Sincerely,Heterosexual human sexual intercourse (also called coitus, or copulation) involves inserting the man's erect penis into a woman's vagina. 'Coitus' derives from the Latin 'coire', which means 'to go together'. The purpose of this act may be for reproduction, or for pleasure and the expression of intimacy and love.
Sexual intercourse can also be broadly defined as "any communication between persons that involves a sexual response." Oral sex, anal sex and other non penetrative sex may be included in this definition of 'having sex'.
Typically, foreplay before sexual intercourse (behaviour such as kissing, cuddling, fondling) leads to sexual arousal in which the man's penis becomes erect and the woman's vagina naturally lubricates. The penis then enters the vagina and both partners move their hips and bodies in a mutually satisfying manner. The man may thrust his penis in and out of the vagina. This generally leads to orgasm and the male ejaculation of semen.
Through the human reproductive act of sexual intercourse, reproduction may be successful if the male orgasms and ejaculates semen into the woman's vagina. Semen is composed of sperm (male gametes) and seminal fluid from the seminal vesicles, Cowper's glands and the prostate. Once inside the woman's vagina, sperm may then be transported up through the cervix and uterus to the fallopian tubes. If an female ovum has been released (ovulation), the male gamete may join with the ovum and fertilisation occurs. The fertilised egg then travels to the uterus, where if it imbeds on the uterine lining, will develop into a fetus and pregnancy begins.
What is exciting evolutionary biologists today is the realisation of the active participation of the female and her genitalia in controlling a species' successful copulation, conception and birth of offspring. A female's genitals are not a simple thoroughfare for sperm, but comprise an exquisitely structured organ with the complexity and sensitivity to determine the paternity of a female's offspring. (Blackledge, Catherine "The Story of V: A Natural History of Female Sexuality" Rutgers University Press 2004 p. 101)
The internally fertilising female, courtesy of her genitalia, does not need to invest heavily in egg production. Her eggs are not fated to be tossed away by the tide, like broadcast spawners. A vagina, or its equivalent, makes sure that not all males have access to a female's eggs. In doing so, internal fertilisation hands a female the capacity to control which sperm get to fertilise her eggs. This is a very important ability. Courtesy of internally fertilising genitalia, the female of many species can exercise reproductive choice. She can be selective. (Blackledge, Catherine 2004. p. 95)
Features common to the genitalia of internally fertilising species also
provides clues as to the true function of a female's genitals. In internally
fertilisng females, it is an almost universal fact that sperm are never
deposited directly on top of the female's eggs. Instead, the sperm deposition
site (the vagina in humans) is typically separated from that of fertilisation.
In this basic way, the design of female genitalia simply and effectively
uncouples copulation from fertilisation. The act of depositing sperm does
not ensure sperm will reach the correct destination for reproductive success.
Another result of this separation of the two sites is that reproductive
ducting - the lengths of internal tubing or canal connecting vagina with
uterus or oviduct (egg tube) with ovary - is an elementary feature of female
genitalia. Hidden from view, these internal tunnels, or gen italic plumbing,
map out the subterranean route sperm must progress along if they are to
reach a female's eggs.
The style and shape of the reproductive tracts of internally fertilisng
females tells another important story. If the idea of female genitalia as
passive vessel - structured merely to assist and smooth the passage of sperm
to egg - is correct, then the design of female reproductive canals appears
perverse in the extreme. Not only are these ducts extremely narrow and extraordinarily
convoluted in design; they place a female's eggs far beyond the immediate
research of sperm. (Blackledge, Catherine 2004. p. 96)
Ejaculate can contain hundreds of millions of sperm, yet the vast majority
will fail to reach the female's ova. Typically only between 2 and 20 sperm
get anywhere near the female's eggs.
Post-ejaculation, females set to work immediately to bring sperm numbers
down. In many insects, birds and mammals, the vagina, the immediate environment
sperm find themselves within, is no safe haven. On the contrary, the vagina
is an extremely hostile acidic arena, which easily destorys newcomers. It
has to be if it wants to be selective. If low pH doesn't pulverise the sperm,
then killer cells appear to digest the intruders. Digestion of sperm by
spermicidal phagocytic cells occurs in insects, worms, fish and mammals.
(Blackledge, Catherine 2004. p.110)
Female mediated sperm transport is a sophisticated affair, consisting of
a series of interactions, controlled predominately by the female's nervous
system, which together tug and pull sperm to the target of the female's
choosing. When the female reproductive tracts are viewed as a structure
to selectively transport some sperm and not others, the engineering begins
to make sense.
Muscles placed strategically along the female's duct system, underpin her
ability to move sperm. Waves of muscular contraction (predominately moving
in the direction of her uterus) typically force sperm along the reproductive
tract. Also manipulating the movement of sperm is the suck and push of the
absorption and secretion of vaginal and cervical gland fluids. These female
fluids can sometimes smooth the sperm's passage, but can also coagulate
and accumulate, effectively smothering sperm. The net result from a female
perspective, is a finely tuned transport system. (Blackledge, Catherine
2004. p. 113)
As well as avoiding too many sperm reaching the vicinity of the egg, barriers in place along the female's reproductive tract must be able to filter out damaged, old, malformed or merely less competent, or unfit sperm and prevent them from reaching the egg. The successful sexual reproduction of a species relies on the vagina being able to perform this sorting and screening process effectively. (Blackledge, Catherine 2004)
A very interesting essay from the journal of Evolutionary Psychology on sperm competition and its role in shaping the human penis, (size, length, width, forceful ejaculation), evolutionary strategies for successful fertilization i.e. semen displacement and coagulation. On sexual intercourse and evolutionary benefits of deep penis thrusting, function of the foreskin and effects of circumcision, premature ejaculation, female reproductive strategies i.e. copulation with multiple partners, human sexual behaviour and wife rape (males having sex with their partner if suspecting them of infidelity).
From the essay: 'The average duration of coitus is 7.9 minutes with 100 to 500 thrusts per encounter (Hrdy and Whitten, 1987). ... In contrast to organisms that walk on all fours, the assumption of an upright posture and the emergence of bipedalism brought the human female reproductive tract, and the vagina in particular, into a perpendicular orientation with gravity that is poorly suited to semen retention. Copulation with the female in a supine position brings the female reproductive tract back into a more primitive parallel orientation with gravity, and enhances the likelihood that semen will be retained. However, due to the effects of gravity, the resumption of an upright posture following coitus has the potential to endanger semen retention. Consistent with this hypothesis, there are several mechanisms that appear to postpone getting up after a sexual encounter, such as post-copulatory petting, patterns of nocturnal copulation, and the sedative-like effects of orgasm (Gallup and Suarez, 1983).'
By Gordon G. Gallup, Jr., Department of Psychology, State
University of New York at Albany, Albany, NY 12222, USA.
Rebecca L. Burch, Department of Psychology, State University of New York
at Oswego, Oswego, NY 13126, USA.
http://www.epjournal.net/filestore/ep021223.pdf
This
Page Title: 'Human Sexual Intercourse: Evolution, Anatomy, Method
and Function of Coitus. Anal and Vagina'.
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