Sexual Reproduction in Animals

Sexual reproduction (also called amphimixis) is the process by which offspring are produced through the formation and fusion of gametes, and in animals gamete formation primarily involves meiosis. Sexually reproducing animals pass through two broad phases in their lifetime. The earlier juvenile phase is mainly a period of physical growth beginning at birth, during which the animal cannot reproduce sexually. This is followed by the reproductive maturity phase, attained once physical growth is essentially complete, during which the sex organs grow and become active and the animal becomes capable of sexual reproduction. The duration of both phases varies considerably between species. Once sexual maturity is reached, the animal passes through a sequence of events: pre-fertilization events (gametogenesis and the transfer of gametes), fertilization itself (the fusion of male and female gametes), and post-fertilization events (formation of the zygote and its embryonic development).

Sexually reproducing animals also differ in their breeding pattern. Some, such as goat, sheep and donkey, are seasonal breeders, restricting mating to particular times of year, while humans and apes are continuous breeders capable of breeding throughout the year. …