We explain what viviparous animals are, 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 into specialized organs within the mother’s womb. The embryos leave the maternal body at the end of gestation, through a birth canal. In this they differ from oviparous and ovoviviparous animals.
The embryos of viviparous animals are connected to the mother’s body from which they receive nutrition and defense. Birth occurs when the embryos have already 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, most of the species continue to require care. In the case of mammals they even require suckling 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 agree on the advantages over risk factors such as predation, cold weather or the physical risk involved in keeping the embryo inside the warm maternal body.
It is estimated that it was also the evolutionary step that would allow reptiles to adapt to cold climates in which egg laying was too thermally risky.
Examples of viviparous animals
All mammals are perfect examples of viviparous animalsregardless of how long your gestation period lasts. From cats, dogs, rats, pigs and rabbits, to lions, giraffes, elephants, apes and even humans.
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.
How is the viviparous gestation?
Gestation It is the time that the fertilized embryo lasts inside the mother’s womb, until its 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 entails important changes in the mother’s metabolism and behavior.
The length of this gestation period can vary according to the species, but it 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.
How do they reproduce?
Reproduction of viviparous animals it is generally and mostly sexual, that is, through intercourse between males and females, during which the internal fertilization of the female occurs. To do this, the male penetrates it with his penis and deposits his seminal fluid inside, in which the sperm go.
When the sperm enter the egg, that is, they fertilize it, the embryo is produced. The latter grows inside the mother’s 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
Oviparous animals, unlike viviparous, 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, fertilization is internal, that is, the 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 lays her unfertilized eggs and then the male sprays them with his sexual fluids, fertilizing them outside the maternal body.
In both cases, fertilized eggs allow the embryo to grow in a protected environment isolated from the outside by means of a waterproof shell, inside which are all the materials necessary for its development.
The relationship between parents and their fertilized eggs can be very diverse. Some species jealously guard them or even transport them from one place to another. The mother can watch over her eggs, hatch them (warm them with her body, like birds do) or bury them in a safe place, waiting for the eggs to hatch.
In other species the female leaves them to their own devices, depositing large amounts to ensure that at least some percentage of them survive.
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Ovoviviparous animals
Ovoviviparous animals are a kind of intermediate category between oviparous and viviparous. In their case, the eggs are produced within the mother through internal fertilization through intercourse, but these remain within the mother’s body until the embryos are sufficiently developed.
The laying is done when the eggs are close to hatching, or directly at hatching, giving the wrong impression that the young have been calved.
Unlike viviparous animals, these animals are not connected to their young through the placenta, so that the development of embryos does not depend on the nutritional resources of your body, but on the content of each egg. At most, the maternal body allows gas exchange, as in the case of sharks and rays.
This is a common breeding method for many species of fish, sharks, rays, some reptiles (such as chameleons), and certain invertebrate animals.