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Among
animal Animals (also called Metazoa) are multicellular A multicellular organism is an organism In biology, an organism () is any organic, life, living system that functions as an individual entity. All organisms are composed of cells ...

animal
s, viviparity is development of the embryo inside the body of the parent. This is opposed to
oviparity Oviparous animals are female animals that lay their egg An egg is the organic vessel containing the in which an develops until it can survive on its own, at which point the animal hatches. An egg results from of an . Most s, (excludi ...
which is a reproductive mode in which females lay developing eggs that complete their development and hatch externally from the mother. The term 'viviparity' and its adjective form 'viviparous' derive from the Latin ''vivus'' meaning "living" and ''pario'' meaning "give birth to".


Reproductive mode

Five
modes of reproduction Animals make use of a variety of modes of reproduction to produce their young. Traditionally this variety was classified into three modes, oviparity (embryos in eggs), viviparity (young born live), and ovoviviparity (intermediate between the first ...
have been differentiated in animals based on relations between
zygote A zygote (, ) is a eukaryotic Eukaryotes () are organism In biology, an organism () is any organic, life, living system that functions as an individual entity. All organisms are composed of cells (cell theory). Organisms are ...

zygote
and parents. The five include two nonviviparous modes:
ovuliparity Oviparous animals are animals that lay their eggs, with little or no other embryonic development within the mother. This is the Biological reproduction, reproductive method of most fish, amphibians, most reptiles, pterosaurs, dinosaurs, birds, an ...
, with external fertilisation, and
oviparity Oviparous animals are female animals that lay their egg An egg is the organic vessel containing the in which an develops until it can survive on its own, at which point the animal hatches. An egg results from of an . Most s, (excludi ...
, with internal fertilisation. In the latter, the female lays zygotes as eggs with a large
yolk Among animals which produce eggs, the yolk (; also known as the vitellus) is the nutrient-bearing portion of the egg whose primary function is to supply food for the development of the embryo. Some types of egg contain no yolk, for example beca ...
; this occurs in all birds, most reptiles, and some fishes. These modes are distinguished from viviparity, which covers all the modes that result in live birth: *Histotrophic viviparity: the zygotes develop in the female's
oviduct The oviduct is the passageway in animals from an ovary The ovary is an organ found in the female reproductive system The female reproductive system is made up of the internal and external sex organs that function in reproduction of new of ...
s, but find their nutriments by
oophagy Oophagy ( ) sometimes ovophagy, literally "egg eating", is the practice of embryos feeding on eggs produced by the ovary while still inside the mother's uterus. The word oophagy is formed from the classical Greek (, "egg") and classical Greek ...
or
adelphophagy Cannibalism is the act of consuming another individual of the same species as food Food is any substance consumed to provide nutritional support for an organism In biology, an organism (from Ancient Greek, Greek: ὀργανισμ ...
(
intra-uterine cannibalism Cannibalism is the act of consuming another individual of the same species as food Food is any substance consumed to provide nutritional support for an organism In biology, an organism (from Ancient Greek, Greek: ὀργανισμ ...
of eggs or sibling embryos in some sharks or in the black salamander ''
Salamandra atra The alpine salamander (''Salamandra atra'') is a shiny black salamander found in the Alps from the Swiss-French border at the western end of its range through Austria to the Dinaric Alps at the eastern end of its range, at altitudes above . The ...
''). *Hemotrophic viviparity: nutrients are provided by the female, often through some form of
placenta The placenta is a temporary fetal organ Organ may refer to: Biology * Organ (anatomy) An organ is a group of Tissue (biology), tissues with similar functions. Plant life and animal life rely on many organs that co-exist in organ systems. ...

placenta
. In the frog ''
Gastrotheca ovifera ''Gastrotheca ovifera'' (vernacular names: pouched frog and giant marsupial frog; es, rana marsupial comun or ) is a species of frog A frog is any member of a diverse and largely Carnivore, carnivorous group of short-bodied, tailless amphibi ...
'',
embryo An embryo is the early stage of development of a multicellular organism A multicellular organism is an organism In biology, an organism () is any organic, life, living system that functions as an individual entity. All organisms ar ...

embryo
s are fed by the mother through specialized
gill A gill () is a respiratory organ that many aquatic Aquatic means relating to water Water (chemical formula H2O) is an inorganic, transparent, tasteless, odorless, and nearly colorless chemical substance, which is the main constituent ...
s. The
skink Skinks are lizard Lizards (suborder Lacertilia) are a widespread group of Squamata, squamate reptiles, with over 6,000 species, ranging across all continents except Antarctica, as well as most oceanic island chains. The group is paraphyleti ...

