Animal reproduction takes two overall forms. In the first form, called asexual reproduction, animals produce offspring without needing a partner. Asexual reproduction is most common in simple animals such as flatworms and cnidarians. In flatworms, the parent often develops a constriction in its body, and the rear part eventually tears itself free.
The rear part grows a new head, while the front part grows a new tail. Some cnidarians can also divide in two, but many reproduce by a different process, called budding. During budding, a small outgrowth of the body slowly develops into a complete new animal, which eventually takes up life on its own.
Asexual reproduction also occurs in insects such as aphids and in a few unusual vertebrates, such as whiptail lizards. However, in general, it is rarely used as an animal's sole method of reproduction. This is because asexual reproduction produces offspring that are genetically identical to their parent. They inherit all their parent's weak points and are equally vulnerable if a disease or other changes in the environment threaten the groupís survival.
A second and much more common form of reproduction, sexual reproduction, involves two parents. The parents produce sperm and egg cells (gametes), which are brought together to form a fertilized cell (zygote) with a new and unique combination of genes. In this genetic lottery, offspring inherit unique combinations of characteristics that increase the likelihood that at least some individuals in the population can survive changes in the environment.
Sexual reproduction is used by the vast majority of the world's animals. However, a significant number of species, particularly in the world of insects, use both forms of reproduction at different stages of their life cycles. They reproduce asexually when food is abundant, but turn to sexual reproduction when conditions become more severe.