Sex

Sex refers to the male and female duality of biology and reproduction. Unlike organisms that only have the ability to reproduce asexually, many species have the ability to produce offspring through meiosis and fertilization. Often, individuals of the two sexes attract one another and communicate their readiness to procreate through biological changes, or, in social species, through courtship behaviours.

An organism's sex is defined by its biological role in reproduction, not according to its sexual or other behavior. The female sex is defined as the one which produces the larger gamete and which typically bears the offspring. In contrast, the male sex has a smaller gamete and rarely bears offspring. In some animals and many plants sex may be assigned to specific structures rather than the entire organism. Earthworms, for example, are normally hermaphrodites.

Contents [hide]

1 Sexual reproduction

2 Animal species

3 Humans

3.1 Social and psychological issues

4 In fiction

5 See also

6 References

7 Sources

8 External links and further reading

Sexual reproduction

Hoverflies matingSexual reproduction is a prevalent system for producing new individuals within various species. Individuals of sexually reproducing species produce special kinds of cells called gametes, whose function is specifically to fuse with one unlike gamete and hence form a new individual. This fusion of two gametes is called fertilization. The condition of having types of gametes that are externally similarÐ'--particularly in sizeÐ'--is isogamy; having gametes that are somewhat dissimilar is anisogamy. The condition of having greatly dissimilar gametesÐ'--particularly a large, immotile cell and a much smaller, motile oneÐ'--is oogamy. By convention, the larger gamete cell is associated with female sex. Thus an individual that produces exclusively large gametes (ova in humans) is said to be female, and one that produces exclusively small gametes (spermatozoa in humans) is said to be male. An individual that produces both types of gametes is called hermaphrodite (a name applicable also to people with one testis and one ovary). In some species hermaphrodites can self-fertilize, in others they can achieve fertilization with females, males or both. Some species, like the Japanese Ash, Fraxinus lanuginosa, only have males and hermaphrodites, a rare reproductive system called androdioecy‎.

What is considered defining of sexual reproduction is the difference between the gametes and the binary nature of fertilization. Multiplicity of gamete types within a species would still be considered a form of sexual reproduction. However, of more than 1.5 million living species,[1] recorded up to about the year 2000, "no third sex cell Ð'-- and so no third sex Ð'-- has appeared in multicellular animals."[2][3][4] Why sexual reproduction has an exclusively binary gamete system is not yet known. A few rare species that push the boundaries of the definitions are the subject of active research for light they may shed on the mechanisms of the evolution of sex. For example, the most toxic insect,[5] the harvester ant Pogonomyrmex, has two kinds of female and two kinds of male. One hypothesis is that the species is a hybrid, evolved from two closely related preceding species.

Fossil records indicate that sexual reproduction has been occurring for at least one billion years.[6] However, the reason for the initial evolution of sex, and the reason it has survived to the present are still matters of debate; there are many plausible theories. It appears that the ability to reproduce sexually has evolved independently in various species. There are also cases where it has been lost. The flatworm, Dugesia tigrina, and a few other species can reproduce either sexually or asexually depending on various conditions.[7]

Animal species

Main articles: Animal sexuality and Mating system

Animal sexual behavior takes many different forms, even within the same species. Researchers have observed monogamy, promiscuity, sex between species, sexual arousal from objects or places, rape, necrophilia, sexual orientation (heterosexuality, homosexuality, bisexuality and situational sexual behaviour) and a range of other practices among animals other than humans. Related studies have noted diversity in sexed bodies and gendered behaviour, such as intersex and transgender animals.

The study of animal sexuality (and primate sexuality especially) is a rapidly developing field. It used to be believed that only humans and a handful of species performed sexual acts other than for procreation, and that animals' sexuality was instinctive and a simple response to the "right" stimulation (sight, scent). Current understanding is that many species believed monogamous have now been proven to be promiscuous or opportunistic in nature, a wide range of species appear to both masturbate and to use objects as tools to help them do so, in many species animals try to give and get sexual stimulation with others where procreation is not the aim, and homosexual behavior has now been observed among 1,500 species, and in 500 of those it is well documented.[citation needed] A few species have particularly complex sex determination systems. Although two sexes is the official maximum, these complex species could reasonably be said to have 3, 4 or 5 sexually distinct phenotypes. For example:

the clam shrimp Eulimnadia texana has no females, but two types of hermaphrodite and one male phenotype, a system call androdioecy.[8]

harvester ant genus Pogonomyrmex has two types of female and two types of male, with an acknowledged claim to these being considered as constituting at least three distinct sexes,[9] or possibly four.[10]

the reptile tuatara might have four sexes[11]

Coprinus macrorhizus (Pers.) Rea might have three or four sexes[12]

Coprinus lagopus has four sexes[13]

A notable minority view regarding humans has been put forward by Anne Fausto-Sterling, who suggested various disorders of sexual development could be classified into an additional three human sexes.[14][15]