Questions about women’s orgasms are, alas, still common, but evolutionary biologists are more concerned with the why than the how. One theory holds that it represents an adaptation of the mechanism seen in other animals where sex stimulates ovulation. For the first time, evidence has been found for this idea, although it’s still far from conclusive.
Orgasm involves a complex set of neurological triggers, making it something unlikely to evolve by chance. Such phenomena usually require strong pressure from natural selection, with those who experience orgasm being more likely to have healthy offspring. Yet we know that, unlike for men, a female orgasm is not necessary for conception.
It’s certainly possible to see how, once women started having orgasms it might encourage more sex, and therefore more children. Once childbirth became as dangerous and painful a process as it is for Homo sapiens (walking upright came at the price of narrow hips and large heads) an extra incentive to get down and dirty was probably particularly useful, but Dr Mihaela Pavlicev of the University of Cincinnati was more interested in deeper origins. After all, natural selection needs something to act on, a sort of proto-orgasm in this case.
Some animals maximize reproductive efficiency by waiting until they have sex to ovulate and use the act of copulation to get things started. Why waste all the biological effort of producing an egg if there is no partner around to fertilize it? Rabbits are one species that do this, and their capacity to reproduce rapidly when conditions are right is legendary.
The rabbits in Pavlicev’s study were 30 percent less likely to ovulate after copulation when treated with fluoxetine compared to controls. Further tests showed the drug was having its effect through the central nervous system, rather than directly affecting the ovaries. Combined these suggest sex stimulates orgasms in women and ovulation in rabbits through similar neuroendocrine pathways, making it more likely the two have a common origin.
In a previous study, Pavlicev and colleagues plotted the species that use sex to induce ovulation on a mammalian family tree (leading to a sharp exchange with fame). They concluded the first eutherian (non-marsupial) mammals probably ovulated in this way, making it something that existed in humanity’s deep ancestral past. Although many of our closest relatives don’t have female orgasms, the biological pathways used to stimulate ovulation survived, ready to be re-purposed for pleasure.
The study leaves many questions unanswered, such as why orgasm is so easy for some women and hard for others, and why for most women the trigger isn’t the sort of sex that leads to conception, but it stimulates research on both.
Female ejaculation is even less well understood than female orgasms – but are the evolutionary causes the same?