Viviparity has two different meanings. In animals, it means development of the embryo inside the body of the mother, eventually leading to live birth, as opposed to laying eggs. In plants, where the term vivipary is more usual, it means reproduction via embryos, such as buds, that develop from the outset without interruption, as opposed to germinating externally from a seed. In both animals and plants, the adjective viviparous is used to describe the condition.
Five modes of reproduction can be differentiated in animals based on relations between zygote and parents, including 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 which result in live birth:
- Histotrophic viviparity: the zygotes develop in the female’s oviducts, but find their nutriments by oophagy or adelphophagy (intra-uterine cannibalism 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.
The relatively less developed form of animal vivipary, ovoviviparity, occurs in most vipers for instance, and also in most live-bearing bony fishes (Poeciliidae); the more developed form of vivipary is called placental viviparity. Placental mammals are the best example, but other animals have also adapted by incorporating this principle. Examples include some species of scorpions and cockroaches, certain genera of sharks and snakes, and velvet worms.
While incipient transport of nutrients appears to be common in all viviparous species, some viviparous species receive substantial amounts of nutrients via the placenta (present in therians, some skinks and some fish). In such species there is direct juxtaposition of maternal and embryonic tissue. In at least one species of skink in the large genus Trachylepis placental transport accounts for nearly all provisioning of nutrients to embryos before birth. In the uterus the eggs are very small, about 1mm in diameter, with very little yolk and very thin shells. The shell membrane is vestigial and transient; its disintegration permits the absorption of nutrients from uterine secretions. The embryo then produces invasive chorionic tissues that grow between the cells of the uterine lining till they can absorb nutrients from maternal blood vessels. As it penetrates the lining, the embryonic tissue grows aggressively till it forms sheets of tissue beneath the uterine epithelium. They eventually strip it away and replace it, making direct contact with maternal capillaries. In several respects the phenomenon is of considerable importance in theoretical zoology. The authors remark that such an endotheliochorial placenta is fundamentally different from that of any known viviparous reptile.
There is no relationship between sex-determining mechanisms and whether a species bears live young or lays eggs. Temperature-dependent sex determination, which cannot function in an aquatic environment, is seen only in terrestrial viviparous reptiles. Therefore, marine viviparous species, including sea snakes and, it now appears, the mosasaurs, ichthyosaurs, and plesiosaurs of the Cretaceous, use genotypic sex determination (sex chromosomes), much as birds and mammals do. 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).
Viviparous plants produce seeds that germinate before they detach from the parent. In many mangroves, for instance, the seedling germinates and grows under its own energy while still attached to its parent. Some drop into the water and are dispersed by currents, but others develop a heavy, straight taproot that commonly penetrates mud when the seedling drops, thereby effectively planting the seedling. In some trees, like Jackfruit, some citrus, and avocado, the seeds can be found already germinated while the fruit goes overripe; strictly speaking this condition cannot be described as vivipary, but the moist and humid conditions provided by the fruit mimic a wet soil that encourages germination. However, the seeds can germinate under moist soil too.
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 as to the sequence of evolutionary steps leading to viviparity is a linear model that can be restated rather simply. Initially, just an increase in the length of time that the egg remained in the reproductive tract of the mother may have gradually allowed for the evolution of egg retention, provided that fertilization was internal. Through continued generations of egg retention, viviparous lecithotrophy may have gradually appeared; in other words, the entire development of the embryo (with nutrients provided by the yolk) occurred inside the mother’s reproductive tract, after which she would give birth to the hatched young. The next evolutionary step would be incipient matrotrophy, where yolk supplies are gradually substituted with nutrients seeping in from the mother's reproductive tract.
In many ways, viviparity can be more strenuous and more physically and energetically taxing on the mother than oviparity. However, its numerous evolutionary origins signify that there must be worthwhile benefits to this mode of reproduction in some scenarios, as well as selective pressures that allowed it to convergently evolve over 150 times in vertebrates. One of the most profoundly advantageous features of viviparity is thermoregulation of the embryo. Since the developing offspring remains within the mother’s body, she becomes, in essence, a walking incubator. This is very useful when climate change imposes cooler average temperatures on a current habitat, or when a migration event necessitates adaptation to a new, cooler environment. When considering squamate reptiles in particular, there is a correlation between high altitudes or latitudes, colder climates and the frequency of viviparity. This tendency for egg-retention, and consequently viviparity, to be selectively favored under cooler conditions due to its thermoregulatory benefits is termed "the cold climate hypothesis."
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