Both men and women experience orgasm. Measurable signs include rapid, cyclical contractions of muscles around the sex organs and anus, accompanied by characteristic changes in heart rate, blood pressure and often breathing. In both sexes, orgasm is involuntary — controlled by the autonomic nervous system. Scans of brain activity during orgasm show that energy turnover is maintained or increased in areas connected with the autonomic nervous system. By contrast, turnover temporarily declines in the cortex. Could this account for “mindless” pleasure? In any event, the ensuing relaxed feeling has been linked to release of the hormones oxytocin and prolactin.
Male versus female orgasms
Although male and female orgasms share a common biological basis, important differences exist. Ejaculation in men is typically short-lived and followed by an unresponsive spell lasting up to several hours. In contrast, female orgasms generally last longer and the refractory period is brief or completely absent. Orgasms can occur in rapid succession, often with rising intensity; but their occurrence is highly variable. Reportedly, one in three women rarely or never experience orgasm during coitus. Occurrence is frequently irregular even in women who do have orgasms. Studies indicate that only one in 10 women consistently experiences coital orgasm.
Orgasm is generally connected with stimulation of the penis in men and of its counterpart, the clitoris, in women. Female orgasm may also result when nipples or other erogenous zones are stimulated. Nipples become erect when stimulated and oxytocin is released.
A 1970s comic book introducing the Facts of Life to schoolchildren likened orgasms to sneezes, also abrupt involuntary responses. In fact, this is less far-fetched than it might seem. Links between orgasm and sneezing, first reported in medical literature in 1875, have been sporadically identified ever since. A 2008 revisitation by physicians Mahmood Bhutta and Harold Maxwell concluded that a connection between orgasms and sneezing or yawning may be quite common. Erectile tissues of the nose, penis and clitoris all share a diffuse influence of the autonomic nervous system. This perhaps casts new light on snuff-taking, which reportedly originated in France.
Functions for female orgasm
Evolutionary biologists rarely discuss male orgasm. Its function is seemingly obvious, with roots presumably extending back at least to the origin of copulation in ancestral vertebrates. It is typically accompanied by rapid, rhythmic contractions of the anal sphincter, prostate and penis muscles. The brief ejaculation process is essential for conception. By contrast, the female orgasm has no self-evident function and at least 20 different explanations have been offered.
It has been widely assumed that female orgasm is unique to women and hence needs special explanation. In The Naked Ape, Desmond Morris proposed two complementary evolutionary functions. The first is strengthening the pair bond between partners by promoting physical intimacy. The second (“poleax hypothesis”) is that orgasm facilitates conception by physically exhausting the woman, keeping her horizontal after coitus and reducing semen leakage. Other suggested evolutionary explanations include adaptation for bipedal locomotion, assessment of mate quality and increased insemination efficiency. British biologists Robin Baker and Mark Bellis suggested an “upsuck” action for female orgasm, proposing that this promotes retention of semen from a preferred partner, increases the likelihood of conception, and even plays a role in sperm competition by expelling sperm from an immediately preceding partner. However the notion of “sperm wars” in humans is highly questionable. [See my previous blogs Sperm Wars: Dispatch from a Conscientious Objector (August 7, 2013) and Kamikaze Sperms or Flawed Products (September 3, 2013).]
Elisabeth Lloyd — an American biology philosopher — challenged the inherent assumption that orgasm in women serves a particular function. She suggested that the clitoris, variably associated with orgasms, is a functionless evolutionary vestige. In The Case of the Female Orgasm (2005), she argues that orgasm in women is a mere by-product of evolutionary, developmental and physiological processes underlying its male equivalent. She suggested a parallel with functionless nipples in men, as noted by her mentor Stephen J. Gould In a Natural History article (included in a 1992 anthology) mockingly entitled “Male nipples and clitoral ripples”. The absence of any specific function of female orgasm would account for its great variability and fits better with the lack of any hard evidence associating orgasm with reproductive success. It is also striking that the ratio of female orgasms to acts of coitus remains steady across the human menstrual cycle, with no mid-cycle peak.
