Sexual intercourse

This article is primarily about sexual intercourse in humans. For other animals, see Animal sexual behaviour

Sexual intercourse, also known as copulation or coitus, commonly refers to the act in which a male's penis enters a female's vagina for the purposes of sexual pleasure or reproduction.[1][2] The term may also describe other sexual penetrative acts, such as anal sex, oral sex and fingering, which can be practiced by both heterosexual and homosexual pairings.[2]

Sexual intercourse typically plays a powerful role in human bonding, often being used solely for pleasure and leading to stronger emotional bonds.[3] Non-penetrative sex (for example, non-penetrative cunnilingus) has been referred to as "outercourse",[4][5][6][7] but may also be among the sexual acts contributing to human bonding and considered intercourse. The term sex can be taken to mean any mutual genital stimulation or to mean any form of sexual activity (i.e. all forms of intercourse and "outercourse").[2][8] As with most forms of sexual interaction, individuals are at risk of contracting sexually transmitted diseases,[9][10] and thus safe sex practices are advised.[9]

In human societies, some jurisdictions have placed various restrictive laws against certain sexual activities such as gay sex, sex with minors, incest, extramarital sex, position-of-trust sex, pornography, prostitution, sodomy, public lewdness and non-consensual sex. Religious beliefs can play a role in decisions about sex, or its purpose, as well; for example, with virginity pledges. Some sections of Christianity commonly view sex between a married couple for the purpose of reproduction as holy, while other sections may not. Modern Judaism and Islam view sexual intercourse between husband and wife as a spiritual and edifying action. Hinduism and Buddhism views on sexuality have differing interpretations.

Sexual intercourse between non-human animals is more often referred to as copulation; for most, mating and copulation occurs at the point of estrus (the most fertile period of time in the female's reproductive cycle),[11][12] which increases the chances of successful impregnation. However, bonobos,[13] dolphins,[14] and chimpanzees are known to engage in sexual intercourse even when the female is not in estrus, and to engage in sex acts with same-sex partners.[14][15] Like humans engaging in sex primarily for pleasure,[3] this behavior in the above mentioned animals is also presumed to be for pleasure,[16] and a contributing factor to strengthening their social bonds.[3]

Contents

Practice

See also: Human sexual activity and Human sexuality

Bonding and affection

In animals, copulation ranges from a purely reproductive activity to one of emotional bonding between mated pairs. Sexual intercourse and other sexual activity typically plays a powerful role in human bonding. For example, in many societies, it is normal for couples to have frequent intercourse while using some method of birth control, sharing pleasure and strengthening their emotional bond through sex even though they are deliberately avoiding pregnancy.[3]

In humans and bonobos, the female undergoes relatively concealed ovulation so that both male and female partners commonly do not know whether she is fertile at any given moment. One possible reason for this distinct biological feature may be formation of strong emotional bonds between sexual partners important for social interactions and, in the case of humans, long-term partnership rather than immediate sexual reproduction.[3]

Humans, bonobos, dolphins, and chimpanzees are all intelligent social animals, whose cooperative behavior proves far more successful than that of any individual alone. In these animals, the use of sex has evolved beyond reproduction, to apparently serve additional social functions.[13][14][15] Sex reinforces intimate social bonds between individuals to form larger social structures. The resulting cooperation encourages collective tasks that promote the survival of each member of the group.[3]

Reproduction and sexual practices

Vaginal sexual intercourse, also called coitus, is the human form of copulation. While its main natural purpose and result is reproduction, it is often performed entirely for pleasure and/or as an expression of love and emotional intimacy.[2][3] Coitus is the basic reproductive method of humans, and may be preceded by foreplay, which leads to sexual arousal of the partners, resulting in the erection of the penis and (usually) natural lubrication of the vagina. To engage in coitus, the erect penis is inserted into the vagina and one or both of the partners move their hips to move the penis backward and forward inside the vagina to cause friction, typically without fully removing the penis. In this way, they stimulate themselves and each other, often continuing until orgasm in either or both partners is achieved. For human females, stimulation of the clitoris plays a significant role in sexual intercourse; most women (70-80%) achieve orgasm only with direct clitoral stimulation, though indirect clitoral stimulation (for example, via vaginal intercourse) may also be sufficient (see orgasm in females).[17][18][19][20]

