VIVIPAROUS: CHARACTERISTICS, REPRODUCTION AND EXAMPLES OF SPECIES - BIOLOGY - 2025

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 the developing fertilized eggs inside their reproductive cavities, to later give birth to young with some independence or capacity for "free life" (depends on the species).

Gray kangaroo mother and calf, a viviparous species. JJ Harrison (https://www.jjharrison.com.au/)

Viviparity is perhaps one of the most significant adaptations in the reproduction of vertebrate animals, since it eliminates the "environmental pressures" imposed on the offspring, since they develop inside the mother's body, escaping from 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 growth and viability of the young.

In addition to not laying eggs, viviparous animal species are distinguished from oviparous in that reproductive behaviors in relation to parental care are somewhat more evident, especially in mammals (both placental and non-placental).

General characteristics

Photograph of an elephant, a viviparous animal that is 660 days gestation (Source: "Axel Tschentscher" via Wikimedia Commons)

The term "viviparous" is used especially to denote species in which 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 species of amphibians, it is typical 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 “bud” or yolk (lecithotrophy), 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, the eggs are retained within the reproductive tract of females, but an important part of their development depends on the nutrients contained within (yolk).

Mammals, by contrast, 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 cell specification event, which results in the formation of a polarized epithelial cell monolayer known as the trophectoderm, which surrounds the blastocelic cavity (see embryonic development).

The process of formation of this structure is controlled by different hormonal and genetic signals. Within the cavity of the blastocele, the cells present there form the embryo (fetus) and are also responsible for the formation of the membranes of the amniotic sac and the allantois, extra-embryonic membranes.

The placenta is, then, an organ that is formed by "apposition" of special tissues of 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 (production of hormones), 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 sex cells fuse: the ovum and the sperm; that give rise to a zygote and this, in turn, to an embryo.

However, viviparous ones differ from oviparous ones in that, in the former, the fertilization of the egg 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 that contact between the sex cells or the gametes of the male and female is "ensured".

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

Fishes

Viviparous male and female guppy fish (Source: Anton Melqkov via Wikimedia Commons)

Most fish are oviparous, only a few species are viviparous and these are mostly freshwater species. Among some of the viviparous fish that are saltwater we can find several sharks.

The guppy fish (Poecilia reticulata) is one of the most widely used fish in ornamental aquaculture. It reproduces viviparously and can have between 30 and 200 fingerlings per litter, depending on the species.

Once the eggs of the female are fertilized, the development of the fry occurs inside the mother, feeding through the placenta. These fish have the ability to store sperm inside the body and therefore they can have several spawn from a single mating.

Viviparous sharks are one of the few saltwater fish that have this characteristic; “less ancestral” sharks are considered to be those with oviparous or ovoviviparous behavior.

In general, sharks have one or two offspring per birth, these develop inside the mother once the egg cells are fertilized. The embryos feed on the mother through the umbilical cord that connects them to her.

Amphibians

Viviparous lizard (Source: Ocrdu via Wikimedia Commons)

Within the amphibian group, viviparous young gestation is not the most common. However, about 100 species, including these newts and salamanders, gestate their young in this way.

In these organisms, neonates are born in a highly developed larval stage, and some even have the developed traits as they would be seen in adulthood. Most larvae already have lungs to breathe efficiently in the terrestrial environment.

Young born 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 the sperm in 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 "spermatheca."

Fertilization and embryo development can occur long after the female collects the spermatophore.

Reptiles

Viviparous snakes (Source: Internet Archive Book Images via Wikimedia Commons)

Viviparity is rare in reptiles, 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 provide sustenance for their young until birth. Through the placenta, the embryo can feed, carry out its gas exchange and excrete waste from its body.

All the snakes of the Boido family are viviparous, they can give birth between 40 and 70 young in a single litter. 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 lived at high altitudes and at 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 mammalian animals are viviparous. Most of these animals show complex reproductive and parental care patterns.

Although it is not a general rule, the young of mammals depend to some extent on their mothers to survive during the early stages of their juvenile life. In addition, a close bond between mothers and young has been shown.

Most mammals are placental, that is, their embryos feed through the placenta or, what is the same, through maternal fluids. In addition, a distinctive feature of mammals is that their young feed on the milk produced by the breasts of females.

Marsupials, a subgroup within mammals, are animals that give birth to underdeveloped offspring, which complete their growth inside a pouch 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 viviparism, since the vast majority lay eggs (oviparous) or raise the eggs inside them until the hatchlings (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 that, as it grows and matures, produces multiple viviparous females.

The new females that emerged from viviparism differ little from the parent female that gave rise to them. These are smaller and have a much lower degree of fertility.

References

1. Brusca, RC, & Brusca, GJ (2003). Invertebrates (No. QL 362. B78 2003). Basingstoke.
2. Hickman, CP, Roberts, LS, Larson, A., Ober, WC, & Garrison, C. (2001). Integrated principles of zoology (Vol. 15). New York: McGraw-Hill.
3. Kardong, KV (2002). Vertebrates: comparative anatomy, function, evolution (No. QL805 K35 2006). New York: McGraw-Hill.
4. Lodé, T. (2012). Oviparity or viviparity? That is the question…. Reproductive Biology, 12 (3), 259-264.
5. Solomon, EP, Berg, LR, & Martin, DW (2011). Biology (9th edn). Brooks / Cole, Cengage Learning: USA.
6. Tremblay, E. (1997). Embryonic development; oviparity and viviparity, (pp. 257-260). Ben-Dov Y., Hodgson Ch. J. (Eds). Soft scale insects – their biology, natural enemies and control. Amsterdam, New York.