Vivipary is giving birth to live young. Sometimes, true eggs are still created but are kept inside the mother’s body until they hatch and the infants emerge. This occurs in relatively few species of snakes, fish, cockroaches, scorpions, and various other animals. In seahorses, the eggs are transferred from the female to the male, and he carries them until they hatch. In mammals, the egg shells do not form at all, and the embryo is developed directly inside the mother, who provides it with nutrients from her body via a placenta except in marsupials, where the infant is born while still an embryo and nursed in a pouch. Vivipary requires much more energy from the mother than oviparity (egg-laying), but it allows longer and finer development which is restricted by eggs and so more complex offspring are possible. Vivipary leads naturally to parental involvement by teaching their offspring, allowing still more complex and specifically-adapted behavior to be learned. Oviparous animals are mostly born with all the knowledge and skills they need genetically wired into them, but a mammal can be taught. To varying extents, a mammal can therefore learn to live in a much greater range of environments than can a single species of oviparous animal.