thumb|An [[aphid giving viviparous birth, an unusual mode of reproduction among insects]]

In animals, viviparity is the development of the embryo inside the body of the mother, with the maternal circulation providing for the metabolic needs of the embryo's development, until the mother gives birth to a fully or partially developed juvenile that is at least metabolically independent. This is opposed to oviparity, where the embryos develop independently outside the mother in eggs until they are developed enough to break out as hatchlings; based on relations between zygote and parents. The five include two nonviviparous modes: ovuliparity, with external fertilisation, and oviparity, with internal fertilisation. In the latter, the female lays zygotes as eggs with a large yolk; this occurs in all birds, most reptiles, and some fishes. These modes are distinguished from viviparity, which covers all the modes that result in live birth:

  • Histotrophic viviparity: the zygotes develop in the female's oviducts, but find their nutrients by oophagy or adelphophagy (intra-uterine cannibalism of eggs or sibling embryos in some sharks or in the black salamander Salamandra atra).
  • Hemotrophic viviparity: nutrients are provided by the female, often through some form of placenta. In the frog Gastrotheca ovifera, embryos are fed by the mother through specialized gills. The skink Pseudemoia entrecasteauxii and most mammals exhibit a hemotrophic viviparity.
  • Placental viviparity is arguably the most highly developed form of viviparity. Placental mammals, including humans, are the best-known example, but adaptations in some other animals also have incorporated this principle or close analogies. Other examples include some species of scorpions and cockroaches, certain genera of sharks and snakes, and some species of velvet worms.
  • Ovoviviparity, a less developed form of viviparity, occurs in most vipers, and in most live-bearing bony fishes (Poeciliidae). However, the term is poorly and inconsistently defined, and may be obsolete. In several respects, the phenomenon is of considerable importance in theoretical zoology. Blackburn & Flemming (2011) Genotypic sex determination is also found in most reptiles, including many viviparous ones (such as Pseudemoia entrecasteauxii), whilst temperature dependent sex determination is found in some viviparous species, such as the montane water skink (Eulamprus tympanum).

Evolution

In general, viviparity and matrotrophy are believed to have evolved from an ancestral condition of oviparity and lecithotrophy (nutrients supplied through the yolk). One traditional hypothesis concerning the sequence of evolutionary steps leading to viviparity is a linear model. According to such a model, provided that fertilization was internal, the egg might have been retained for progressively longer periods in the reproductive tract of the mother. Through continued generations of egg retention, viviparous lecithotrophy may have gradually developed; in other words the entire development of the embryo, though still with nutrients provided by the yolk, occurred inside the mother's reproductive tract, after which she would give birth to the young as they hatched. The next evolutionary development would be incipient matrotrophy, in which yolk supplies are gradually reduced and are supplemented with nutrients from the mother's reproductive tract.

In many ways, depending on the ecology and life strategy of the species, viviparity may be more strenuous and more physically and energetically taxing on the mother than oviparity. However, its numerous evolutionary origins imply that in some scenarios there must be worthwhile benefits to viviparous modes of reproduction; selective pressures have led to its convergent evolution more than 150 times among the vertebrates alone.

There is no one mode of reproduction that is universally superior in selective terms, but in many circumstances viviparity of various forms offers good protection from parasites and predators and permits flexibility in dealing with problems of reliability and economy in adverse circumstances. Variations on the theme in biology are enormous, ranging from trophic eggs to resorption of partly developed embryos in hard times or when they are too numerous for the mother to bring to term, but among the most profoundly advantageous features of viviparity are various forms of physiological support and protection of the embryo, such as thermoregulation and osmoregulation.

Reversion of viviparity

Through ancestral state reconstruction, scientists have shown that the evolution of viviparity to oviparity may have occurred a maximum of eight times in the genus Gerrhonotus of anguid lizards.