skink
''
Pseudemoia entrecasteauxii The southern grass skink (''Pseudemoia entrecasteauxii)'' is a species In biology, a species is the basic unit of biological classification, classification and a taxonomic rank of an organism, as well as a unit of biodiversity. A species is ...
''and most mammals exhibit a hemotrophic viviparity. * is arguably the most highly developed form of viviparity.
Placental mammals Placentalia is one of the three extant subdivisions of the class of animals Mammalia; the other two are Monotremata and Marsupial Marsupials are any members of the mammal Mammals (from Latin language, Latin , 'breast') are a group ...

Placental mammals
, including
humans Humans (''Homo sapiens'') are the most abundant and widespread species In biology, a species is the basic unit of biological classification, classification and a taxonomic rank of an organism, as well as a unit of biodiversity. A speci ...
, are the best-known example, but adaptations in some other animals also have incorporated this principle or close analogies. Other examples include some species of
scorpion Scorpions are predatory Predation is a biological interaction In ecology Ecology (from el, οἶκος, "house" and el, -λογία, label=none, "study of") is the study of the relationships between living organisms, including hum ...

scorpion
s and
cockroach Cockroaches (or roaches) are insects of the order Blattodea, which also includes termites. About 30 cockroach species out of 4,600 are associated with human habitats. Some species are well-known as Pest (organism), pests. The cockroaches are ...

cockroach
es, certain genera of
sharks Sharks are a group of elasmobranch Elasmobranchii () is a subclass of Chondrichthyes or cartilaginous fish, including shark Sharks are a group of elasmobranch fish characterized by a Chondrichthyes#Skeleton, cartilaginous skeleton, ...
and
snake Snakes are elongated, limbless, carnivore, carnivorous reptiles of the suborder Serpentes . Like all other Squamata, squamates, snakes are ectothermic, amniote vertebrates covered in overlapping Scale (zoology), scales. Many species of snakes ...

snake
s, and
velvet worm Onychophora (from Ancient Greek ονυχής, ''onyches'', "claws"; and φέρειν, ''pherein'', "to carry"), commonly known as velvet worms (due to their velvety texture and somewhat wormlike appearance) or more ambiguously as peripatus (aft ...

velvet worm
s. *
Ovoviviparity Ovoviviparity, ovovivipary, ovivipary, or aplacental viviparity is an outmoded term used as a "bridging" form of reproduction between egg-laying oviparous and live-bearing viviparity , viviparous reproduction. Ovoviviparous animals have the embr ...
, a less developed form of viviparity, occurs in most
vipers The Viperidae (vipers) are a family of venomous snake Venomous snakes are species In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical ...
, and in most live-bearing bony fishes (
Poeciliidae The Poeciliidae are a family In human society, family (from la, familia) is a group of people related either by consanguinity (by recognized birth) or affinity (by marriage or other relationship). The purpose of families is to maintain t ...
). However, the term is poorly and inconsistently defined, and may be obsolete. This term has been redefined and more commonly referred to as oviparous egg retention or prolonged egg retention. At least some transport of nutrients from mother to embryo appears to be common to all viviparous species, but those with fully developed placentas such as found in the
Theria Theria (; Greek#REDIRECT Greek Greek may refer to: Greece Anything of, from, or related to Greece Greece ( el, Ελλάδα, , ), officially the Hellenic Republic, is a country located in Southeast Europe. Its population is approximatel ...
, some skinks, and some fish can rely on the placenta for transfer of all necessary nutrients to the offspring and for removal of all the metabolic wastes as well once it has been fully established during the early phases of a pregnancy. In such species, there is direct, intimate contact between maternal and embryonic tissue, though there also is a
placental barrier The placenta is a temporary fetal organ (anatomy), organ that begins developing from the blastocyst shortly after implantation. It plays critical roles in facilitating nutrient, gas and waste exchange between the physically separate maternal and f ...

placental barrier
to control or prevent uncontrolled exchange and the transfer of
pathogen In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecular interactions, Physiology, physiological mechanism ...
s. In at least one species of
skink Skinks are lizard Lizards (suborder Lacertilia) are a widespread group of Squamata, squamate reptiles, with over 6,000 species, ranging across all continents except Antarctica, as well as most oceanic island chains. The group is paraphyleti ...

