We explain what are viviparous animals, how is their reproduction and gestation. In addition, differences with oviparous and ovoviviparous animals.
What are viviparous animals?
Viviparous animals are those that reproduce through internal fertilization and their embryos develop in specialized organs within the mother’s womb . The embryos leave the mother’s body at the end of gestation, through a birth canal. In this they are distinguished from oviparous and ovoviviparous animals.
Embryos of viviparous animals are connected to the mother’s body from which they receive nutrition and defense . Birth occurs when the embryos have matured and the beginning of their individual existence corresponds.
That they can have an existence outside the maternal body means that they are able to subsist as an independent organism, which can feed, extract, breathe, etc. However, a large part of the species continue to require care. In the case of mammals they even require breastfeeding until later.
It is assumed that the emergence of viviparity in animals occurred with the appearance of the first mammals, since reptiles are oviparous. There are several hypotheses to explain this evolutionary change , but they coincide in the advantages over risk factors such as predation, cold weather or the physical risk of preserving the embryo within the warm maternal body.
It is estimated that it was also the evolutionary step that would allow reptiles to adapt to cold climates where egg laying was too thermally risky.
Examples of viviparous animals
All mammals are perfect examples of viviparous animals , regardless of how long their gestation period lasts. From cats, dogs, rats, pigs and rabbits, to lions, giraffes, elephants, apes and even the human being.
Marine mammals are not exempt: killer whales, dolphins, whales, seals, narwhals or sperm whales, as well as some specific types of amphibians such as salamanders and newts.
Gestation of the viviparous
Gestation is the time that the embryo fertilized within the womb lasts, until maturation and expulsion through the birth canal . During this period, the maternal body nourishes the embryo through an umbilical cord or equivalent, sharing blood, fluids and nutrients, which involves important changes in the mother’s metabolism and behavior.
The duration of this gestation period may vary according to the species, but usually ends when the embryo is sufficiently developed to be born. In the case of humans this gestation is around 9 months, while in lions it does not exceed 110 days, and in the case of mice, only about 20.
Reproduction of the viviparous
The reproduction of viviparous animals is generally and mostly sexual, that is, through intercourse between males and females , during which the internal fertilization of the female occurs. For this, the male penetrates it with his penis and deposits its seminal fluid inside, in which the sperm go.
When sperm enter the ovule, that is, they fertilize it, the embryo is produced. The latter grows inside the womb, wrapped in a placenta, for a certain period of time and is finally expelled through the birth canal, to begin its existence as an independent organism.
Oviparous animals, unlike viviparous animals, are those that lay eggs, as do lizards, birds or fish, among many others. This form of reproduction is much older than viviparism.
In some cases, the fertilization is internal, that is to say that the already fertilized eggs are deposited by the female and hatch on their own later, when the embryos are mature. In other cases, fertilization is external: the female deposits her eggs without fertilizing and then the male sprays them with her sexual fluids, fertilizing them outside the mother’s body.
In both cases, fertilized eggs allow the growth of the embryo in a protected area and isolated from the outside by means of an impermeable shell, within which are all the materials necessary for its development.
The relationship between the parents and their fertilized eggs can be very diverse . Some species take care of them jealously or even transport them from side to side. The mother can watch her eggs, hatch them (warm them with her body, like birds) or bury them in a safe place, waiting for the eggs to hatch.
In other species the female leaves them to their fate, depositing large amounts to ensure that at least some percentage of them survive.
Ovoviviparous animals are a sort of intermediate category between oviparous and viviparous . Where appropriate, eggs are produced within the mother through internal fertilization through intercourse, but these remain within the maternal body until the embryos are sufficiently developed.
The laying is done when the eggs are already close to hatching, or directly to hatching, giving the erroneous impression that the offspring have been calved.
Unlike viviparous animals, these animals are not connected with their young through the placenta , so the development of embryos does not depend on the food resources of their organism, but on the content of each egg. At most, the maternal body allows the exchange of gases, as in the case of sharks and rays.
This is a common breeding method in many species of fish, sharks, rays, some reptiles (such as chameleons) and certain invertebrate animals .