Editing Sex (section)

===Animals===
{{Main|Sexual reproduction#Animals}}
[[File:Hoverflies mating midair.jpg|thumb|left|''[[Simosyrphus grandicornis]]'' mating]]

Most sexually reproducing animals spend their lives as diploid, with the haploid stage reduced to single-cell gametes.<ref>{{cite book|vauthors=[[Bruce Alberts|Alberts B]], [[Alexander D. Johnson|Johnson A]], [[Julian Lewis (biologist)|Lewis J]], [[Martin Raff|Raff M]], Roberts K, [[Peter Walter|Walter P]]|title=Molecular Biology of the Cell|edition=4th|year=2002|isbn=978-0-8153-3218-3|publisher=Garland Science|location=New York | chapter = Mendelian genetics in eukaryotic life cycles | chapter-url = https://www.ncbi.nlm.nih.gov/books/NBK21836 | archive-url = https://web.archive.org/web/20170402185423/https://www.ncbi.nlm.nih.gov/books/NBK21836/ | archive-date = 2 April 2017}}</ref> The gametes of animals have male and female forms—[[spermatozoa]] and egg cells, respectively. These gametes combine to form [[embryos]] which develop into new organisms.

The male gamete, a [[spermatozoon]] (produced in vertebrates within the [[testes]]), is a small cell containing a single long [[flagellum]] which propels it.<ref>{{cite book|vauthors=[[Bruce Alberts|Alberts B]], [[Alexander D. Johnson|Johnson A]], [[Julian Lewis (biologist)|Lewis J]], [[Martin Raff|Raff M]], Roberts K, [[Peter Walter|Walter P]]|title=Molecular Biology of the Cell|edition=4th|year=2002|isbn=978-0-8153-3218-3|publisher=Garland Science|location=New York | chapter = Sperm | chapter-url = https://www.ncbi.nlm.nih.gov/books/bv.fcgi?rid=mboc4.section.3729 | archive-url = https://web.archive.org/web/20090629222617/http://www.ncbi.nlm.nih.gov/books/bv.fcgi?rid=mboc4.section.3729 | archive-date= 29 June 2009 }}</ref> Spermatozoa are extremely reduced cells, lacking many cellular components that would be necessary for embryonic development. They are specialized for motility, seeking out an egg cell and fusing with it in a process called [[Fertilization#Fertilisation in animals|fertilization]].

Female gametes are egg cells. In vertebrates, they are produced within the [[ovary|ovaries]]. They are large, immobile cells that contain the nutrients and cellular components necessary for a developing embryo.<ref>{{cite book|vauthors=[[Bruce Alberts|Alberts B]], [[Alexander D. Johnson|Johnson A]], [[Julian Lewis (biologist)|Lewis J]], [[Martin Raff|Raff M]], Roberts K, [[Peter Walter|Walter P]]|title=Molecular Biology of the Cell|edition=4th|year=2002|isbn=978-0-8153-3218-3|publisher=Garland Science|location=New York | chapter = Eggs | chapter-url = https://www.ncbi.nlm.nih.gov/books/bv.fcgi?rid=mboc4.section.3718 | archive-url = https://web.archive.org/web/20090629074430/http://www.ncbi.nlm.nih.gov/books/bv.fcgi?rid=mboc4.section.3718 | archive-date= 29 June 2009}}</ref> Egg cells are often associated with other cells which support the development of the embryo, forming an [[Egg (biology)|egg]]. In mammals, the fertilized embryo instead develops within the female, receiving nutrition directly from its mother.

Animals are usually<!--h'm. but not Sponges, Corals, Bryozoa, Barnacles, ...--> mobile and seek out a partner of the opposite sex for [[mating]]. Animals which live in the water can mate using [[external fertilization]], where the eggs and sperm are released into and combine within the surrounding water.<ref>{{cite book|vauthors=[[Bruce Alberts|Alberts B]], [[Alexander D. Johnson|Johnson A]], [[Julian Lewis (biologist)|Lewis J]], [[Martin Raff|Raff M]], Roberts K, [[Peter Walter|Walter P]]|title=Molecular Biology of the Cell|edition=4th|year=2002|isbn=978-0-8153-3218-3|publisher=Garland Science|location=New York | chapter = Fertilization | chapter-url = https://www.ncbi.nlm.nih.gov/books/bv.fcgi?rid=mboc4.section.3738 | archive-url = https://web.archive.org/web/20081219005819/http://www.ncbi.nlm.nih.gov/books/bv.fcgi?rid=mboc4.section.3738 | archive-date= 19 December 2008 }}</ref> Most animals that live outside of water, however, use [[internal fertilization]], transferring sperm directly into the female to prevent the gametes from drying up.<!--Dragonflies use indirect method, transferring a spermatheca.-->

In most birds, both excretion and reproduction are done through a single posterior opening, called the [[cloaca]]—male and female birds touch cloaca to transfer sperm, a process called "cloacal kissing".<ref>{{cite web |title=Avian Reproduction |url=http://people.eku.edu/ritchisong/avianreproduction.html |publisher=Eastern Kentucky University | vauthors = Ritchison G |access-date=3 April 2008 |archive-date=12 April 2008 |archive-url=https://web.archive.org/web/20080412231002/http://people.eku.edu/ritchisong/avianreproduction.html |url-status=live }}</ref> In many other terrestrial animals, males use specialized [[sex organ]]s to assist the transport of sperm—these male sex organs are called [[intromittent organ]]s. In humans and other mammals, this male organ is known as the [[penis]], which enters the female reproductive tract (called the [[vagina]]) to achieve [[insemination]]—a process called [[sexual intercourse]]. The penis contains a tube through which [[semen]] (a fluid containing sperm) travels. In female mammals, the vagina connects with the [[uterus]], an organ which directly supports the development of a fertilized embryo within (a process called [[gestation]]).

Because of their motility, [[animal sexual behavior]] can involve coercive sex. [[Traumatic insemination]], for example, is used by some insect species to inseminate females through a wound in the abdominal cavity—a process detrimental to the female's health.