The reproductive system is complex and varies between different families of animals. While all species reproduce in one way or another, the way in which the eggs are fertilized and the young come into the world differ greatly. Development of animals also varies; some animals go through several stages of development.
Types of Reproduction
Asexual reproduction involves one parent without the fusion of sperm and egg. In essence, the female creature clones herself and can create an entire population without the presence of males. Sexual reproduction, on the other hand, is highly favored throughout the animal kingdom. Sexual reproduction requires a sperm to fertilize an egg, creating offspring. Some creatures exhibit both asexual and sexual reproduction.
Sexual reproduction typically requires a male and a female, however there are hermaphroditic creatures throughout the animal kingdom. Some hermaphrodites---those possessing male and female reproductive organs---switch sexes later on in life, while some are born with both types of sexual organs.
Types of Bearers
Besides sexual and asexual categories, animals can also be categorized by how they bear their young. Oviparous (egg layer) and viviparous (live bearer) are the scientific names for each of these categories. Birds are oviparous, along with most fish and reptiles. Sharks, pit vipers and various other reptiles and amphibians are live bearers. All mammals, with the exception of the platypus and very few others, are viviparous.
Time Frame of Egg Development
In the case of sexual reproduction, the development of the embryo varies widely. For example, a human pregnancy lasts about nine months, while elephants are pregnant for nearly two years. Oviparous creatures lay their eggs and wait for a specific amount of time for them to hatch. Live-bearing animals most generally have the embryo develop within the womb, and the young is then born into the world.
Viviparous creatures in the wild can be born blind, deaf and hairless; others are born able to walk or swim. Oviparous creatures, however, are often at the mercy of their parents while developing within the egg. Generally in the animal kingdom, the parents keep close eyes on their newborns or eggs to protect them from predators. Few species take no care of their young; many fish and reptiles, for example, guard the eggs fearlessly until they hatch.
The length of time a young animal remains in its mother's care depends greatly on the species. Often times, it is until the baby is sexually reproductive so that it may find a new mate and start their own family, pack, herd or other structure.
Function of Reproduction
Throughout the course of time, and with the varying methods of reproduction, it remains vital to the sustainability of a species to be able to reproduce. Whether the young is a clone of its mother or an embryo developing within an egg, it is a new member of the species.