Iiiiiiiii 2 4 6 8 10 12 14 16 18 Vaginal length

Figure 4.3 Relationship between vaginal length and length of the (erect) penis in nine species of primates, including H. sapiens. 1. Lemur catta; 2. Callithrixjacchus; 3. Lophocebus aterrimus; 4. Mandrillus sphinx; 5. Macaca mulatta; 6. M. arctoides; 7. Pan troglodytes; 8. Gorilla g. gorilla; and 9. H. sapiens.

human penile length is exceptional among the primates. This is not the case.

Does the thickness of the human penis have any significance as regards sexual selection during human evolution? If human penile morphology evolved to promote pleasurable stimulation of the female partner, there might be at least two avenues of selective advantage. Firstly, if enduring relationships between the sexes with long-term reproductive benefit in terms of offspring survival are facilitated by pleasurable sexual activity, then penile morphology might be adaptive in this context. However, it will be recalled that in the monogamous and polygynous non-human primates, males tend to have the least specialized penile morphologies, despite the occurrence of long-term sexual relationships in such species. There is some evidence that women rate the thickness and length of a partner's penis as significant factors in their sexual satisfaction (Stulhofer 2006). Human mate choice and long-term relationship decisions are immensely complicated, however. Cross-cultural studies indicate that qualities such as kindness, a good sense of humour, and ability to provide resources are valued more highly by women than physical attractiveness alone (Buss 2003). This is not to deny the importance of physical attractiveness in human evolution and I shall return to this subject in Chapter 7.

A second possible function of pleasurable penile stimulation might be to promote female orgasm, which in turn might play a role in relation to sperm transport and sperm competition (Fisher 1982; Small 1993; Baker and Bellis 1993b; 1995; Miller 2000). Baker and Bellis in particular have advanced the hypothesis that women's orgasms play an important role in sperm competition. However, the functions and evolution of orgasm in the human female have been much debated and remain controversial. The reader is referred to an excellent analysis and critique of this field by Lloyd (2005). Here I shall concentrate upon the question of whether female orgasm evolved in association with sperm competition and cryptic female choice in human beings.

Baker and Bellis (1993b) proposed that 'by altering the occurrence, sequence and timing of the different types of orgasm [nocturnal, masturbatory and copulatory orgasms], the female can influence both the probability of conception in monandrous situations and the outcome of sperm competition in polyandrous situations . . . much of this influence will be cryptic to the male partner(s).' These ideas arose from the notion that female orgasm might produce an 'upsuck' response, drawing sperm through the cervix and into the uterus (Fox, Wolff, and Baker 1970; Singer, 1973). Baker and Bellis conducted experiments with human subjects in order to estimate sperm numbers in 'flowbacks:' the fluid ejected from the vagina after copulation. Their goal was to define the effects of female orgasm upon sperm retention by women. The highest sperm retentions occurred when women experienced orgasm during a window of time extending from 1 min before ejaculation, until 45 min afterwards. Estimates of sperm retention were significantly greater for women who engaged in extra-pair copulations, as compared to estimates involving their usual (long-term) partners. This increase was attributed to a higher incidence of sperm retention orgasms during (or after) extra-pair copulations.

Baker and Bellis also attributed important functions to female orgasms which occurred during intervals between matings. However, these 'inter-copulatory orgasms' were viewed as activating the upsuck mechanism to draw acidic (vaginal) material into the cervix and to negatively affect existing sperm or to reduce the likelihood of sperm retention at a subsequent mating. Even in the absence of copulation, female orgasms were posited to have a beneficial (antibiotic) effect, by drawing acidic secretions from the vagina into the cervical canal. Thus during pregnancy or in young (virgin) females masturbatory orgasms might assist in 'combating cervical infection'.

Before examining some of these findings in more detail, it will be helpful to consider the whole question of the inelegantly named 'upsuck hypothesis'. What evidence is there that female orgasms draw sperm from the vagina into the cervix, or that sperm transport is affected by orgasms in women or females of other mammalian species? This is an old theory and Dickinson provided some interesting comments on it in his (1949) volume on Human Sex Anatomy.

Suction pump action in orgasm is accepted behavior in lay literature. It may some day be proven to be usual, but calls for confirmation. The mucus held within or produced for the occasion by the glands of the healthy cervix is asserted to be extruded during the peak of the climax in order to tanglefoot the sperms and then is said to be wholly dragged back through the external os.

