Question: Why do some animals lay eggs and some just have the baby?

Carrie Ijichi answered on 17 Mar 2017:

They’re two really good solutions to the problem of how to bring babies into the world safely. Even in mammals that carry their babies, the process starts with a fertilised egg – the difference is that a mammal doesn’t lay that egg but lets it develop internally. This is great because the developing baby is very well protected and gets nutrition from the mother. The problem is that she will need to deliver a bigger, fully developed baby and she’ll be vulnerable when she’s heavily pregnant. Birds and reptiles lay the fertilised egg and then care for it, or just leave it, outside of their bodies. They can’t bring the eggs with them so are tied to the nest and have to protect them because they’re very vulnerable.

Both systems work well, they’re just two different solutions to having babies!