As viviparous animals are grouped those creatures whose embryos evolve within the womb before birth. Although there are exceptions to this rule, their bodies usually come into the world well formed despite the fact that they still show some dependence on their parent. By continuing reading you will learn much more about these species.
viviparous animals
Viviparity (from the Latin vivus "alive" and parire "to give birth") is known as that form of reproduction by means of which embryonic development takes place within the mother. Under this scheme meets most of the mammals, frequently called as viviparous animals. However, there are also other classes of animals that reproduce based on viviparity, such as some amphibians, reptiles and fish.
What is a biosimilar
The animal whose embryonic development, after fertilization, is conducted within a specialized organ of the maternal womb, is called viviparous. Within such a structure, known as the placenta, it will be provided with the nutrients and oxygen required to form its organs, for its growth and maturation until the moment of birth.
The most evolved form of viviparism is called placental viviparism, which is the one that occurs in practically all mammals except monotremes, which release eggs, and marsupials, which, lacking a placenta, the fetus is born in a very early condition. , and must continue its development in a sack or outer bag called a marsupium. Likewise, certain insects, reptiles, fish and urodele amphibians are viviparous.
There are also viviparous plants, which generate seeds that sprout before being released from the mother plant. In numerous mangrove swamps, for example, the seed germinates and manages to grow, providing itself with its own energy while it remains attached to its mother and then falls into the water in which it will be transported.
Origin of Viviparism
Evolutionarily, viviparity emerged as a means to protect the young from predators of certain species such as cartilaginous fishes and the extinct placodonts. For a long time, such cartilaginous fish (including sharks, rays and chimaeras) have been optimizing this form of reproduction.
Some time later, in the Triassic period, two reptiles developed viviparity in very different ways. A group, that of the advanced cynodonts or cynognathians, improved this way of reproducing to supplement the care of their young. The other group that developed it was that of sea reptiles, since land animals or amniotes cannot release eggs into the water.
Evolution of Viviparous Animals
It is generally accepted that viviparity and matrotrophy (form in which the embryo feeds) have evolved based on an ancestral situation of oviparity and lecytotrophy (nutrients provided through the yolk). A traditional theory about the order of evolutionary steps leading to viviparity is a linear pattern. According to this pattern, as long as fertilization was internal, the egg could have been preserved for longer and longer periods in the mother's reproductive duct.
For uninterrupted generations in which eggs were retained, viviparous lecytotrophy may have gradually evolved; In other words, all the embryonic development, despite the fact that the nutrients were still supplied by the yolk, occurred within the reproductive duct of the mother, after which she would give birth to the young at birth.
The next evolutionary development would be a rudimentary matrotrophy, in which what is supplied by the yolk progressively decreases and is supplemented with nutrients from the mother's reproductive duct.
In many ways, and according to the ecology and life plan of the species, viviparity can be considered more exhausting and of greater physical and energetic expenditure for the mother than oviparity. However, its extensive evolutionary origins mean that in certain scenarios there must be beneficial benefits for viviparous forms of reproduction. Selection pressures have led to their convergent development on more than 150 occasions among vertebrates alone.
There is no single mode of reproduction that is esteemed superior as far as selection is concerned, but in many circumstances viviparity in various ways provides desirable protection against parasites and predators and allows flexibility to account for reliability and economics drawbacks in adverse conditions. .
The variations on the subject in biology are immense, from trophic eggs to the reabsorption of moderately evolved embryos in unfavorable times or when they are too numerous for the mother to complete them, however, among the most advantageous particularities of viviparity are various forms of physiology. The support and protection of the embryo through thermoregulation and osmoregulation.
Since the developing offspring remain inside the mother's body, they essentially become a walking incubator, providing protection for the evolving young from excessive heat, cold, drought, or waterlogging. This provides powerful alternatives to deal with exaggerated changes in climate or when migratory events leave populations exposed to adverse temperatures or humidity.
In scaled reptiles, especially, there is a correspondence between high elevations or latitudes, colder climates, and how frequent viviparity is. The notion that the propensity to selectively promote egg retention in cooler conditions emerges from the benefits of thermoregulation, and by virtue of this favors the evolution of viviparity as an adaptation, which is known as "The Cold Climate Hypothesis".
Embryonic Development in Animals
But to truly understand what viviparous animals are, it is essential to refer to embryonic development, which is the period that passes from fertilization to the birth of a new being. In such a way that, in the sexual reproduction of animals, three categories of embryonic development can be distinguished:
- viviparous animals: after internal fertilization, the embryos grow within a specialized organism within the maternal body (placenta), which protects and nourishes them until they are fully formed and ready for childbirth.