However, Dickinson also noted a lack of connection between orgasmic responsiveness and fertility in women:

Insuck is supposed to depend on orgasm. Orgasm is absent in the frigid. Yet in my study of a thousand office patients the frigid were not notably infertile, having the expected quota of living children, and somewhat less than the average incidence of sterility.

In 1950, Grafenberg published the results of a study to determine whether female orgasm might affect transfer of fluid from the vagina and through the cervix during copulation. Grafenberg asked women to wear a plastic cervical cap throughout a menstrual cycle; the cap was filled with a radiopaque oil. Orgasm occurred as a result of sexual intercourse in these subjects, but X-ray examinations failed to reveal any transfer of the radiopaque oil into the cervix or uterus. Masters and Johnson (1966) also used radiopaque fluid held in position using contraceptive diaphragms. However, they attempted to produce a fluid which more closely matched human semen in its physical properties. Six subjects participated in experiments to determine the effects of (manually induced) orgasms upon fluid movement. As in the Grafenberg studies, however, X-ray examinations revealed no effects. Masters and Johnson were able to take X-rays during orgasm, as well as 10 minutes afterwards.

All experiments have methodological limitations, especially so where clinical research and sexology are concerned. Thus, it might be objected that the use of a contraceptive cap could interfere with the natural orgasmic mechanism and transfer of fluid from the vagina into the cervix. Fox et al. (1970) criticized the results of earlier studies and examined effects of sexual intercourse upon intra-uterine pressure in a single female subject. They employed telemetry to measure a marked post-orgasmic fall in intrauterine pressure. However, no attempt was made to ascertain whether female orgasm, and associated changes in uterine pressure, had any effect upon fluid transport from the vagina into the cervix. Lloyd (2005) in her detailed critique of theories of human female orgasm, points out that Fox et al.'s data are therefore limited and do not address the major question of whether orgasm influences sperm transport. Given the considerable individual variability in orgasmic responsiveness which occurs among women, it is also regrettable that only one subject was studied by Fox et al., and that only two experiments were reported. Yet when Smith (1984) wrote his influential review on human sperm competition he cited Fox et al.'s studies as important evidence for the notion that female orgasm results in semen transport and facilitates fertilization. Subsequently, Baker and Bellis (1993b; 1995) continued to cite this study in support of the upsuck hypothesis. The negative findings of Grafenberg (1950) and Masters and Johnson (1966) were accorded much less weight.

Rapid transportation of spermatozoa to the oviducts has been recorded in various mammals and (non-orgasmic) contractions of the female reproductive tract are important in facilitating this process. Table 4.1 provides information on the time taken for sperm to reach the oviduct after copulation or (artificial insemination) in ten species, including Homo sapiens. Thus, in the absence of orgasm, sperm may reach the oviduct within 5 min (rabbit), 15 min (rat, mouse, pig), or 5-68 min (women).

Table 4.1 Time taken by spermatozoa to reach the oviduct after copulation or artificial insemination in mammals



Region of tube


5-68 min



15 min



15-20 min



2-60 min



Several min


Guinea pig

15 min


Domestic dog

2 min to several hours



15 min


Domestic cattle

2-13 min



6 min-5 h


Source: Data from Harper (1994); after Dixson (1998a).

Source: Data from Harper (1994); after Dixson (1998a).

Various physiological mechanisms have been proposed to account for sperm transport under these conditions, including coitus-induced release of oxy-tocin in women (Carmichael et al. 1987; Komisaruk, Beyer-Flores, and Whipple 2006) and the presence of prostaglandins in semen (which is unlikely to affect sperm transport: Mortimer 1983). Wildt et al. (1998) conducted experiments to determine the possible effects of oxytocin upon sperm transport through the reproductive tract of the human female. They positioned pressure recorders in the uterus and showed that rhythmic contractions occurred (3 times per minute) prior to any hormonal treatment. Administration of oxytocin resulted in an increase in the strength and frequency of uterine contractions; the highest pressures occurred lower in the uterus (near the cervix) and lower pressures were recorded close to openings of the oviducts. Under these conditions, artificially formulated semen containing radiopaque particles was rapidly transported from the vagina, through the cervix and into the lumen of the uterus. This occurred promptly after administration of oxytocin (intravenously or by nasal spray) and in the absence of sexual stimulation or orgasm. Lloyd (2005) has summarized these findings as follows:

Wildt et al attribute rapid transport of sperm to the peristaltic contractions of the uterus—much like the contractions documented in dogs and cows— and to the muscular layers of the fallopian tubes. The relevant peristaltic contractions occur with a frequency of 2 to 5 per minute in healthy women at all times.