Unsurprisingly, Lloyd’s book unleashed a storm of protests from proponents of evolutionary explanations for orgasm. One female sociobiologist perversely accused her of sexism, while a review in Nature unkindly (if amusingly) described her book as an “anticlimax.”
Female orgasm not unique to women
Contributors to the debate have generally accepted that female orgasms occur only in women. A 2008 paper co-authored by Lloyd and reproductive biologist Kim Wallen explicitly stated: “Humans appear relatively unique among animals in that both males and females can experience orgasm...” Yet this claim is problematic for both sides. If female orgasm evolved to serve one or more specific functions, why should it be unique to humans unless the proposed functions are also unique to our species? Why, for instance, would only humans need to reinforce pair bonds with female orgasms? Conversely, if female orgasms are just a by-product, why do they occur only in humans?
In fact, it has been known for over 40 years that at least some nonhuman primates show a mating response resembling orgasm in women. In 1968, reproductive biologists Doris Zumpe and Richard Michael reported that orgasm occurs during mating in female Rhesus macaques, accompanied by backward clutching. A decade later, American psychologist David Goldfoot and colleagues reported behavioural and physiological indicators of a sexual climax in female stumptailed macaques. The most convincing evidence came from direct recordings indicating intense uterine contractions and abruptly increased heart rate. A characteristic round-mouthed facial expression was also seen in four out of 10 females during copulations. A year later, American primatologists Mel Allen and William Lemmon published a landmark review reporting female orgasms for numerous nonhuman primate species. In fact, if we rely on mere external observation, we may fail to recognize the occurrence of orgasm in other species, especially if it is subtle. Direct measurement of physiological indicators such as heart rate and uterine contractions is essential, but such measurements are seemingly limited to macaques. Nonetheless, it has been abundantly clear for some time that female orgasm is not unique to humans. Authors who fail to acknowledge this have simply not done their homework.
Allen, M.L. & Lemmon, W.B. (1981) Orgasm in female primates. Am. J. Primatol. 1:15-34.
Bhutta, M.F. & Maxwell, H. (2008) Sneezing induced by sexual ideation or orgasm: an under-reported phenomenon. J. Roy. Soc. Med. 101:587-591.
Dixson, A.F. (2012) Primate Sexuality: Comparative Studies of the Prosimians, Monkeys, Apes and Human Beings (Second Edition). Oxford: Oxford University Press.
Goldfoot, D.A., Westerborg-van Loon, H. Groeneveld, W., & Slob, A.K. (1980) Behavioral and physiological evidence of sexual climax in the female stump-tailed macaque (Macaca arctoides). Science 208:1477-1479.
Gould, S.J. (1992) Bully for Brontosaurus: Reflections in Natural History. New York: W.W. Norton & Co.
Lloyd, E.A. (2005) The Case of the Female Orgasm: Bias in the Science of Evolution. Cambridge, MA: Harvard University Press.
Morris, D. (1967) The Naked Ape: A Zoologist's Study of the Human Animal. London: Jonathan Cape.
Slob, A.K., Groeneveld, W.H. & Van der Werff Ten Bosch, J.J. (1986) Physiological changes during copulation in male and female stumptail macaques (Macaca arctoides). Physiol. Behav. 38:891-895.
Udry, J.R. & Morris, N.M. (1968) Distribution of coitus in the menstrual cycle. Nature 220:593-596.
Wallen, K. & Lloyd, E.A. (2008) Clitoral variability compared with penile variability supports nonadaptation of female orgasm. Evol. Develop. 10:1-2.
Zumpe, D., & Michael, R.P. (1968) The clutching reaction and orgasm in the female rhesus monkey (Macaca mulatta). J. Endocrinol. 40:117-123.
A videotaped interview discussing the breastfeeding Chapter of my book How We Do It was recently posted on YouTube. The interview was conducted at The Field Museum by Emily Graslie, who has developed an impressive following with her regular series BrainScoop. Here is the interview: