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The placental mammals evolved to be able to get nutrients directly to their developing young, which meant none need give birth to their babies until they were very well developed.

Baby rabbits develop within the mother for 28 days - twice as long as an opossum, which is a primitive marsupial of about the same size.

The baby rabbits don't open their eyes until several days after birth, but when necessary, a young placental mammal can be ready for action almost as soon as it leaves its mother's body.