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Asked by sofiegrey to Ed, Sam, Steve, Vera on 22 Jun 2011.
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Ed Morrison answered on 22 Jun 2011:
Most animals mate as they need a male and female to produce young. All mammals and some fish give birth to live young, most other animals lay eggs, but I don’t know why. Perhaps it is because there is only limited room inside the mother for young to develop, so if you want loads and loads of babies you have to lay eggs. For example, salmon lay 1 million eggs, although most never become adults.
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Vera Weisbecker answered on 22 Jun 2011:
The “default condition” is to lay eggs, which is why most animals lay eggs. It takes a fair bit of evolution to get an embryo to develop entirely inside the mother, which is why most animals have not evolved this strategy. For example, inside the mother you don’t have a lot of oxygen whereas eggshells let oxygen through. So for babies to develop inside the mother, there needs to be a way of providing them with oxygen. In mammals and many live-bearing reptiles, you get a connection between baby and mother through what is called a placenta – but that is very complicated and takes a long time and a lot of selection pressure to evolve.
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Steven Daly answered on 22 Jun 2011:
I think the reason, or a reason, why birds lay eggs is that they wouldnt be able to fly very well with the additional weight of carrying their young around inside them. This at least can explain why they have never evolved giving birth to live young. It is interesting that there are some egg laying mammals, do you guys know if they never evolved to give birth to live young, and why?
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Sam Tazzyman answered on 22 Jun 2011:
Monotremes are the mammals that lay eggs – echidnas and platypusses (platypi?).
I don’t know why mammals evolved to give birth to live young, and I’d be interested to find out.
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