Sex

Overview
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.

Sexual reproduction
Sexual 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 similar—particularly in size—is isogamy; having gametes that are somewhat dissimilar is anisogamy. The condition of having greatly dissimilar gametes—particularly a large, immotile cell and a much smaller, motile one—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, recorded up to about the year 2000, "no third sex cell — and so no third sex — has appeared in multicellular animals." 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, 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. 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 on many occasions. There are cases where it has also been lost. The flatworm, Dugesia tigrina, and a few other species can reproduce either sexually or asexually depending on various conditions.

Animal species
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. 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.
 * 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, or possibly four.
 * the reptile tuatara might have four sexes
 * Coprinus macrorhizus (Pers.) Rea might have three or four sexes
 * Coprinus lagopus has four sexes
 * 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. Despite mainstream opposition, including the Intersex Society of North America, Fausto-Sterling and others still maintain this view in current publications.
 * Other species have exhibited evidence of 5 sexes

Humans

 * See Human sexuality for information about sexual activities, sexual sensation, sexual gratification, and sexual intimacy between human beings

In humans, "sex" is often perceived as a dichotomous state or identity for most biological and social purposes - such that a person can only be female or male. But many factors, including one's biology, environment, psychology and social context, have a role in determining how a particular person, and those around them, view their sex. Although the table below shows common differences between males and females, many people do not correspond to "male" or "female" with regard to every criterion. Additionally, about 1 to 1.7 percent of human beings exhibit biological sexual ambiguity to the degree that they cannot be physically classified as exclusively male or female. This is known as intersex. A person with intersex may have biological characteristics of both the male and female sexes.

"Primary" sexual characteristics are typically present at birth and directly involved in reproduction. "Secondary" sexual characteristics typically develop later in life (usually during puberty) and are not directly involved in reproduction. Differences between the sexes are known as sexual dimorphism. At the biological level these differences are usually:

The relationship between the various levels of biological sexual differentiation is fairly well understood. Many of the biological levels are said to cause, or at least shape, the next level. For example, in most people, the presence of a Y chromosome causes the gonads to become testes, which produce hormones that cause the internal and external genitalia to become male, which in turn lead parents to assign 'male' as the sex of their child (assigned sex), and raise the child as a boy (gender of rearing). However, the degree to which biological and environmental factors contribute to the psychosocial aspects of sexual differentiation, and even the interrelationships between the various psychosocial aspects of differentiation, is less well understood as illustrated by the ongoing nature versus nurture debate. Unfortunately, because of a lack of focus on this area, studies may use data from research not designed to discern the role of sex. One sample of 432 papers publishing the results of gender-related genetics found that only 66.6% of them had set out to deal with the subject before conducting any research and 87.3% used unsound statistics.

Social and psychological issues
Gender, in common usage, refers to the differentiation between men and women. It is individual's self-conception or social conception as being male or female, though gender is commonly used interchangeably with sex. Within the social sciences it often refers to specifically social differences, known as gender roles.
 * Gender discordance:

Discordance is the term used to describe the extent to which people differ from the usual biological and psychosocial types described above. Some discordances are biological, such as when the sex of the chromosomes (genetic sex) does not match the sex of the external genitalia (anatomic sex), such as in Swyer syndrome, a type of intersex condition. Discordances between the biological and psychosocial levels (such as when the gender identity does not match the anatomic sex) or between the various psychosocial levels (such as when the gender role does not match the gender identity) are even more common, but less well understood.

In gender theory, the term "heteronormativity" refers to the idea that human beings fall into two distinct and complementary categories, male and female; that sexual and marital relations are normal only when between two people of different genders; and that people should follow roles determined by their gender. Instead, some people have sought to define their sexuality and sexual identity in non-polar terms, in the belief that the simple division of all humans into "males" and "females" does not fit their individual conditions. A proponent of this movement away from polar oppositions, Anne Fausto-Sterling, recognized five sexes: male, female, merm (male pseudohermaphrodite), ferm (female pseudohermaphrodite) and herm (true hermaphrodite). Although she was heavily criticized, her idea demonstrates the difficulty and imperfection of the current social responses to these variations.


 * Social and legal considerations:

Forms of legal or social distinction or discrimination based on sex include sex segregation and sexism. Notably, some businesses, public institutions, and laws may provide privileges and services for one sex and not another, or they may require different sexes to be physically separated. Recently, western societies have moved towards greater sexual equality.

External links and further reading

 * Francoeur, Robert T. (ed.), The International Encyclopedia of Sexuality [full text]
 * Janssen, D. F., Growing Up Sexually. Volume I. World Reference Atlas [full text]
 * Francoeur, Robert T. and Rayond J. Noonan, (eds.) "Continuum Complete International Encyclopedia of Sexuality". Continuum, August 2003, ISBN 0-8264-1488-5
 * Human Sexual Differentiation by P. C. Sizonenko
 * New Scientist article on Sex chromosomes in the platypus
 * Human Sexuality Info by University of California Santa Barbara

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