Viviparity and oviparity: Evolution and reproductive strategies
Viviparity is a reproductive pattern in which females retain developing eggs inside their reproductive tracts or body cavity and give birth to offspring capable of a free-living existence. Oviparity, in contrast, is a pattern in which females deposit eggs that develop and hatch in the external environment. These patterns can be viewed as "reproductive strategies," patterns that have advantages as well as disadvantages that affect their evolution. An advantage of viviparity, for example, is that embryos are protected and physiologically maintained by the pregnant female. In many viviparous species, the mother provides nutrients to the embryo during gestation, a pattern known as "matrotrophy." Viviparity has originated on over 160 times among animals and is found among bony fishes, cartilaginous fishes, amphibians, mammals, and squamate reptiles, as well as in several invertebrate groups. Viviparity and matrotrophy are phenomena of considerable biological interest. They have been studied from the standpoints of morphology, physiology, endocrinology, ecology, and evolution.