Oviparity (biology)

Oviparity is a form of sexual reproduction in which animals lay eggs. Oviparity is one of the three traditional modes of reproduction, with the others including viviparity and ovoviviparity. In oviparous animals, eggs can be fertilized either internally or externally, but hatching always takes place outside the body. Oviparity is common among birds, fish, amphibians, and reptiles. It is comparatively rare among mammals, with only the platypus and some species of echidnas incubating their eggs externally. Oviparity is often thought to be the ancestral reproductive condition of many animals, with other modes of reproduction arising later through evolution. This means that at least some contemporary oviparous species remain oviparous simply because they are unable to transition to a more evolved mode of reproduction for one reason or another. In any event, oviparity remains a common mode of reproduction that has key advantages and disadvantages for species that multiply in that manner.

Background

There are three traditional modes of sexual reproduction in animals. These include oviparity, ovoviviparity, and viviparity. All three are distinguished from one another based on the manner in which a given species incubates its fertilized eggs.

Perhaps the oldest and most common mode of sexual reproduction is oviparity. While fertilization can be internal or external among oviparous species, incubation and hatching always occur outside the mother’s body. In other words, oviparous animals lay embryo-containing eggs that eventually hatch after being incubated externally. A large number of animals are oviparous, including all birds.

Ovoviviparity is similar to oviparity but differs in a number of key ways. Unlike oviparity, which begins with eggs that can be fertilized either internally or externally, ovoviviparous fertilization is strictly internal. Ovoviviparity is also distinct from oviparity in that the fertilized eggs are not laid but instead remain inside the mother until after they hatch. Among most ovoviviparous species, the eggs are fertilized in the oviducts and remain there throughout their development. Before they hatch, the embryos rely on the egg yolk inside the egg for nourishment. When the eggs eventually hatch, the fetuses stay in the oviduct until they develop enough to be born. During this period, the fetuses subsist on secretions or unfertilized egg yolks. Some examples of ovoviviparous animals include rays, certain sharks, and various snakes, fish, and insects.

Viviparity is the most complex mode of reproduction. Unlike either oviparity or ovoviviparity, viviparity does not involve the laying or hatching of eggs. Instead, fetuses gestate inside the mother’s womb and are eventually born live. This gestation period typically lasts for at least several months. Further, the offspring of viviparous animals usually stay with the mother for months or even years after birth. Shelled eggs in the sense of chicken or lizard eggs are not involved in the process at all. Many viviparous species normally give birth to only one child or pup at a time, although some can give birth to multiple young simultaneously. Some examples of viviparous species include humans, primates, felines, canines, whales, dolphins, and certain sharks and fish.

Overview

Reproduction in oviparous animals occurs in much the same way it does in most other animals. As with other animals, the egg cells produced by female oviparous animals begin as gametes, or germ cells that form a zygote when united with another germ cell of the opposite sex. An egg cell, or ovum, is fertilized when it comes in contact with sperm from a male of the same species. Depending on the species, fertilization may take place inside the female’s body or in the external environment. Once fertilization takes place, an embryo begins to form inside the egg.

Most oviparous animals produce multiple eggs at once when they mate. While this may seem like a random occurrence, it is actually a crucial part of the reproductive process. Because the eggs produced by oviparous species are fragile and must be incubated outside the body, there is a significant risk of potentially catastrophic damage. There is also a chance that some eggs could be lost to predators. As a result, producing more eggs at once means a greater chance of ensuring the survival of developing offspring.

There are a number of drawbacks to oviparous reproduction. Most of these disadvantages are tied to the fact that the eggs of oviparous species are incubated outside the body. This leaves the eggs exposed to all sorts of dangers. As a result, oviparous species usually have to take special measures to protect their eggs. For birds, this usually means building nests and sitting on the eggs as much as possible. Similarly, lizards and other oviparous reptiles typically bury their eggs in sand.

Oviparous species can be divided into two main categories based on the structure of their eggs. These include anamniotes and amniotes. Most aquatic oviparous species are anamniotes. This means that their eggs develop in water and with only a single, soft embryonic membrane. Such membranes allow the eggs of anamniotes to directly exchange gas and waste to and from the external environment through diffusion. Birds, reptiles, and other amniotes lay eggs that have a more complex structure. Because they are laid on land and face more potential dangers, the eggs of amniotes have a protective shell that surrounds the embryonic membrane. The nature of the shell varies by species. While bird eggs have hard shells, reptile eggs typically have soft, leathery shells. The shell, which forms only after the egg has been fertilized, allows for the embryo within to develop in a wet environment even though it is not actually in water. This means that gas and waste can only be exchanged through the shell with the aid of several special membranes. These include the amnion, allantois, chorion, and yolk sac. The amnion surrounds the embryo and is filled with amniotic fluid. The allantois aids with gas exchange and the management of liquid waste. The chorion, which is located immediately beneath the shell, surrounds both the allantois and the amnion. The yolk sac is a key membrane that provides the embryo with the sustenance and nutrition it needs to complete its development inside the egg.

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