Sex

SEX STORY

This article is about biological sexes: male, female, etc. For alternate uses, such as gender, sexual intercourse, or human sexual behaviour, see Sex (disambiguation)

Sex refers to the male and female duality of biology and reproduction, a process in biological DNA that dates back 3.5 - 4.6 billion years. Unlike organisms that only have the ability to reproduce asexually, sexed male and female pairs have the ability to produce offspring through meiosis and fertilization. The two sexes attract one another and communicate their readiness to procreate through differences in their biology.

An organism's sex category reflects its biological reproductive function rather than its sexuality 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, sex may be assigned to specific structures rather than the entire organism. Earthworms, for example, are normally hermaphrodites.

Evolution of sex

Main article: Evolution of sex

The evolutionary origin of sex is still uncertain, although plausible explanations exist. The most likely cause is thought to be the advantage conferred to a species by the creation of genetic variation itself. Beneficial traits in the form of mutations can be spread more rapidly through sexual reproduction. Likewise, deleterious mutations are more rapidly culled from the population when they are brought together to produce unfit individuals. Finally, combinations of genes that together result in fitter individuals are more likely to occur as a result of sexual reproduction.

One of the limitations of sexual reproduction in most species is that only one of the sexes is capable of producing offspring. By comparison, every member of an asexual species can reproduce. Another restriction is the requirement that two members of the same species (and of the opposite sex) must seek each other out in order to reproduce.

Non-animal species

Holly has unisexual reproductive units occurring on different individuals: (above) shoot with flowers from male plant; (top right) male flower enlarged, showing stamens with pollen and reduced, sterile stigma; (below) shoot with flowers from female plant; (lower right) female flower enlarged, showing stigma
and reduced, sterile stamens with no pollen

Main article: Plant sexuality

Plants are hermaphrodites, but this terminology is quickly complicated by variations in the degree of sexuality. As with animals, there are only two types of gametes. These are generally called male and female based on their relative sizes and motility. In flowering plants, flowers bear the gametes. In some cases, flowers may contain only one type of gamete, while in others they may contain both.

In other varieties of multicellular life (e.g. the fungi division, Basidiomycota), sexual characteristics can be much more complex, and may involve many more than two sexes. For details on the sexual characteristics of fungi, see: Hypha and Plasmogamy.

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.

More than two sexes

There are species that have 3, 4 or 5 sexes. For example:

• the clam shrimp Eulimnadia texana has three sexes, that is, it is a triploid organism [2]
• harvester ant genus Pogonomyrmex has three sexes [3]or maybe four sexes [4]
• the reptile tuatara might have four sexes [5]
• Coprinus macrorhizus (Pers.) Rea might have three or four sexes [6]
• Coprinus lagopus has four sexes [7]
• Fausto-Sterling proposes that there are five sexes of humans [8]
• Other species have exhibited evidence of 5 sexes [9]

Humans

Stylised illustration of humans on the Pioneer plaque, showing both male and female.

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 criteria.

"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:

Level	Characteristics	Female	Male
Primary
	Sex chromosomes	XX	XY
	Gametes	Ova	Spermatozoon
	Sex organs	Ovaries	Testes
	Predominant Sex hormones	Estrogen and Progesterone	Testosterone
	Anatomy of internal genitalia	clitoral crura, vagina, uterus, fallopian tubes	corpora cavernosa, urethra, prostate, seminal vesicles
	Anatomy of external genitalia	glans clitoris, labia, vulva, clitoral hood, perineal urethra	glans penis, scrotum, phallus, foreskin fused perineum
Secondary
	Height	Relatively shorter	Relatively taller
	Body fat	Relatively more	Relatively less
	Body form development	"Hourglass" shape: 8	"Triangular" shape: ▼
	Other	Breasts and menstrual cycle	Beard and body hair

At the psychosocial level, the sexes differ with regard to the way society treats them and expects them to behave:

Characteristics	Female	Male
Assigned sex, gender of rearing and gender identity	Girl, Woman	Boy, Man
Gender role	Feminine	Masculine
Sexual orientation	Androphilic	Gynephilic

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.

Discordance

See also: homosexuality, cross-dressing, transvestism, transgender, and third gender

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) - this is known as intersex. 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.

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 each gender has certain natural roles in life.

See also

• Human sexuality
• List of gender names
• Mammalian gestation
• Sexual differentiation
• Sex ratio
• sexual intercourse

External links and further reading

• Were Bacteria the First Forms of Life on Earth having sex?
• Francoeur, Robert T. (ed.), The International Encyclopedia of Sexuality [full text]
• Janssen, D. F., Growing Up Sexually. Volume I. World Reference Atlas [full text]
• Raymond J. Noonan, Robert T. Francoeur, and Martha Cornog, "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