Release of oxytocin in women may occur as a result of vaginal dilation, cervical stimulation, and coital stimulation in the absence of orgasm. Thus, as Wildt et al. (1998) point out, there is no requirement to posit that female orgasm induces the genital contractions which influence sperm transport.

The absence of robust evidence for the occurrence of uterine 'upsuck' in relation to female orgasm casts considerable doubt upon the existence of 'sperm retention orgasms' in human females (Baker and Bellis 1993b, 1995). Many women require additional (manual) stimulation to achieve orgasm and do not necessarily achieve orgasm during (or after) intercourse in the absence of such stimulation (Kinsey et al. 1953; Fisher 1973; Hite 1977). Cross-cultural evidence also casts doubt on the proposition that female orgasm is a widespread or inevitable part of human sexual response (Mead 1967). Baker and Bellis (1993b; 1995) did not measure directly the effects of female orgasm on female fertility and reproductive success. Numbers of sperm lost in flowbacks were measured, but sperm numbers inseminated and retained after orgasm were only estimated mathematically. The accuracy of these procedures is highly questionable, however. Thus, Figure 1 in Baker and Bellis' (1993b) paper in Animal Behaviour includes examples where numbers of sperm in flowbacks actually exceed the numbers estimated at insemination! Lloyd (2005) critiques the selective use of data in these studies and concludes that 'there are such serious problems with the fundamental data set on flowbacks used by Baker and Bellis that it fails to meet basic scientific standards of evidence.' For example, of 11 couples who contributed data on flowbacks, a single couple accounted for 73 per cent of the 127 measurements analysed, whilst four couples supplied only one measurement each.

Given such serious criticisms and doubts concerning Baker and Bellis's reports, it is regrettable that their results have often been cited in the literature as if they represented the established facts of human physiology. This is not the case, and they do not support arguments concerning possible effects of cryptic female choice or sperm competition during human evolution.

Why does orgasm occur in human females and what is its evolutionary basis? Although answers to these questions are unlikely to advance our understanding of the origins of human mating systems, I believe that some discussion of these questions may be helpful to readers. Firstly, female orgasm is not confined to Homo sapiens. Putatively homologous responses been recorded in a number of non-human primates, including stump-tail and Japanese macaques, rhesus macaques, and chimpanzees (Dixson 1998a). Pre-human ancestors of Homo sapiens, such as the australopithecines, probably possessed a capacity to exhibit female orgasm, as do various extant monkey and ape species. The best documented example concerns the stump-tail macaque (Macaca arctoides), in which orgasmic uterine contractions have been recorded during female-female mounts (Goldfoot et al. 1980) as well as during copulation (Slob et al. 1986). Not every mount is accompanied by a female orgasm, however. De Waal (1989) estimates that female stump-tails show their distinctive climax face (which correlates with occurrence of uterine contractions) once in every six copulations. Vaginal spasms were noted in two female rhesus monkeys as a result of extended periods of stimulation (using an artificial penis) by an experimenter (Burton 1971). Likewise, a female chimpanzee exhibited rhythmic vaginal contractions, clitoral erection, limb spasms, and body tension in response to manual stimulation of its genitalia (Allen and Lemmon 1981). Masturba-tory behaviour, accompanied by behavioural and physiological responses indicative of orgasm, has also been noted in Japanese macaques (Wolfe 1991) and chimpanzees (Goodall 1986).

In human beings, men and women experience similar sensations during orgasm; the process appears to be physiologically homologous in the two sexes. Male and female descriptions of orgasm are often indistinguishable, except for references to sexually dimorphic structures, such as the genitalia (Vance and Wagner 1976; Bancroft 1989). The main sensate focus for tactile stimulation resulting in orgasm is a homologous structure; the penis in the male and the clitoris in the female. Interestingly, Wallen and Lloyd (2008) have shown that variability in the length of the human clitoris greatly exceeds variation in penile or vaginal length. They conclude that this is due to co-evolution of penile and vaginal length, and a relative absence of selective pressure upon the evolution of clitoral size in humans.