Penetration by the hardened, erect penis is additionally known as intromission, or by the Latin name immissio penis (Latin for "insertion of the penis"). During ejaculation, which usually accompanies male orgasm, a series of muscular contractions delivers semen containing male gametes known as sperm cells or spermatozoa from the penis into the vagina. The subsequent route of the sperm from the vault of the vagina is through the cervix and into the uterus, and then into the fallopian tubes. Millions of sperm are present in each ejaculation, to increase the chances of one fertilizing an egg or ovum (see sperm competition). When a fertile ovum from the female is present in the fallopian tubes, the male gamete joins with the ovum, resulting in fertilization and the formation of a new embryo. When a fertilized ovum reaches the uterus, it becomes implanted in the lining of the uterus - known as the endometrium - and a pregnancy begins. Unlike most species, human sexual activity is not linked to periods of estrus and can take place at any time during the reproductive cycle, even during pregnancy.[21] Where a sperm donor has sexual intercourse with a woman who is not his partner, for the sole purpose of impregnating the woman, this may be known as "natural insemination", as opposed to artificial insemination. However, most sperm donors donate their sperm through a sperm bank and pregnancy is achieved through artificial insemination.[22]

The World Health Organization (WHO) estimated in 2005 that each year 123 million women become pregnant world-wide. A substantial number of those pregnancies – around 87 million – are unintentional. About 46 million pregnancies yearly reportedly end in induced abortion.[23] About 6 million U.S. women become pregnant per year. Out of known pregnancies, two-thirds result in live births and roughly 25% in abortions; the remainder end in miscarriage. However, many more women become pregnant and miscarry without even realizing it.[24] The U.S. teenage pregnancy rate fell by 27 percent between 1990 and 2000, from 116.3 pregnancies per 1,000 girls aged 15–19 to 84.5. This data includes live births, abortions, and fetal losses. Almost 1 million American teenage women, 10% of all women aged 15–19 and 19% of those who report having had intercourse, become pregnant each year.[25] Britain has been stated to have a teenage pregnancy rate similar to America's.[26]

Reproduction methods and pregnancy also extend to gay and lesbian couples. For gay male pairings, there is the option of surrogate pregnancy; for lesbian couples, there is donor insemination in addition to choosing surrogate pregnancy.[27][28] Further, developmental biologists have been researching and developing techniques to facilitate biological same-sex reproduction, though this has yet to be demonstrated in humans (see same-sex reproduction).[29][30] Surrogacy and donor insemination remain the primary methods. Surrogacy is an arrangement in which a woman carries and delivers a child for another couple or person. The woman may be the child's genetic mother (traditional surrogacy) or she may carry a pregnancy to delivery after having another woman's eggs transferred to her uterus (gestational surrogacy). Gay and lesbian pairings who want the host to have no genetic connection to the child may choose gestational surrogacy and enter into a contract with an egg donor. Gay male couples might decide that they should both contribute semen for the in vitro fertilisation (IVF) process, which further establishes the couple's joint intention to become parents.[28] Lesbian couples often have contracts drafted to extinguish the legal rights of the sperm donor, while creating legal rights for the parent who is not biologically related to the child.[31]

In addition to vaginal intercourse, sexual intercourse can encompass other forms of insertive sexual behavior, such as oral sex and anal intercourse. Sexual acts, other than as a means of reproduction, are varied: Oral sex consists of all the sexual activities that involve the use of the mouth, tongue, and possibly throat to stimulate genitalia. It is sometimes performed to the exclusion of all other forms of sexual activity, and may include the ingestion or absorption of semen or vaginal fluids. While there are many sexual acts involving the anus, anal cavity, sphincter valve and/or rectum, the most common meaning of anal sex is the insertion of a man's penis into another person's rectum.[32][33]

Non-penetrative sex acts are also common. These acts are sometimes seen among heterosexuals as maintaining "technical virginity."[34][35][36][37] Some gay men view frotting and oral sex as maintaining their virginity,[38] while other gay men consider the acts to be their main form of intercourse.[39][40][41] The phrase "to have sex" can mean any or all of these behaviors (intercourse and outercourse), and there are a number of sex positions.