skink
in the large genus ''
Trachylepis ''Trachylepis'' is a skink genus in the subfamily Lygosominae found mainly in Africa. Its members were formerly included in the "wastebin taxon" ''Mabuya'', and for some time in ''Euprepis''. As defined today, ''Trachylepis'' contains the clade ...
'', placental transport accounts for nearly all of the provisioning of nutrients to the embryos before birth. In the uterus, the eggs are very small, about 1 mm 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. Blackburn & Flemming (2011) 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 Temperature-dependent sex determination (TSD) is a type of environmental sex determination Environmental sex determination is the Sex-determination system, establishment of sex by a non-genetic cue, such as nutrient availability, experienced within ...
, which cannot function in an aquatic environment, is seen only in terrestrial viviparous reptiles. Therefore, marine viviparous species, including
sea snake Sea snakes, or coral reef snakes, are a subfamily of venomous snake, venomous Elapidae, elapid snakes, the Hydrophiinae, that inhabit Marine (ocean), marine environments for most or all of their lives. Most are extensively adapted to a fully aqua ...

sea snake
s and, it now appears, the
mosasaur Mosasaurs (from Latin ''Mosa'' meaning the 'Meuse', and Ancient Greek, Greek ' meaning 'lizard') comprise a group of extinct, large marine reptiles from the Late Cretaceous. Their first fossil remains were discovered in a limestone quarry at Ma ...
s,
ichthyosaur Ichthyosaurs (Ancient Greek Ancient Greek includes the forms of the used in and the from around 1500 BC to 300 BC. It is often roughly divided into the following periods: (), Dark Ages (), the period (), and the period (). Ancient ...
s, and
plesiosaur The Plesiosauria (; Greek: πλησίος, ''plesios'', meaning "near to" and ''sauros'', meaning "lizard") or plesiosaurs are an order or clade A clade (; from grc, , ''klados'', "branch"), also known as a monophyletic group or natural ...

plesiosaur
s 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 ''Eulamprus'' is a genus of lizards, commonly known as water skinks, in the subfamily Lygosominae of the Family (biology), family Scincidae. The genus is native to Australia. Taxonomy The genus ''Eulamprus'' belongs to a clade in the ''Sphenomo ...
'').


Evolution

In general, viviparity and
matrotrophy Matrotrophy is a form of maternal care during embryo development. Vegetal matrotrophy Some flowering plants supplied the developing embryos for the first stages. The ovule has no stocks and the nutrients are provided by the "mother". In plants, ...
are believed to have evolved from an ancestral condition of oviparity and lecithotrophy (nutrients supplied through the
yolk Among animals which produce eggs, the yolk (; also known as the vitellus) is the nutrient-bearing portion of the egg whose primary function is to supply food for the development of the embryo. Some types of egg contain no yolk, for example beca ...

yolk
). One traditional hypothesis concerning the sequence of evolutionary steps leading to viviparity is a linear model. According to such a model, provided that fertilization was internal, the egg might have been retained for progressively longer periods in the reproductive tract of the mother. Through continued generations of egg retention, viviparous lecithotrophy may have gradually developed; in other words the entire development of the embryo, though still with nutrients provided by the yolk, occurred inside the mother's reproductive tract, after which she would give birth to the young as they hatched. The next evolutionary development would be incipient matrotrophy, in which yolk supplies are gradually reduced and are supplemented with nutrients from the mother's reproductive tract. In many ways, depending on the ecology and life strategy of the species, viviparity may be more strenuous and more physically and energetically taxing on the mother than oviparity. However, its numerous evolutionary origins imply that in some scenarios there must be worthwhile benefits to viviparous modes of reproduction; selective pressures have led to its convergent evolution more than 150 times among the
vertebrate Vertebrates () comprise all species of animal Animals (also called Metazoa) are multicellular A multicellular organism is an organism In biology, an organism () is any organic, life, living system that functions as an indiv ...
s alone. There is no one mode of reproduction that is universally superior in selective terms, but in many circumstances viviparity of various forms offers good protection from parasites and predators and permits flexibility in dealing with problems of reliability and economy in adverse circumstances. Variations on the theme in biology are enormous, ranging from
trophic eggA trophic egg, in most species that produce them, usually is an unfertilised egg (biology), egg because its function is not reproduction but nutrition; in essence it serves as food for offspring hatched from viable eggs. The production of trophic egg ...
s to
resorption Resorption is the absorption into the circulatory system of cells or tissue, usually by osteoclast, osteoclasts. Types of resorption include: * Bone resorption * Herniated Disc Resorption * Tooth resorption * Fetal resorption * Blood resorption S ...
of partly developed embryos in hard times or when they are too numerous for the mother to bring to term, but among the most profoundly advantageous features of viviparity are various forms of physiological support and protection of the embryo, such as
thermoregulation Thermoregulation is the ability of an organism In biology, an organism () is any organic, life, living system that functions as an individual entity. All organisms are composed of cells (cell theory). Organisms are classified by taxo ...
and
osmoregulation Osmoregulation is the active regulation of the osmotic pressure of an organism In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical ...
. Since the developing offspring remains within the mother's body, she becomes, in essence, a walking incubator, protecting the developing young from excessive heat, cold, drought, or flood. This offers powerful options for dealing with excessive changes in climate or when migration events expose populations to unfavourable temperatures or humidities. In
squamate Squamata (, Latin Latin (, or , ) is a classical language belonging to the Italic languages, Italic branch of the Indo-European languages. Latin was originally spoken in the area around Rome, known as Latium. Through the power of the Roman R ...

