A marsupial's system of reproduction is great evolutionary advance compared with that of egg-laying animals such as reptiles, birds and monotremes. The egg layers provide only scant protection from the environment, sometimes little active care, and only a short period of development. They need to produce more eggs that the number of offspring required to make up for the risk of loss.
In contrast, the marsupial provides the offspring with a very long period of development, while protecting it in the pouch. The first stage of development concentrates on physical preparation for the journey from the womb to the pouch. The offspring must be able to move, breathe, feed and digest food before it undertakes the trip. Other mammals, nourished and provided with oxygen automatically by the placenta, focus their earliest development on preparation of the brain.
Consequently, the brains of marsupials never achieve the high level of development characteristic in placental mammals. For this reason, the reproductive practice observed in mammals is superior that that of the marsupial.
In contrast, the marsupial provides the offspring with a very long period of development, while protecting it in the pouch. The first stage of development concentrates on physical preparation for the journey from the womb to the pouch. The offspring must be able to move, breathe, feed and digest food before it undertakes the trip. Other mammals, nourished and provided with oxygen automatically by the placenta, focus their earliest development on preparation of the brain.
Consequently, the brains of marsupials never achieve the high level of development characteristic in placental mammals. For this reason, the reproductive practice observed in mammals is superior that that of the marsupial.