Viviparous

Viviparous

Viviparous reproduction involves the fertilized egg (or zygote) being retained within the uterus of the female, where it is nourished by nutrients and gases (especially oxygen) passing from the mother across a specialized, blood-rich tissue known as the placenta, which is connected to the embryo by a cord-like umbilicus. Waste chemicals pass the other way across the placenta, from the developing embryo to the parent, who excretes them with her own metabolic wastes. The embryo grows and develops under these conditions of close, maternal nourishment. It is eventually born as a "live" young that can exist externally, although for some time it is still dependent upon its mother for nourishment and care. The time between fertilization of the egg and birth of the young is known as the gestation period.

Viviparous animals are different from oviparous ones, which lay shelled eggs that incubate externally to the mother's body. The developing embryos receive all of their nourishment from protein, fat, and carbohydrate stored in the yolk and albumin of the egg. Viviparous animals also differ from ovoviviparous species, in which the fertilized eggs are retained inside of the body of the female, where they develop (utilizing nutrition of the yolk and albumin) internally until fully independent, living young are born.

Almost all species of mammals are viviparous in their reproduction. The only exceptions are those in the order Edentata, which includes several species of echidnas and the platypus; these occur only in Australia and New Guinea. Edentates are oviparous mammals, laying shelled eggs that are incubated by the female parent. Among the viviparous mammals, the group known as marsupials gives birth to young that are in a relatively early stage of development. Examples of species within this group are kangaroos, wallabies, koalas, and opossums. In essence, marsupials give birth to embryos, which are then nurtured externally in a specialized pouch on the mother's belly (known as the marsupium). The marsupial young suckle and develop in the pouch until they are ready to live a more fully independent existence.

Other species are known as placental mammals. These retain the developing embryos within the body of the female parent until a much later stage of development is reached. However, after birth the young of placental mammals must be fed for some time by the mother. This is done by suckling on one of her teats to feed on a fat- and protein-rich material known as "milk." Familiar examples of placental mammals are cows, horses, dogs, and humans.

A few species of so-called "viviparous" plants retain their seeds on the parent, where they germinate and are then dispersed into the environment. One such example is the red mangrove (Rhizophora mangle), a tree which grows in flat, muddy places along tropical shores. The red mangrove retains its ripe seeds on its branches, where they germinate and grow a root-like radicle. The radicle may reach a length of 4-6 in (10-15 cm) before the germinated seed falls from the parent, to hopefully stick vertically into the mud and establish as a head-started seedling.