Birth

==Other animals==
[[File:Rodząca dafnia.jpg|thumb|A [[Cladocera]] giving birth (100x magnification)]]
Many reptiles and the vast majority of invertebrates, most fish, amphibians and all birds are [[Oviparity|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 have internal fertilization<ref>Sea World, [http://www.seaworld.org/animal-info/info-books/sharks-&-rays/reproduction.htm Sharks & Rays] {{Webarchive|url=https://web.archive.org/web/20131110032029/http://www.seaworld.org/animal-info/info-books/sharks-%26-rays/reproduction.htm |date=2013-11-10 }}; accessed 2013.09.09.</ref>). Millions of eggs may be produced with no further parental involvement, in the expectation that a small number may survive to become mature individuals. Terrestrial invertebrates may also produce large numbers of eggs, a few of which may avoid predation and carry on the species. Some fish, reptiles, and amphibians have adopted a different strategy and invest their effort in producing a small number of young at a more advanced stage which are more likely to survive 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 not occur in birds, given their need to fly.<ref name=Attenborough>{{cite book |title=The Trials of Life |last=Attenborough |first=David |year=1990 |isbn=9780002199124 |pages=26–30 |publisher=Collins }}</ref>

[[Ovoviviparity]] is a mode of reproduction in which [[embryo]]s develop inside [[egg (biology)|egg]]s that remain in the mother's body until they are ready to hatch. Ovoviviparous animals are similar to [[viviparous]] species in that there is [[internal fertilization]] and the young are born in an advanced state, but differ in that there is no [[placenta]]l connection and the unborn young are nourished by [[egg yolk]]. The mother's body provides [[gas exchange]] (respiration), but that is largely necessary for oviparous animals as well.<ref name=Attenborough/> In many [[shark]]s 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.<ref>{{cite journal|last1=Adams|first1=Kye R.|last2=Fetterplace|first2=Lachlan C.|last3=Davis|first3=Andrew R.|last4=Taylor|first4=Matthew D.|last5=Knott|first5=Nathan A.|title=Sharks, rays and abortion: The prevalence of capture-induced parturition in elasmobranchs|journal=Biological Conservation|date=January 2018|volume=217|pages=11–27|doi=10.1016/j.biocon.2017.10.010|s2cid=90834034 |url=http://marxiv.org/k2qvy/|access-date=2019-06-30|archive-date=2019-02-23|archive-url=https://web.archive.org/web/20190223020619/https://marxiv.org/k2qvy/|url-status=dead}}</ref> The [[Lamniforme]] sharks practice ''[[oophagy]]'', where the first embryos to hatch consume the remaining eggs and [[sand tiger shark]] pups [[cannibal]]istically consume neighbouring embryos. The [[requiem sharks]] maintain a ''placental'' link to the developing young, this practice is known as [[viviparity]]. This is more analogous to mammalian gestation than to that of other fishes. In all these cases, the young are born alive and fully functional.<ref>{{cite web |url=http://www.seaworld.org/animal-info/info-books/sharks-&-rays/birth-&-care.htm |title=Birth and care of young |work=Animals: Sharks and rays |publisher=Busch Entertainment Corporation |access-date=2013-08-28 |url-status=dead |archive-url=https://web.archive.org/web/20130803103514/http://www.seaworld.org/animal-info/info-books/sharks-%26-rays/birth-%26-care.htm |archive-date=August 3, 2013 }}</ref> The majority of  [[caecilian]]s 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.<ref>{{cite book |title=A Natural History of Amphibians |last1=Stebbins |first1=Robert C. |last2=Cohen |first2=Nathan W. |year=1995 |publisher=Princeton University Press |isbn=978-0-691-03281-8 |pages=172–173 }}</ref> 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.<ref>{{cite book |title=A Natural History of Amphibians |last1=Stebbins |first1=Robert C. |last2=Cohen |first2=Nathan W. |year=1995 |publisher=Princeton University Press |isbn=978-0-691-03281-8 |page=204 }}</ref>

