Intimately Connected

In evolutionary terms, the female orgasm is one of the most intriguing aspects of human reproduction. Because it varies widely and is often elusive it has proven very difficult to explain its origin convincingly. In her 2005 book, The Case of the Female Orgasm, Elisabeth Lloyd listed no fewer than 21 proposed explanations for the female climax. The only safe interpretation is that a woman’s orgasms are directly linked to her clitoris, the developmental equivalent of a man’s penis. Ultimately, Lloyd concluded that both the clitoris and orgasms are evolutionary by-products that serve no function, comparable to the vestigial nipples of men.
Possible functions of orgasms
Although multiple attempts have been made to identify biological functions for female orgasms and the clitoris, no convincing evidence has yet been produced to support any of them. If orgasms serve a function, they should be somehow linked with reproductive success, perhaps through increasing the chances of fertilization. Yet no such link has ever been documented and many authors simply invoke Aristotle’s comment that women can conceive without orgasm. The “up-suck” hypothesis that orgasm draws semen into the womb has been largely discredited. Moreover, if orgasm has any connection with the likelihood of fertilization, surely it should be more prevalent around ovulation in the middle of a woman’s menstrual cycle? But this is seemingly not the case. A 1968 paper by Richard Udry and Naomi Morris examining the distribution of coitus across the cycle incidentally recorded orgasms too. They occurred in about 70% acts of coitus across all stages, with no evidence of a mid-cycle surge.

Hitherto, little attempt has been made to establish whether female orgasms are linked to reproductive success. One exception is a 2013 paper by Brendan Zietsch and Pekka Santtila, who tested whether orgasm rate in women is connected with offspring number. Analysing data from over 8,000 identical and non-identical female twins in Finland, they showed that both orgasm rate and offspring number have substantial genetic foundations. However, after allowing for relationship duration and coitus frequency, orgasms and offspring number were not directly connected.

Exploring the clitoris
It beggars belief that the anatomy of the clitoris and associated structures was first properly documented less than 20 years ago. Helen O’Connell and colleagues achieved a major breakthrough with a 1998 paper and have since published additional details. Thanks to their research it is now evident that the clitoris is but a small, externally visible part of a much larger, complex system. As most of that “clitoral complex” lies below the surface, it has been aptly likened to an iceberg.
In 2008, Kim Wallen and Elisabeth Lloyd took a simplistic approach to bolster their argument that the clitoris is a functionless vestige. Using large published datasets, they argued that the clitoris is significantly more variable in length than the penis, indicating that natural selection is weak or absent. They linked variability in clitoris size to variability in occurrence of orgasm. But they did add a proviso: Because average clitoris length is less than a sixth of average penis length, precise measurement is more difficult. Nevertheless, in view of the wide range of clitoral sizes, Wallen and Lloyd concluded that measurement error did not explain their results.

Soon afterwards, however, Vincent Lynch challenged this conclusion. For starters, he questioned the unstated assumption that a woman’s capacity to achieve orgasm is directly related to the length of her clitoris (even less plausible because the substantial subsurface portion of the clitoral complex was ignored). But Lynch also repeated their analyses, using clitoris and penis volumes instead of lengths. Having thus reduced the potential influence of measurement error, Lynch found no significant difference in variability between clitoris and penis.

