Ovoviviparity is a method of animal development in which the young grow in eggs inside the mother's body, and the mother gives birth to live young. Ovoviviparity is seen in some fish, reptiles, and amphibians around the world. There are a variety of reasons why animals might have evolved to give birth this way, and ovoviviparity is a topic of some interest among researchers as a result. This method of reproduction is also intriguing to many laypeople, since most people assume that egg-layers cannot give birth to live young.
Most people are familiar with vivipary, in which an animal gives birth to live young after it has developed in the uterus for a period which can range from weeks to almost two years, in the case of elephants. Humans, for example, are viviparous animals. Conversely, ovoparity, in which an animal lays eggs and the young hatch out, is the method of reproduction used by many birds, reptiles, amphibians, and others. Ovoviviparity is a blend of both methods, and it is much more rare.
The primary difference between ovoviviparity and vivipary other than the obvious presence of eggs is that ovoviviparous animals do not provide nutrition to their young through a placenta, as is the case in viviparous animals. Instead, the developing young inside the egg feed on the egg yolk, just as their ovoparous cousins do. The mother's body provides protection for the eggs, allowing them to develop without the risk of being eaten or trampled by other animals, and she also provides some gas exchange to the developing egg.
The advantage to ovoparity is that animals can lay large clutches of eggs, ensuring that at least some of their young survive. Ovoviviparous animals can only incubate limited numbers of eggs at a time, typically giving birth to no more than six young. Therefore, the young are more precious to their parents. In contrast with the young of viviparous animals, ovoviviparous young are typically also very well-developed and precocious, able to navigate the world shortly after birth, although they may still require guidance from their parents.
In some ovoviviparous animals, the young are born in larval form, and they metamorphose outside the bodies of their parents. Others are born fully formed, although they may require several weeks or months to mature to the size of their parents. Sharks are among the most notable of animals which exhibit ovoviviparity, although there are numerous other examples in the fish, reptile, and amphibian world, including manta rays, vipers, and some toads.