Prevalence, safe sex, and contraception

By their late teenage years, at least 3/4 of all men and women have had intercourse, and more than 2/3 of all sexually experienced teens have had 2 or more partners.[42] Among sexually active 15- to 19-year-olds in the U.S., 83% of females and 91% of males reported using at least one method of birth control during last intercourse.[43]

There are a variety of safe sex practices, including non-penetrative sex acts, and heterosexual couples may use oral or anal sex (or both) as a means of contraception.[34][35][36] In 2004, the Guttmacher Institute indicated in 2002 that 62% of the 62 million women aged 15–44 are currently using a contraceptive method, that among U.S. women who practice contraception, the Pill is the most popular choice (30.6%), followed by tubal sterilization (27.0%) and the male condom (18.0%), and that 27% of teenage women using contraceptives choose condoms as their primary method.[44]

In 2006, a survey conducted by The Observer showed that most adolescents in Britain were waiting longer to have sexual intercourse than they were only a few years earlier. In 2002, 32% of British teenagers were having sex before the age of 16; in 2006, it was only 20%. The average age a teen lost his/her virginity was 17.13 years in 2002; in 2006, it was 17.44 years on average for girls and 18.06 for boys. The most notable drop among teens who reported having sex was 14- and 15-year-olds.[45] A 2008 survey conducted by YouGov for Channel 4 showed that 40% of all 14- to 17-year-olds are sexually active, 74% of sexually active 14- to 17-year-olds have had a sexual experience, and 6% of teens would wait until marriage before having sex.[46] Sexually transmitted infections are also on the increase in Britain.[26] One in nine sexually active teens has chlamydia and 790,000 teens have sexually transmitted infections. In 2006, The Independent newspaper reported that the biggest rise in sexually transmitted infections was in syphilis, which rose by more than 20%, while increases were also seen in cases of genital warts and herpes.[47]

The National Survey of Sexual Health and Behavior (NSSHB) indicated in 2010 that 1 of 4 acts of vaginal intercourse are condom protected in the U.S. (1 in 3 among singles), condom use is higher among black and Hispanic Americans than among white Americans and those from other racial groups, and adults using a condom for intercourse were just as likely to rate the sexual extent positively in terms of arousal, pleasure and orgasm than when having intercourse without one.[48]

Duration

Intercourse often ends when the man has ejaculated. Thus the partner might not have time to reach orgasm. In addition, many men suffer from premature ejaculation. Conversely, many women require a substantially longer duration of stimulation than men before reaching an orgasm.[49]

A survey of Canadian and American sex therapists said that the average time for intromission was 7 minutes and that 1 to 2 minutes was too short, 3 to 7 minutes was adequate and 7 to 13 minutes desirable,[50] while 10 to 30 minutes was too long.[51][52]

Coitus difficulties

Anorgasmia is regular difficulty reaching orgasm after ample sexual stimulation, causing personal distress. This is much more common in women than men.[20][53] The physical structure of the act of coitus favors penile stimulation over clitoral stimulation. The location of the clitoris then often necessitates manual stimulation in order for the female to achieve orgasm. About 15 percent of women report difficulties with orgasm, and as many as 10 percent of women in the United States have never climaxed. Even women who orgasm on a regular basis only climax about 50 to 70 percent of the time.[20][54]

Vaginismus is involuntary tensing of the pelvic floor musculature, making coitus distressing, painful, and sometimes impossible for women.[55][56][57] It is a conditioned reflex of the pubococcygeus muscle, sometimes referred to as the "PC muscle". Vaginismus can be a vicious cycle for women, they expect to experience pain during intercourse, which then causes a muscle spasm, which leads to painful intercourse. Treatment of vaginismus often includes both psychological and behavioral techniques, including the use of vaginal dilators. A new medical treatment using Botox is in the testing phase.[57][58] Some women also experience dyspareunia, a medical term for painful or uncomfortable intercourse, of unknown cause.[59][60]

About 40% of males suffer from some form of erectile dysfunction (ED) or impotence, at least occasionally.[61] For those whose impotence is caused by medical conditions, prescription drugs such as Viagra, Cialis, and Levitra are available. However, doctors caution against the unnecessary use of these drugs because they are accompanied by serious risks such as increased chance of heart attack. Moreover, using a drug to counteract the symptom—impotence—can mask the underlying problem causing the impotence and does not resolve it. A serious medical condition might be aggravated if left untreated.[62]

A more common sexual disorder in males is premature ejaculation (PE). The American Urological Association (AUA) estimates that premature ejaculation could affect 21 percent of men in the United States.[63] The U.S. Food and Drug Administration is examining the drug dapoxetine to treat premature ejaculation. In clinical trials, those with PE who took dapoxetine experienced intercourse three to four times longer before orgasm than without the drug. Another ejaculation-related disorder is delayed ejaculation, which can be caused as an unwanted side effect of antidepressant medications such as Fluvoxamine.[64][65]