Origin of female orgasm
In a wide-ranging assessment of reproduction across mammals, a new paper by Mihaela Pavličev and Günter Wagner has radically transformed our understanding of the evolutionary context of female orgasm.
Their findings concern the basic pattern of the ovarian cycle in mammals. The key point is that a cycle always begins with the ripening of a batch of starter follicles. One or more of these may eventually release an egg through ovulation, after which the remnant of any ruptured follicle forms a corpus luteum (yellow body) that supports pregnancy. But there is a fundamental difference among mammals between induced ovulators, with ovulation occurring only in response to mating, and spontaneous ovulators, with internally triggered ovulation occurring regardless of mating. (In certain cases, ovulation is spontaneous but mating is needed to form a corpus luteum, which is effectively the same as induced ovulation.) In fact, most mammals  —  including many marsupials, insectivores, rodents, carnivores, rabbits and tree-shrews  —  are induced ovulators. Spontaneous ovulation is less common, occurring (as far as is known) in all primates, most hoofed mammals, at least some bats and a small group containing elephants, hyraxes and manatees. Analysis by Pavličev & Wagner neatly confirmed a conclusion in my 1990 book Primate Origins: Induced ovulation is the primitive state in placental mammals, while spontaneous ovulation developed as a derived condition in some lineages.Crucially, Pavličev & Wagner speculate that orgasm in women is associated with a hormonal surge (involving prolactin and oxytocin) with similarities to the mating-triggered surge in species with induced ovulation. They go on to infer that the human female orgasm is derived from the mating response that induced ovulation in ancestral mammals. Additional analyses revealed that the evolution of spontaneous ovulation is associated with increasing separation between clitoris and vagina. In most female mammals the lower ends of the urinary and reproductive tracts are combined into a urogenital sinus with a single opening to the outside world. Only relatively few mammals — including all primates and some rodents — have essentially eliminated that confluence, such that the urethra opens separately above the vaginal orifice. In sum, Pavličev & Wagner propose that evolution of spontaneous ovulation together with physical separation between clitoris and vagina freed orgasm to acquire new functions.
An intriguing finding reported by Wallen & Lloyd in 2011 is that the likelihood of a woman experiencing orgasm during coitus declines with the distance between her clitoris and urinary meatus (urethral opening). This was originally proposed in 1924 by “A.E. Narjani” (who outed herself 9 years later as the psychoanalyst Marie Bonaparte) and subsequently confirmed by Carney Landis and colleagues in the 1940 book Sex in Development. When Wallen and Lloyd analysed both datasets, which had never undergone formal statistical treatment, they found that a shorter clitoris-urethra distance was significantly related to orgasm during coitus. They concluded that a woman is very likely to have orgasms solely from coitus if the distance is less than 2.5 cm (1 inch). This difference, they suggested, could reflect different degrees of exposure to androgens (male hormones) during fetal development, with higher levels generating larger distances.

A functionless vestige?
Elisabeth Lloyd and Kim Wallen have championed the interpretation that the clitoris and associated orgasms are evolutionary relics with no functional significance. But this is logically inconsistent, because they also argue that there is little evidence that female orgasm occurs outside of humans. If the clitoris and orgasms of women are functionless — a mere byproduct of a genetic programme for developing the penis and ejaculation in males — surely we would expect orgasm to occur in all female mammals? That is, indeed, the implication of the interpretation proposed by Mihaela Pavličev and Günter Wagner.
There is also a striking sex difference between men and women in the development of orgasm. Whereas male orgasm develops quickly in close association with puberty, the frequency of female organism increases gradually and reaches a plateau only when a woman attains the age of about 35. If female orgasm were a functionless vestige, surely one would expect it to become less likely with age? In any case, now that the full extent of the clitoral complex in women is known, it seems scarcely probable that such complexity would have been preserved over evolutionary time without any accompanying function. Unfortunately, we currently do not know whether the clitoris has a similarly complex structure in other female mammals or whether it is unique to primates or perhaps women alone. Comparative studies are sorely needed to establish the evolutionary background of the human clitoral complex. If, for example, the extensive structure now documented for women originated in the common ancestor of monkeys, apes and humans, it would be exceedingly difficult to argue that it evolved without any selective advantage.

For now, it seems quite likely that the clitoris and orgasms in women do serve some evolved function. Rather than slavishly quoting Aristotle’s perfunctory observation that women can conceive without orgasm, we need to examine subtler aspects of human reproduction such as processes influencing female choice and bonding between partners.