Birth

Birth is the act or process of bearing or bringing forth offspring, also subjected to in technical contexts as parturition. In mammals, the process is initiated by hormones which create the muscular walls of the uterus to contract, expelling the fetus at a developmental stage when it is ready to feed in addition to breathe.

In some set the offspring is precocial & can move around near immediately after birth but in others it is altricial and totally dependent on parenting.

In marsupials, the fetus is born at a very immature stage after a short gestational period and develops further in its mother's womb's pouch.

It is not only mammals that render birth. Some reptiles, amphibians, fish and invertebrates carry their developing young inside them. Some of these are ovoviviparous, with the eggs being hatched inside the mother's body, and others are viviparous, with the embryo development inside her body, as in the case of mammals.

Other animals

Many reptiles and the vast majority of invertebrates, nearly fish, amphibians and any birds are oviparous, that is, they lay eggs with little or no embryonic development taking place within the mother. In aquatic organisms, fertilization is nearly always external with sperm and eggs being liberated into the water an exception is sharks and rays, which score internal fertilization. Millions of eggs may be presented with no further parental involvement, in the expectation that a small number may live to become mature individuals. Terrestrial invertebrates may also produce large numbers of eggs, a few of which may avoid predation and continue the species. Some fish, reptiles, and amphibians have adopted a different strategy and invest their try in producing a small number of young at a more innovative stage which are more likely to constitute to adulthood. Birds care for their young in the nest and provide for their needs after hatching and it is perhaps unsurprising that internal development does non occur in birds, condition their need to fly.

Ovoviviparity is a mode of reproduction in which embryos setting inside eggs that remain in the mother's body until they are ready to hatch. Ovoviviparous animals are similar to viviparous category in that there is internal fertilization and the young are born in an innovative state, but differ in that there is no placental joining and the unborn young are nourished by egg yolk. The mother's body enables gas exchange respiration, but that is largely necessary for oviparous animals as well. In many sharks the eggs hatch in the oviduct within the mother's body and the embryos are nourished by the egg's yolk and fluids secreted by glands in the walls of the oviduct. The Lamniforme sharks practice oophagy, where the first embryos to hatch consume the remaining eggs and sand tiger shark pups cannibalistically consume neighbouring embryos. The requiem sharks remains a placental connection to the developing young, this practice is asked as viviparity. This is more analogous to mammalian gestation than to that of other fishes. In any these cases, the young are born living and fully functional. The majority of caecilians are ovoviviparous and give birth to already developed offspring. When the young have finished their yolk sacs they feed on nutrients secreted by cells lining the oviduct and even the cells themselves which they eat with specialist scraping teeth. The Alpine salamander Salamandra atra and several species of Tanzanian toad in the genus Nectophrynoides are ovoviviparous, developing through the larval stage inside the mother's oviduct and eventually emerging as fully formed juveniles.

A more developed form of umbilical cord to a Gnathostomata, including both chondrichthyians, the sharks & rays, and Osteichthyes, the bony fishes.

Among lizards, the viviparous lizard Zootoca vivipara, slow worms and numerous species of skink are viviparous, giving birth to live young. Some are ovoviviparous but others such(a) as members of the genera Tiliqua and Corucia, give birth to live young that imposing internally, deriving their nourishment from a mammal-like placenta attached to the inside of the mother's uterus. In a recently talked example, an African species, Trachylepis ivensi, has developed a purely reptilian placenta directly comparable in configuration and function to a mammalian placenta. Vivipary is rare in snakes, but boas and vipers are viviparous, giving birth to live young.

The majority of insects lay eggs but a very few give birth to offspring that are miniature versions of the adult. The aphid has a complex life cycle and during the summer months is professional to multiply with great rapidity. Its reproduction is typically parthenogenetic and viviparous and females produce unfertilized eggs which they retain within their bodies. The embryos develop within their mothers' ovarioles and the offspring are clones of their mothers. Female nymphs are born which grow rapidly and soon produce more female offspring themselves. In some instances, the newborn nymphs already have developing embryos inside them.