Although disability-related pain and mobility impairment can hamper intercourse, in many cases the most significant impediments to intercourse for individuals with a disability are psychological.[66] In particular, people who have a disability can find intercourse daunting due to issues involving their self-concept as a sexual being,[67][68] or partner's discomfort or perceived discomfort.[66]

Temporary difficulties can arise with alcohol and sex as alcohol initially increases interest (through disinhibition) but decreases capacity with greater intake.[69]

Health effects

Benefits

In humans, sex has been claimed to produce health benefits as varied as improved sense of smell,[70] stress and blood pressure reduction,[71][72] increased immunity,[73] and decreased risk of prostate cancer.[74][75][76] Sexual intimacy, as well as orgasms, increases levels of the hormone oxytocin, also known as "the love hormone", which helps people bond and build trust.[77][78][79] A long-term study of 3,500 people between 30 and 101 by clinical neuropsychologist David Weeks, MD, head of old age psychology at the Royal Edinburgh Hospital in Scotland, found that "sex helps you look between four and seven years younger", according to impartial ratings of the subjects' photos. Exclusive causation, however, is unclear, and the benefits may be indirectly related to sex and directly related to significant reductions in stress, greater contentment, and better sleep that sex promotes.[80][81][82]

Risks

In contrast to its benefits, sexual intercourse can also be a disease vector.[83] There are 19 million new cases of sexually transmitted diseases (STD) every year in the U.S.,[10] and worldwide there are over 340 million STDs a year.[84] More than half of all STDs occur in adolescents and young adults aged 15–24 years.[85] At least one in four U.S. teenage girls has a sexually transmitted disease.[10][86] In the US, about 30% of 15–17 year old adolescents have had sexual intercourse, but only about 80% of 15–19 year old adolescents report using condoms for their first sexual intercourse.[87] More than 75% of young women age 18–25 years felt they were at low risk of acquiring an STD in one study.[88] Some sexually transmitted infections include:

Safe sex is a relevant harm reduction philosophy.[9][97] Condoms and dental dams are widely recommended for the prevention of STDs. According to reports by the National Institutes of Health (NIH) and World Health Organization, correct and consistent use of latex condoms reduces the risk of HIV/AIDS transmission by approximately 85%–99% relative to risk when unprotected.[98][99]

People, especially those who get little or no physical exercise, have a slightly increased risk of triggering heart attack or sudden cardiac death when they engage in sexual intercourse, or any other vigorous physical exercise which is engaged in on a sporadic basis. Increased risk is temporary with incidents occurring within a few hours of the activity. Regular exercise reduces but does not eliminate the increased risk.[100]

Social effects

Adults

Alex Comfort and others posit three potential advantages of intercourse in humans, which are not mutually exclusive: reproductive, relational, and recreational.[3][101] While the development of the Pill and other highly effective forms of contraception in the mid- and late 20th century increased people's ability to segregate these three functions, they still overlap a great deal and in complex patterns. For example: A fertile couple may have intercourse while contracepting not only to experience sexual pleasure (recreational), but also as a means of emotional intimacy (relational), thus deepening their bonding, making their relationship more stable and more capable of sustaining children in the future (deferred reproductive). This same couple may emphasize different aspects of intercourse on different occasions, being playful during one episode of intercourse (recreational), experiencing deep emotional connection on another occasion (relational), and later, after discontinuing contraception, seeking to achieve pregnancy (reproductive, or more likely reproductive and relational).[101]

Nearly all Americans marry during their lifetime; yet close to half of all first marriages are expected to end in separation or divorce, many within a few years,[102] and subsequent marriages are even more likely to end.[103] Sexual dissatisfaction is associated with increased risk of divorce and relationship dissolution.[103]

According to the National Survey of Sexual Health and Behavior (NSSHB), in 2010, men whose most recent sexual encounter was with a relationship partner reported greater arousal, greater pleasure, fewer problems with erectile function, orgasm, and less pain during the event than men whose last sexual encounter was with a non-relationship partner.[48] According to the Journal of Counseling & Development, many women express that their most satisfying sexual experiences entail being connected to someone, rather than solely basing satisfaction on orgasm.[104]

Adolescents

With regard to adolescent sexuality, sexual intercourse is also often for relational and recreational purposes. However, teenage pregnancy is usually disparaged, and research suggests that the earlier onset of puberty for children puts pressure on children and teenagers to act like adults before they are emotionally or cognitively ready,[105][106] and thus are at risk to suffer from emotional distress as a result of their sexual activities.[106][107][108][109][110] Some studies have found that engaging in sex leaves adolescents, and especially girls, with higher levels of stress and depression.[111] A majority of adolescents in the United States have been provided with some information regarding sexuality,[112] though there have been efforts among social conservatives in the United States government to limit sex education in public schools to abstinence-only sex education curricula.[113]

