SEX

'Sex' refers to the male and female duality of biology and reproduction. 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 reflects its biological function in reproduction, not its sexuality 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, sex may be assigned to specific structures rather than the entire organism. Earthworms, for example, are normally hermaphrodites.
  

Contents

Animal species

Humans

Social and psychological issues

See also

References

External links and further reading

Animal species

Main articles: Animal sexuality, 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.
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.^[1]
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★ harvester ant genus ''Pogonomyrmex'' has three sexes^[2] or possibly four sexes^[3]
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★ the reptile tuatara might have four sexes^[4]
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★ Coprinus macrorhizus (Pers.) Rea might have three or four sexes^[5]
:
★ Coprinus lagopus has four sexes^[6]
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★ Fausto-Sterling proposes that there are five sexes of humans ^[7][8]
:
★ Other species have exhibited evidence of 5 sexes^[9]

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.
"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	Spermatozoa
	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	penis, scrotum, foreskin, fused perineum
Secondary
	Skeletal Structure	Relatively shorter, wider in hips	Relatively taller, wider in shoulders, bigger chest
	Face	Smaller nose, rounded jaw	Bigger nose, brow bone, squarer jaw, facial hair
	Body fat and muscles	Relatively more fat	Relatively more muscles[10]
	Fat Distribution	More in buttocks, hips and thighs	More in abdomen
	Body form development	"Hourglass" shape: '8'	"Triangular" shape: '▼'
	Other	Breasts	Adam's apple and body hair

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.^[11]

Social and psychological issues

;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'') – 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:
Main articles: Sociology of gender

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.

See also

★ Evolution of sex

★ Human sexuality

★ List of gender names

★ Mammalian gestation

★ Sexual differentiation

★ Sexual dysfunction

★ Sex ratio

★ Sexual intercourse

★ Age of consent

References

1. Evolution with three sexes
2. Genetic determination of the queen caste in an ant hybrid zone, G. E. Julian, J. H. Fewell, J. Gadau, R. A. Johnson, D. Larrabee, , , Proceedings of the National Academy of Sciences, 2002
3. Everything You Always Wanted to Know about Sexes, , John, Whitfield, Public Library of Science: Biology, 2004
4. Tuatara
5. Observations on Inheritance in Coprinus macrorhizus (Pers.) Rea
6. The Pycnidia of the Rust Fungi
7. The Five Sexes Revisited, Sciences, Jul/Aug2000, Vol. 40 Issue 4, p18, 6p, 1c, 2bw
8. Two Sexes Are Not Enough
9. Cytoplasmic Fusion and the Nature of Sexes
10. [1], [2]
11. Claims of sex Differences: An empirical assessment in genetic associations, Nikolaos A. Patsopoulos, ''et al.'', , , J. Am. Med. Assoc., 2007

# http://www.columbia.edu/itc/hs/pubhealth/modules/reproductiveHealth/anatomy.html
# http://www.humanhormones.com/category/steroids/androgens/
# http://www.bbc.co.uk/science/humanbody/sex/articles/testosterone.shtml
# http://www.sciencedaily.com/releases/2007/05/070523105948.htm

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]

★ 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