Orgasm occurs during ejaculation in men, and is followed by a quiescent (refractory) period during which penile detumescence occurs and sexual activity usually ceases. In many non-human primates ejaculation is accompanied by behavioural responses (cessation of thrusting movements, body tension and tremor of the limbs, and changes in facial expression and vocalization) indicative of orgasm. A refractory period occurs after ejaculation in monkeys and apes, just as in males of many other mammals (Dixson 1998a). In women (as in females of other primate species) orgasm is not followed by a refractory period. Thus, some women are capable of repeated (multiple) orgasms, whereas such capacities have rarely been recorded in men.

Given the homology of the physiological and anatomical substrates for orgasm in the two sexes, and given also that these homologues exist in younger (prepubertal) individuals as well as in adults, Symons (1979) proposed that female orgasm might represent a non-selected homologue of a primarily masculine response. Thus orgasm in males is associated with important reproductive functions in adulthood (ejaculation, refractory period) and has been selected for on this basis. The female response might represent a homologue of the male process. By analogy, males possess homologues of female traits (e.g. nipples), which play a vital role in human reproduction (e.g. lactation) but have no function in males. Although Symons (1979) uses the male nipples as a case of non-adaptive homol-ogy, there are other examples. Thus human males and male anthropoids such as the macaques and guenons possess a vestigial and functionless homologue of the female uterus: the utriculus prostaticus or uterus masculinus. This tiny blind-ended pocket of tissue, adjoining the prostatic urethra, is all that remains in adult males of the Mullerian ducts, which give rise in females to the uterus and upper portion of the vagina during embryonic development (Gray 1977).

Symons (1979) cites Beach (1976a) to the effect that neural mechanisms mediating sexual patterns typical of one sex may also be capable of expression in the opposite sex, given favourable circumstances for their elicitation. Symons regards human female orgasm as a 'byproduct of female bisexual potential'. This view is not unrealistic or disrespectful to female sexuality, despite feminist criticisms of Symons' hypothesis on the basis that it is 'androcentric' (Wasser and Waterhouse 1983), in 'denial of the significance of female sexual pleasure' (Caulfield 1985) or that it smacks of 'a gentlemanly breeze from the nineteenth century' (Hrdy 1979). Everyone, of both sexes, is entitled to seek a loving, mutually respectful (and pleasurable) sexual relationship with their partner. In certain human cultures, women are reported to attain orgasm regularly during copulation and men are expected to learn the sexual skills required for its elicitation (e.g. among the people of Mangaia, in the Cook Islands: Marshall and Suggs 1971; Le Vay and Valente 2002). This is by no means the rule in human cultures, however. It should be kept in mind that Symons (1979) was examining the evolution of human sexuality, and was not seeking to define a culturally or gender-biased ideal of how people should behave.

The occurrence of orgasm in female non-human primates is better documented than it was in 1979, when Symons's book was published (Slob et al. 1986; Wolfe 1991; Dixson 1998a; Campbell 2007). The factors which influence the expression of female orgasm are still poorly understood, however, and further primatological research on this topic would be beneficial. For example, Troisi and Carosi (1998) reported that lower-ranking female Japanese macaques exhibit orgasms more frequently when copulating with a high-ranking male. The non-human primate data do not support the notion that female orgasm improves 'bonding' and pair formation between the sexes, however. Indeed, the best documented examples of female orgasmic responses occur in species with multi-male/multi-female mating systems (e.g. macaques and chimpanzees) rather than in monogamous forms such as gibbons (Dixson 1998a).

Forty years ago, Morris (1967) advanced the peculiar argument that orgasm exhausts women, and makes it more likely that they will rest after intercourse, rather than walking upright with the resulting loss of semen. There is no comparative depth to the argument, however, as monkeys and apes exhibit female orgasm in association with dorso-ventral copulatory postures and an absence of post-mating rest periods. Lloyd (2005) carefully analysed nineteen adaptive accounts of female orgasm, including Morris's (1967) ideas, and found that all of them are inadequate to explain its evolution in human beings. I agree with Lloyd that the most parsimonious explanation for the available evidence concerning the evolution of female orgasm in H. sapiens is the byproduct hypothesis (Symons 1979). The byproduct hypothesis has little to tell us about the origins and evolution of human mating systems, however. So, we must leave matters there in order to explore another avenue of enquiry: the anatomy and functions of the oviduct in mammals, and its relationship to cryptic female choice.

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