One group of Canadian researchers found a relationship between self esteem and sexual activity. They found that students, especially girls, who were verbally abused by teachers or rejected by their peers were more likely than other students to engage in sex by the end of the Grade 7. The researchers speculate that low self esteem increases the likelihood of sexual activity: "low self-esteem seemed to explain the link between peer rejection and early sex. Girls with a poor self-image may see sex as a way to become 'popular', according to the researchers".[114]

In India, there is growing evidence that adolescents are becoming more sexually active outside of marriage, which is feared to lead to an increase in the spread of HIV/AIDS among adolescents, as well as the number of unwanted pregnancies and abortions, and add to the conflict between contemporary social values. In India, adolescents have relatively poor access to health care and education, and with cultural norms opposing extramarital sexual behavior, "these implications may acquire threatening dimensions for the society and the nation".[115]

Not all views on adolescent sexual behavior are negative, however. Psychiatrist Lynn Ponton writes, "All adolescents have sex lives, whether they are sexually active with others, with themselves, or seemingly not at all," and that viewing adolescent sexuality as a potentially positive experience, rather than as something inherently dangerous, may help young people develop healthier patterns and make more positive choices regarding sex.[105] Likewise, others state that long-term romantic relationships allow adolescents to gain the skills necessary for high-quality relationships later in life[116] and develop feelings of self-worth. Overall, positive romantic relationships among adolescents can result in long-term benefits. High-quality romantic relationships are associated with higher commitment in early adulthood[117] and are positively associated with self-esteem, self-confidence, and social competence.[118][119]

Ethical, religious, and legal views

See also: Sexual ethics and Religion and sexuality

While sexual intercourse is the natural mode of reproduction for the human species, humans also have intricate moral and ethical guidelines which regulate the practice of sexual intercourse and that vary according to religious and governmental laws. Some governments and religions also have strict designations of "appropriate" and "inappropriate" sexual behavior, which include restrictions on the types of sex acts which are permissible. A historically prohibited or regulated sex act is anal sex.[120][121]

Consent and sexual offenses

Sexual intercourse with a person against their will, or without their informed legal consent, is referred to as rape, and is considered a serious crime in most countries.[122] More than 90% of rape victims are female, 99% of rapists male, and only about 5% of rapists are strangers to the victims.[123]

Most 'developed' countries have age of consent laws specifying the minimum legal age a person may engage in sexual intercourse with substantially older persons, usually set at about 16–18, while the legal age of consent ranges from 12–20 years of age or is not a matter of law in other countries.[124] Sex with a person under the age of consent, regardless of their stated consent, is often considered to be sexual assault or statutory rape depending on differences in ages of the participants.

Some countries codify rape as any sex with a person of diminished or insufficient mental capacity to give consent, regardless of age.[125]

The expression "sexual intercourse" has been used as a term of art in England and Wales and New York State. In England and Wales, from its enactment to its repeal on the 1 May 2004,[126] section 44 of the Sexual Offences Act 1956 read:

Where, on the trial of any offence under this Act, it is necessary to prove sexual intercourse (whether natural or unnatural), it shall not be necessary to prove the completion of the intercourse by the emission of seed, but the intercourse shall be deemed complete upon proof of penetration only.
Unnatural

This expression referred to buggery (including both buggery with a person and buggery with an animal).[127]

Penetration

According to cases decided on the meaning of the statutory definition of carnal knowledge under the Offences against the Person Act 1828, which was in identical terms to this definition, the slightest penetration was sufficient.[128] The book "Archbold" said that it "submitted" that this continued to be the law under the new enactment.[129]

Continuing act

See Kaitamaki v R [1985] AC 147, [1984] 3 WLR 137, [1984] 2 All ER 435, 79 Cr App R 251, [1984] Crim LR 564, PC (decided under equivalent legislation in New Zealand).

Section 7(2) of the Sexual Offences (Amendment) Act 1976 contained the following words: "In this Act . . . references to sexual intercourse shall be construed in accordance with section 44 of the Sexual Offences Act 1956 so far as it relates to natural intercourse (under which such intercourse is deemed complete on proof of penetration only)". The Act made provision, in relation to rape and related offences, for England and Wales, and for courts-martial elsewhere.

