An animal that is viviparous gives birth to developed live young. The embryos are raised with special organs in the parents as they develop, that supply nutrients to the growing embryos. This condition is known as matrotrophy, when the embryo receives nutrients directly from the mother and not from the yolk. The difference between ovoviviparity and viviparity is that viviparous animals feed their embryos with nutrients from the mother. Without a large egg, there is no large yolk sac for the embryo to survive on. Different groups of viviparous animals have evolved throughout time, weighing in on the various pros and cons of being viviparous.
Viviparous animals exist in almost every vertebrate taxa, while there are few, if any, in the invertebrate taxa (though many invertebrates are ovoviviparous). Of the recognizable groups of animals, only the birds do not show some form of viviparity. Fish, amphibians, reptiles, and mammals all have viviparous members, while none of the group are exclusively viviparous. Developing the young viviparously appears to be a derived trait from oviparous animals. The theory is that some oviparous animals tend to develop eggs longer than others internally. In some species, this leads to the young being hatched within the mother, or ovoviviparity. In ovoviviparous animals, it is often the case that the young feed on each other for nourishment after hatching but while in the womb. Some of these ovoviviparous species began supplementing their offspring with nutrients secreted by the oviducts or other parts of the reproductive tract. In some primitive viviparous animals, the yolk sac still plays a role, although the sac receives extra nutrients from special tissues in the oviduct. In some other viviparous animals, the young develop into young larvae inside the mother, and feed on special secretions from her reproductive tract. Mammals have taken this concept one step further, birthing young earlier and feeding them on a nutritive substance from the mammary glands. This combines benefits of viviparous animals with the ability to shorten the gestation time and demand on the mother.
Viviparous animals must reproduce sexually through internal fertilization, for that is where their fertilized eggs develop. Males must have some structure to fertilize the female. This can be a penis in mammals, claspers in sharks, or even the strange gel-like structure that male salamanders leave behind for their mates to find, which the females deposit internally. It is interesting that birds are the only group of animals which does not contain any viviparous species. This is thought to be because of how birds evolved. Early in the evolution of birds, they became endothermic. This means they could simply brood their eggs and receive similar results as viviparous animals. Some snakes and reptiles, on the other hand, developed viviparity and ovoviviparity in order to move their eggs to the sun to warm them up in colder climates. While the groups of animals that are viviparous vary widely, it has common pros and cons.
All viviparous animals are able to move their developing young, which is of utmost importance in areas with high amounts of predators. Another important benefit of viviparity is the ability to reproduce at any time of the year. Unlike oviparous animals, which must produce yolk sacs when their food intake is the highest, viviparous animals can nourish their young with fat reserves. This allows viviparous animals to mate any time they get a suitable interaction. On the downside, being viviparous is extremely costly to the female mother. Depending on the exact mode of viviparity, the female’s organs and reproductive structure can be severely damaged by the young offspring. Carrying and feeding offspring is a much more energetically costly endeavor than simply laying an egg. Further, carrying a brood of embryos makes females slower, and they become an easier target for predators. Because of these and other advantages and disadvantages, viviparous animals are not the most widespread animals. Viviparity has developed many different times, in response to very different environmental conditions that favored the benefits of viviparity over its downfalls.
Examples of Viviparous
Humans, like most mammals, are viviparous animals. Humans reproduce via internal fertilization. As in all higher mammals, the egg implants in the uterine wall while it develops. The uterine wall develops a structure known as the placenta, containing many blood vessels. This tissue surrounds the embryo, supplies it with nutrients, and removes waste products. The embryo develops into a small fetus, and the fetus into a baby. Unlike some viviparous animals, humans are born well before they are fully developed. This is seen in many mammals, as milk provides a rich and nutritive substance for babies. Further, it lessens the burden of the mother as she doesn’t have to carry the baby for the many years it would take for it to become fully developed. This frees her up to gather food for the baby and become impregnated again. While most mammals show high levels of parental care, it is not a requirement of viviparous animals.
Sharks are an example of an animal that is viviparous and exhibits very little to no parental care. While sharks range widely in their reproductive strategies, a few of them have evolved advanced methods of viviparity, resembling that of mammals. Sharks such as the Great White have tissues that behave much like the placenta of mammals. These tissues form as outgrowths of the oviducts, and send spaghetti-like strands into the gills of each developing shark. The tissues exchange oxygen and pass nutrients to the young sharks in the form of a secreted milky substance. Once the sharks reach a stage in which they could survive on their own, they detach from the oviduct and find their way to the cloaca. After being born into the environment, the sharks need very little parental care as they can start eating fish and other prey species right away.
Much like the sharks, the amphibians have several derived groups that form placenta-like structures during their viviparous reproduction cycles. Many salamanders and some frogs reproduce using this method. Again, the oviducts are specially formed to not only transport eggs to the cloaca, but to nourish them along the way. Young embryos attach to the oviduct and scrape its surface with special embryonic teeth. The scraping stimulates the oviduct to produce a nutritive substance, and the young subsist on the substance until they are developed enough to be born. Many amphibians are still born in their larval stage, and must develop further and undergo metamorphosis before they become adults.