynne Thomas

      Lynne Thomas answered on 17 Jun 2014:

      I couldn’t answer this question any better than Jenny or Daren. Evolution drives the differences between species though. Calcium carbonate is an amazingly strong material and can be found in all sorts of places. In fact it is also found in worm poo and can tell you something about where the worms have been and if there are any pollutants in the soil! Not at all relevant to the question but an interesting fact!

    • Photo: Phillip Manning

      Phillip Manning answered on 17 Jun 2014:

      Eggs were a wonderful adaptation that evolved in a group called the amniotes over 300 million years ago. This allowed animals to move away from lakes and rivers and colonise land, as they no longer needed to pay their eggs in water (such as amphibians do today) given the shell protects the embryo growing inside. It is like having your own private pond! The female of a species often takes calcium from their own bones to help rapidly build the egg shell inside their body, which they then replace through their diet. We can see changes in the bones of animals that produce young inside their bodies (including us!). While we do not lay eggs, we evolved from ancestors who did….some mammals, such as the duck bill platypus, still lay eggs! However we adapted to keep our babies inside our bodies to then give live birth….but, some reptiles can also give birth to babies (minus any egg!). While some argue that live birth allows for larger babies….this did not stop dinosaurs laying eggs and growing enormous!

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