- Oviparous animals: on this occasion internal fertilization also occurs, however, embryonic development occurs outside the mother's body, inside an egg.
- Ovoviviparous Animals: also through internal fertilization, the embryos of ovoviviparous animals grow inside an egg, although this time the egg is also housed inside the mother's body, until hatching and, therefore, the birth of the pups.
Types of Reproduction of Viviparous Animals
However, in addition to distinguishing between the different categories of embryonic development, we must know that there are different types of reproduction among viviparous animals:
- Placental viviparous: they are those that have their development within the placenta, an organism attached to the uterus that expands through gestation to provide space for fetuses. An example of this is the human being.
- Marsupial viviparous: In contrast to other mammals, marsupials are born before they are fully developed, and their missing formation takes place within the marsupium, an external sac that has a placenta-like function. The best known example is the kangaroo.
- Ovoviviparous: as already mentioned, it is a combination between viviparism and oviparism. On this occasion, the mother releases the eggs inside her body, where they develop until they are fully formed. The birth of the young can take place inside the mother's body or outside.
Characteristics of Viviparous Animals
The viviparism of the placenta, is the one carried out by most mammals, it is a gestation procedure with greater evolution and development than that used by oviparous animals, since their fetus grows within a specialized organism called the placenta. In addition to obtaining oxygen and nutrients, embryonic development inside the parent gives them greater protection when compared to oviparous animals.
Another important feature is that developing viviparous creatures do not have a solid outer shell. The placenta is a membranous structure containing a rich and vigorous supply of blood that covers the uterus of pregnant females. The fetus is fed through a supply line called the umbilical cord. The time between fertilization and the birth of viviparous animals is called the gestation period or pregnancy and varies between different species.
Among the most relevant aspects of mammals as viviparous animals is the transcendental transition experienced by females after an ovule is fertilized and the period of gestation or pregnancy begins. In this phase, the uterus grows in size in relation to the development of the zygote, and the female begins to undergo a succession of changes inside her as well as outside her, in complete natural preparation for this entire procedure.
The duration of the gestation period varies according to the species, but it usually ends when the embryo is sufficiently evolved to be born. Regarding the human species, this gestation can last around 9 months, while in lions it does not exceed 110 days, and for mice, only about 20 days.
Most viviparous animals are quadrupedal, which means that they require four legs to stand, walk and move. Most mammalian mothers have a powerful and close maternal instinct to feed and protect their young until it can fend for itself. The female will have to know exactly the arrival of that moment.
Existing outside the womb means that they have the ability to survive as an autonomous organism that can obtain food, breathe, etc. However, in a significant number of species some care is still required. With regard to mammals, they even need to be breastfed for a certain time.
In the animal environment there is also another form of viviparism, which is less frequent. We are talking about marsupials, whose most notorious case is that of the kangaroo. Marsupials are beings that give birth to their young in an immature condition and subsequently retain and nurse them in the bags or sacs they have in their abdomen. The newborns remain in this space until they complete their formation and, then, they will no longer need breast milk to survive.
Examples of Viviparous Animals
What are the viviparous species? Practically all mammalian animals have viviparous reproduction, there are only a few exceptional cases of oviparous mammals, which are called monotremes, which are mainly represented by the echidna and the platypus. In this group we must also incorporate marine varieties such as dolphins, whales and narwhals, as well as the solitary species of mammal that flies: the bat.
Terrestrial Mammals
Among them are the well-known examples of: Dog, Cat, Rabbit, Horse, Cow, Pig, Giraffe, Lion, Chimpanzee, Elephant, etc.
Aquatic Mammals
The species that are worth noting in this group are: Dolphin, Whale, Sperm Whale, Orca, Narwhal, etc.
viviparous fish
Although biologically they are ovoviviparous animals, the species of guppies, platys or mollies are classified as viviparous animals. Let's get to know some of the most common viviparous fish: Poecilia reticulata, Poecilia sphenops, Poecilia wingei, Xiphophorus maculatus, Xiphophorus helleri, Dermogenys pusillus, Nomorhamphus liemi, etc.
viviparous amphibians
As in the case of viviparous fish, viviparous amphibians are not particularly common, however, two representative creatures can be recognized in the Caudata order: Triton and Salamander
viviparous reptiles
To conclude our list of viviparous animals we must refer to certain viviparous reptiles. Despite most of the reptiles are oviparous, we also find specific species that practice viviparism: Boa (Boidae) and Sea Serpent (Hydrophiinae).
Other articles that may interest you are:
- Oviparous animals
- Mammals
- Vertebrate animals