Expert: John Locke - 7/12/2007

Question
why do echidnas lay eggs?

Answer
Thanks for using AllExperts, Jeremy.

Echidnas and platypi together form a very old group of animals known as the monotremes (please note that all of what I'm about to say here applies both to echidnas and platypi). They are warm-blooded mammals, like humans, and they grow body hair and produce milk to nurse their young; they differ from other mammals in that they lay eggs instead of giving birth to live young. However, their method of laying eggs is different from other egg-laying animals like snakes and frogs. Monotremes keep their eggs inside their body for some time and nourish them by a placenta, but eventually expel them, at which point the eggs then continue to develop outside the mother. You'll recall that placental mammals nourish their offspring inside their bodies until they're ready to be born. Most reptiles and amphibians expel their eggs before they're even fertilized; both fertilization and development then take place outside the body.

The ability to give birth to live young is a more recent development than laying eggs--mammals, reptiles, and amphibians alike had been laying eggs for millions of years before the first placental mammals ever came along. The first mammals that could to give birth to live young lived about 130-150 million years ago during the Cretaceous period. Echidnas and platypi, however, evolved before placental mammals and have survived even though placental mammals have become the dominant form of mammalian life on the planet (except for the monotremes, all mammals give birth to live young). Monotremes are old; they evolved before mammals could give birth to live young, and have kept that egg-laying ability ever since. Because of that, echidnas today lay eggs.