From 3 November 1994 to 1 May 2004, section 1(2)(a) of the Sexual Offences Act 1956 (as substituted by section 142 of the Criminal Justice and Public Order Act 1994) referred to "sexual intercourse with a person (whether vaginal or anal)". This section created the offence of rape in England and Wales.

The penal code in New York State provides: § 130.00 Sex offenses; definitions of terms: 1. "Sexual intercourse" has its ordinary meaning and occurs upon any penetration, however slight.[130]

Romantic relationships

Sexual orientation and gender

There is considerable legal variability regarding definitions of and the legality of sexual intercourse between persons of the same sex or gender. For example, in 2003 the New Hampshire Supreme Court ruled that female same-sex relations did not constitute sexual intercourse, based on a 1961 definition from Webster's Third New International Dictionary, in Blanchflower v. Blanchflower, and thereby an accused wife in a divorce case was found not guilty of adultery based on this technicality. Some countries, such as Islamic countries, consider homosexual behavior to be an offense punishable by imprisonment or execution.[131]

Marriage and relationships

Sexual intercourse has traditionally been considered an essential part of a marriage; many religious customs required consummation of the marriage by sexual intercourse, and the failure for any reason to consummate the marriage was a ground for annulment, which did not require a divorce process. Annulment declaration implied that the marriage was void from the start – i.e. there was in law no marriage. Furthermore, continuing sexual relations between the marriage partners is commonly considered a 'marital right' by many religions, permissible to married couples, generally for the purpose of reproduction. Today, there is wide variation in the opinions and teachings about sexual intercourse relative to marriage and other intimate relationships by the world's religions. Examples:

In some cases, the sexual intercourse between two people is seen as counter to religious law or doctrine. In many religious communities, including the Catholic Church and Mahayana Buddhists, religious leaders are expected to refrain from sexual intercourse in order to devote their full attention, energy, and loyalty to their religious duties.[149]

Opposition to same-sex marriage is largely based on the belief that sexual intercourse and sexual orientation should be of a heterosexual nature.[150][151][152][153][154][155] The recognition of such marriages is a civil rights, political, social, moral, and religious issue in many nations, and the conflicts arise over whether same-sex couples should be allowed to enter into marriage, be required to use a different status (such as a civil union, which either grant equal rights as marriage or limited rights in comparison to marriage), or not have any such rights. A related issue is whether the term marriage should be applied.[136][156][157]

In other animals

See also: Animal sexual behaviour

In zoology, copulation is often termed as the process in which a male introduces sperm into the female's body. Spiders have separate male and female sexes. Before mating and copulation, a male spins a small web and ejaculates on to it. He then stores the sperm in reservoirs on his large pedipalps, from which he transfers sperm to the female's genitals. Females can store sperm indefinitely.[158]

Many animals which live in the water use external fertilization, whereas internal fertilization may have developed from a need to maintain gametes in a liquid medium in the Late Ordovician epoch. Internal fertilization with many vertebrates (such as reptiles, some fish, and most birds) occur via cloacal copulation (see also hemipenis), while mammals copulate vaginally, and many basal vertebrates reproduce sexually with external fertilization.

However, some terrestrial arthropods do use external fertilization. For primitive insects, the male deposits spermatozoa on the substrate, sometimes stored within a special structure, and courtship involves inducing the female to take up the sperm package into her genital opening; there is no actual copulation. In groups such as dragonflies and spiders, males extrude sperm into secondary copulatory structures removed from their genital opening, which are then used to inseminate the female (in dragonflies, it is a set of modified sternites on the second abdominal segment; in spiders, it is the male pedipalps). In advanced groups of insects, the male uses its aedeagus, a structure formed from the terminal segments of the abdomen, to deposit sperm directly (though sometimes in a capsule called a "spermatophore") into the female's reproductive tract.

Humans, bonobos,[13] chimpanzees and dolphins[14] are species known to engage in heterosexual behaviors even when the female is not in estrus, which is a point in her reproductive cycle suitable for successful impregnation. These species, and others, are also known to engage in homosexual behaviors.[15] Humans, bonobos and dolphins are all intelligent social animals, whose cooperative behavior proves far more successful than that of any individual alone. In these animals, the use of sex has evolved beyond reproduction, to apparently serve additional social functions. Sex reinforces intimate social bonds between individuals to form larger social structures. The resulting cooperation encourages collective tasks that promote the survival of each member of the group.[3]

See also

References

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