Is viviparity the best means of reproduction?
Successful reproduction in the context of evolution demands no more than perpetuation of the species, but an ideal system of reproduction should also aim for high efficiency. Two basic systems have predominated: egg laying and viviparity with placental attachment of the embryo to the mother. Egg-laying is inefficient in allowing huge losses and greatly limits the size of the newborn, although it has the advantage of causing little physiological upheaval to the mother. Viviparity, on the other hand, allows the growth of a much larger fetus, offers great protection and is highly efficient. Its major disadvantage is the enormous disturbance of the mother's physiology in the interests of ensuring that the fetus will not be rejected immunologically and that a continuous supply of nutrients can be provided and waste products removed. Only the marsupial seems to have achieved a sensible compromise by producing the young at a very early stage when it is living as an egg embryo without the need for placentation, and then giving it continuous nourishment and great protection by milk feeding in an external pouch.
D005075:Biological Evolution; D012098:Reproduction