por What are viviparous?
The viviparous are those animals that are «born alive», that is, those that develop within specialized structures of the mother, from whom they receive metabolic sustenance and through which they can carry out gas exchange and eliminate their waste products.
In other words, viviparity is a reproductive pattern by which females retain developing fertilized eggs inside their reproductive cavities, to later give birth to offspring with some independence or «free-living» capacity (depending on the species).
Viviparity is perhaps one of the most significant adaptations in the reproduction of vertebrate animals, since it eliminates the «environmental pressures» imposed on the young, since they develop inside the mother’s body, escaping predation, dehydration, freezing, famine, etc.
Unlike oviparous animals, which reproduce through the formation of structures known as «eggs», within which there is usually enough food for the development of embryos, viviparous animals depend on the placenta, an internal structure that supports hatchling growth and viability.
In addition to not laying eggs, viviparous animal species are distinguished from oviparous in that reproductive behaviors in relation to parental care are a bit more evident, especially in mammals (both placental and non-placental).
Viviparous characteristics
The term «viviparous» is used especially to denote species in which the embryos are retained within the female until their full development, after which they may or may not hatch and leave the female’s body.
The condition of viviparity has arisen more or less 160 times among the different groups of animals. It is represented in some bony and cartilaginous fish, in some amphibian species, it is characteristic of practically all mammals, squamata reptiles and some groups of invertebrate animals, including some insects.
The form of nutrition of the embryos, characteristic of viviparous species, is considerably different from that of oviparous species, since in the former, feeding does not occur due to the presence of a «yolk» or yolk (lecitotrophy), but depends mainly on the mother. (matrotrophy or placentotrophy).
Some authors consider that many viviparous species (with the exception of mammals) are also lecithotrophic, that is, that the eggs are retained within the female reproductive tract, but an important part of their development depends on the nutrients contained inside (yolk). ).
Mammals, on the contrary, are exclusive matrotrophic viviparous, in which absolutely all nutrients are provided by the mother during gestation and through the placenta or some substances in the reproductive tract.
development of the placenta
The placenta, the tissue that nourishes viviparous embryos, is formed during embryogenesis from the first event of cell specification, which results in the formation of a monolayer of polarized epithelial cells known as the trophectoderm, which surrounds the blastocoel cavity ( see embryonic development).
The process of formation of this structure is controlled by different hormonal and genetic signals. Inside the blastocoel cavity, the cells present there form the embryo (fetus) and are also responsible for the formation of the membranes of the amniotic sac and allantois, extraembryonic membranes.
The placenta is, then, an organ that is formed by «apposition» of special tissues from the mother and the embryo. It works specifically in the exchange of nutrients, in the protection of the embryo, in the production of immunomodulatory agents that promote the immune acceptance of the fetus by the mother, in the endocrine support of pregnancy (hormone production), etc.
Reproduction
Just as it is true for oviparous animals, the offspring of viviparous animals are formed thanks to sexual reproduction, through which two different sexual cells fuse: the ovule and the spermatozoon; which give rise to a zygote and this, in turn, to an embryo.
However, the viviparous differ from the oviparous in that, in the former, the fertilization of the ovocell is strictly internal, that is, external fertilization does not occur as in many oviparous fish and amphibians.
This condition implies the development of somewhat more complex reproductive structures, so as to «ensure» contact between the sex cells or gametes of the male and the female.
The most common site for the development of the embryo in viviparous animals is represented by the oviduct (the uterus in mammals), which means that, evolutionarily, an organ has been «recruited» for the «accommodation» of the egg during its development.
The gestation times of viviparous species are highly variable, however, it is a general rule that they are much longer than those of oviparous species. Only in mammals, for example, the gestation time can vary from 20 days to 660 and the size and weight of the young is also highly variable.
Examples of viviparous species
Fish
Most fish are oviparous, only a few species are viviparous, and these are mostly freshwater species. Among some of the viviparous saltwater fish we can find several sharks.
The «guppy» fish (Poecilia reticulata) is one of the most used fish in ornamental aquaculture. It reproduces viviparously and can have between 30 and 200 fry per litter, depending on the species.
Once the female’s eggs are fertilized, the development of the fry occurs inside the mother, feeding through the placenta. These fish have the ability to store sperm inside their bodies and therefore can have several clutches from a single mating.
Viviparous sharks are one of the few saltwater fish that have this feature; Sharks that exhibit oviparous or ovoviviparous behavior are considered “less ancestral” sharks.
In general, sharks have one or two pups per birth, these develop inside the mother once the egg cells are fertilized. The embryos feed from the mother through the umbilical cord that joins them to it.
amphibians
Within the group of amphibians, the gestation of pups viviparously is not the most common. However, about 100 species, including newts and salamanders, bear their young in this way.
In these organisms, the hatchlings are born in a highly developed larval stage, and some even already have the developed features as would be observed in adulthood. Most larvae already have lungs to breathe efficiently in the terrestrial environment.
Young hatched in this way are almost completely independent of aquatic environments. Viviparity is very common in the Salamandridae family and scientists have linked the development of viviparity in this family with the fertilization of the male inside the female.
The male secretes sperm into a gelatinous sac called a «spermatophore» (which carries the sperm). The female takes the spermatophore with her cloacal lips and stores it in a specialized structure called the «spermatheca».
Fertilization and embryo development can occur long after the female has collected the spermatophore.
reptiles
In reptiles, viviparity is rare, it is only found in the Squamata group, in some species of snakes and lizards. Among the most recognized viviparous species are rattlesnakes, anacondas, and sea snakes.
Viviparous snakes have a placenta to support their young until they are born. Through the placenta, the embryo can feed itself, carry out its gas exchange, and excrete waste from its body.
All the snakes of the Boido family are viviparous, they can give birth to between 40 and 70 young in a single litter. The newborn pups measure between 60 and 80 cm and, from the moment of their birth, they have to fend for themselves, since the mothers do not have parental care.
Scientists speculate that the origin of viviparous species occurred in reptile species that inhabited high altitudes and very low temperatures. This mode of gestation provided advantages over oviparous reptile species to survive in these conditions.
mammals
With the exception of the platypus, all mammals are viviparous. Most of these animals show complex reproductive and parental care patterns.
Although it is not a general rule, young mammals depend to some extent on their mothers for subsistence during the early stages of their juvenile lives. In addition, a close bond between mothers and pups has been demonstrated.
Most mammals are placental, that is, their embryos feed through the placenta or, what is the same, through maternal fluids. Furthermore, a distinctive feature of mammals is that their young feed on the milk produced by the females’ breasts.
Marsupials, a subgroup within mammals, are animals that give birth to underdeveloped young, which complete their growth inside a bag in the female known as a «marsupium», where the breasts are also found. They differ from other placental mammals by the premature birth of their young.
insects
Among insects there are few examples of viviparity, since the vast majority lay eggs (oviparous) or raise the eggs inside until the young hatch (ovoviviparous).
Aphids or aphids, as they are commonly known, are insects that can present both types of reproduction, oviparous or viviparous, and exhibit very complex life cycles.
One of the cycles implies that these insects live on a single plant; the female (of sexual origin) lays a single egg before winter; when the egg hatches, it gives rise to the founding female which, when it grows and matures, gives rise to multiple viviparous females.
The new females that arose from viviparity differ little from the parent female that gave rise to them. These are smaller and have a much lower fertility rate.
References
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