An oviparous animal is one that produces eggs, and the young hatch after being expelled from the body. While fertilization of the egg can occur internally or externally, oviparous animals always hatch their young outside of their body. Many amphibians, birds, fish and reptiles are oviparous and often make nests to protect their eggs. This can be contrasted to ovoviviparous animals, which hatch eggs inside of their bodies, then expel live young. This can be seen in some sharks, snakes, and other animals.
Being oviparous is an evolutionary strategy for reproduction. In this strategy, one or many eggs can be produced. Each egg is a gamete that has the female’s contribution of the genetic material. In many species, the male supplies his gamete in the form of sperm, which must find its way to the egg. Once fertilized, the cells within the egg will begin to subdivide as an embryo is formed. Many oviparous animals choose to make many small, fragile eggs. Other oviparous animals choose to protect a few very strong, large eggs. There are advantages to both. Many eggs results in many offspring at once, and many offspring can overcome a few predators. On the other hand, a large protected egg increased the development of the offspring and the chances it will survive until birth. This advantage may make the offspring large enough to escape potential predators and accidents after birth.
Much like the other reproductive strategies, being oviparous has its downsides as well. Unlike viviparous and ovoviviparous animals which carry their developing young with them, oviparous animals must protect or hide their eggs for the duration of development. Many birds must sit on their eggs frequently to keep them warm, or even constantly in the case of cold-climate birds like penguins. In the case of animals that don’t watch their eggs, there is always the chance of a predator stumbling upon the nest and eating their whole clutch (batch of eggs).
Examples of Oviparous
The most recognizable oviparous animal is the chicken. Chickens develop an egg in one of their ovaries, which will descend to be laid whether or not it becomes fertilized. If it does become fertilized, the young embryo develops inside the egg, feeding off of the nutrient-rich yolk sack inside the egg. Once mostly developed, the small bird hatches, ready to walk and eat. Birds are oviparous in general, and lay hard-shelled eggs that have been fertilized internally. Many of the young are precocial, or have the ability to walk and feed immediately upon hatching.
Reptiles use very similar methods of developing their young. The main difference is that reptile eggs often have a much softer shell, often leathery to the touch. Still, like birds, the eggs are incubated in a nest. Where birds prefer to sit on their nests to provide warmth to the eggs, reptiles tend to bury their eggs completely in burrows or mounded nests. This tends to keep the eggs at a stable temperature. Reptiles tend to need a stable environment for their eggs because the sex of the young is dependent on the temperature during critical periods of the embryotic development. This is known as temperature dependent sex determination.
Oviparous Fish and Amphibians
While birds and reptiles use internal fertilization, it is not necessary to be oviparous. Many female fish lay eggs in a nest. The males immediately swoop in to fertilize the eggs by casting their sperm over the nest. In this case both male and female cast their gametes (eggs and sperm) into the environment in the hopes that they will find each other. Some fish are very successful in this, and have complex nests and mating strategies to ensure the gametes meet. Other fish use complex mating dances to release their gametes in unison, thereby increasing the chances of fertilization.
Most amphibians are oviparous as well, laying their eggs in ponds or other sources of standing water. Fertilization in amphibians is mostly external. Unlike reptiles and birds, amphibians often emerge from the egg in a larval form. This form has a tail and gills, which allow it to continue developing in the pond or body of water it was born in. Eventually the tadpole or larva will metamorphose into the adult form, losing its tail and growing large limbs.