A more developed form of [[viviparity]] called [[placenta|placental viviparity]] is adopted by  some species of [[scorpion]]s<ref>Capinera, John L., ''Encyclopedia of entomology''. Springer Reference, 2008, p. 3311.</ref> and cockroaches,<ref>Costa, James T., ''The Other Insect Societies''. Belknap Press, 2006, p. 151.</ref> certain genera of [[sharks]], [[snake]]s and [[velvet worm]]s. In these, the developing embryo is nourished by some form of placental structure. The earliest known placenta was found recently in a group of extinct fishes called [[Placodermi|placoderms]]. A fossil from [[Australia|Australia's]] [[Gogo Formation]], laid down in the [[Devonian]] period, 380&nbsp;million years ago, was found with an embryo inside it connected by an [[umbilical cord]] to a [[yolk sac]]. The find confirmed the hypothesis that a sub-group of placoderms, called [[ptyctodontida|ptyctodontids]], fertilized their eggs internally. Some fishes that fertilize their eggs internally also give birth to live young, as seen here. This discovery moved our knowledge of live birth back 200&nbsp;million years.<ref name=dennis2008>{{cite journal |title= Nature News: The oldest pregnant mum: Devonian fossilized fish contains an embryo |first= Carina |last= Dennis |date= 2008-05-28 |journal= Nature |doi= 10.1038/453575a |volume= 453 |issue= 7195 |page= 575 |pmid= 18509405 |bibcode= 2008Natur.453..575D |doi-access= free }}</ref> The fossil of another genus was found with three embryos in the same position.<ref name=long2008 >{{cite journal |title= Live birth in the Devonian period |first1= John A. |last1= Long |first2= Kate |last2= Trinastic |first3= Gavin C. |last3= Young |first4= Tim |last4= Senden |date= 2008-05-28 |journal= Nature |doi= 10.1038/nature06966 |volume= 453 |issue= 7195 |pages= 650–652 |pmid=18509443|bibcode= 2008Natur.453..650L |s2cid= 205213348 }}</ref> Placoderms are a sister group of the ancestor of all living jawed fishes ([[Gnathostomata]]), including both [[Chondrichthyes|chondrichthyans]], the sharks & rays, and [[Osteichthyes]], the bony fishes.

Among lizards, the [[viviparous lizard]] ''Zootoca vivipara'', the [[Jackson's chameleon]],<ref>{{Cite web |last=Kundinger |first=Heather |title=Chamaeleo jacksonii (Jacksons chameleon) |url=https://animaldiversity.org/accounts/Chamaeleo_jacksonii/ |access-date=2023-02-27 |website=Animal Diversity Web |language=en |archive-date=2023-02-05 |archive-url=https://web.archive.org/web/20230205212943/https://animaldiversity.org/accounts/Chamaeleo_jacksonii/ |url-status=live }}</ref> [[Anguis|slow worms]] and many species of [[skink]] are viviparous, giving birth to live young. Some are ovoviviparous but others such as members of the genera ''[[Tiliqua]]'' and ''[[Corucia]]'', give birth to live young that develop internally, deriving their nourishment from a mammal-like placenta attached to the inside of the mother's uterus. In a recently described example, an African species, ''Trachylepis ivensi'', has developed a purely reptilian [[placenta]] directly comparable in structure and function to a mammalian placenta.<ref>{{cite journal |vauthors=Blackburn DG, Flemming AF |title=Invasive implantation and intimate placental associations in a placentotrophic African lizard, Trachylepis ivensi (scincidae) |journal=J. Morphol. |volume=273 |issue=2 |pages=137–59 |year=2012 |pmid=21956253 |doi=10.1002/jmor.11011 |s2cid=5191828 }}</ref> Vivipary is rare in [[snake]]s, but [[Boidae|boas]] and [[Viperidae|viper]]s are viviparous, giving birth to live young.<ref>{{cite journal |last1=Neill |first1=Wilfred T. |title=Viviparity in Snakes: Some Ecological and Zoogeographical Considerations |journal=The American Naturalist |date=January 1964 |volume=98 |issue=898 |pages=35–55 |doi=10.1086/282299|s2cid=85209921 }}</ref>

[[File:Aphid-giving-birth.jpg|thumb|Female [[aphid]] giving birth]]
The majority of insects lay eggs but a very few give birth to offspring that are miniature versions of the adult.<ref name=Attenborough/> The [[aphid]] has a complex life cycle and during the summer months is able to multiply with great rapidity. Its reproduction is typically [[parthenogenetic]] and viviparous and females produce unfertilized eggs which they retain within their bodies.<ref>{{Cite journal| doi = 10.1111/j.1095-8312.1979.tb00038.x| issn = 1095-8312| volume = 11| issue = 3| pages = 259–277| last = Blackman| first = Roger L| title = Stability and variation in aphid clonal lineages| journal = Biological Journal of the Linnean Society | year = 1979 }}</ref> The embryos develop within their mothers' [[ovarioles]] and the offspring are [[Cloning|clones]] of their mothers. Female [[Nymph (biology)|nymphs]] are born which grow rapidly and soon produce more female offspring themselves.<ref>{{cite web |url=http://www.backyardnature.net/aphid_lc.htm |title=The aphid life cycle |author=Conrad, Jim |date=2011-12-10 |publisher=The Backyard Nature Website |access-date=2013-08-31 |archive-date=2019-03-18 |archive-url=https://web.archive.org/web/20190318150003/http://www.backyardnature.net/aphid_lc.htm |url-status=live }}</ref>  In some instances, the newborn nymphs already have developing embryos inside them.<ref name=Attenborough/>