squamate
reptiles in particular, there is a correlation between high altitudes or latitudes, colder climates and the frequency of viviparity. The idea that the tendency to favour egg-retention selectively under cooler conditions arises from the thermoregulatory benefits, and that it consequently promotes the evolution of viviparity as an adaptation, is known as "the cold climate hypothesis".


Reversion of viviparity

Through ancestral state reconstruction, scientists have shown that the evolution of viviparity to oviparity may have occurred a maximum of eight times in the genus ''
Gerrhonotus ''Gerrhonotus'' is a genus of anguidae, anguid lizards that are commonly referred to as alligator lizards, due to a vague resemblance to the alligator. Along with glass lizards (''Ophisaurus'') and many other lizards, alligator lizards have the ab ...
'' of
lizard Lizards (suborder In biological classification, the order ( la, wikt:ordo#Latin, ordo) is # a taxonomic rank used in the classification of organisms and recognized by the nomenclature codes. The well-known ranks in descending order are: lif ...

lizard
s. Advanced ancestral state reconstruction was used to more accurately prove that the reverse evolution of viviparity to oviparity is true. In the analysis, the authors use a maximum likelihood tree to reveal that parity mode is a labile trait in the Squamata order. They also further show through analysis that viviparity is also strongly associated with cooler climates which suggests the previously stated "cold-climate hypothesis" is true. However, others directly refute this notion that parity is a
labile Lability refers to something that is constantly undergoing change or is likely to undergo change. Biochemistry In reference to biochemistry Biochemistry or biological chemistry, is the study of chemical process In a scientific Sci ...
trait. In their critique, they show that ancestral state reconstruction analyses are reliant on the underlying phylogenetic information provided. The use of a maximum likelihood tree which is vulnerable to phylogenetic error may cause an artificial inflation of the number of viviparity to oviparity occurrences. Additionally, they state that the previous study does not take into account the morphological and behavioral modifications that would have to occur for reversion to occur. Some of these modifications would be the redevelopment of uterine glands to synthesize and secrete shell fibers, the restoration of the careful timing of oviposition due to eggshell thickness, etc. The degradation and loss of function of oviparous genes during viviparous evolution suggests that these genes would have to re-evolve in order for the reversion of this evolution to occur. Since this re-evolution is near impossible due to the complexity of oviparous reproductive mode, the simple labile characteristic of parity cannot be sufficiently supported.


References

*


See also

*
Apomixis In botany, apomixis is asexual reproduction without fertilization. Its etymology is Greek for "away from" + "mixing". This definition notably does not mention meiosis. Thus "normal Plant reproduction#Asexual reproduction, asexual reproduction" o ...
*
Placental mammal The infraclass Placentalia is one of the three extant subdivisions of the class of animals Mammalia; the other two are Monotremata and Marsupialia. Placentalia contains the vast majority of extant mammals. Placentals are partly distinguished fro ...
s *
Marsupial Marsupials are any members of the mammal Mammals (from Latin Latin (, or , ) is a classical language A classical language is a language A language is a structured system of communication Communication (from Latin ' ...
s *
Live-bearing aquarium fish Guppy fry Livebearers are aquarium fish Fish are Aquatic animal, aquatic, craniate, gill-bearing animals that lack Limb (anatomy), limbs with Digit (anatomy), digits. They form a sister group to the tunicates, together forming the Chordate#T ...
* False vivipary {{Authority control Reproduction in animals