Billy Crystal may have been shocked when Meg Ryan so effectively—and amusingly—faked an orgasm in a restaurant during the movie When Harry Met Sally , but surveys suggest only one-third of women are regularly fully aroused during intercourse. And although poor partner performance, psychological issues, or physiological shortfalls are often cited as the reason, two evolutionary biologists now offer a provocative new explanation. In a paper published today, they argue that female orgasm is an evolutionary holdover from an ancient system, seen in some other mammals, in which intercourse stimulated important hormonal surges that drive ovulation. And because those hormonal surges no longer confer a biological advantage, orgasms during intercourse may be lost in some women.

Follow our live coverage for the latest news on the coronavirus pandemic. But actually all female mammals have a clitoris, the highly sensitive organ that is linked with pleasure and orgasm in women. For example, a research paper presented at a biology conference this week showed that the clitoris in dolphins is very large, and more complex than we previously thought. Let's take a look at the biology and evolution of the clitoris — for science. All babies, regardless of whether they are destined to become a boy or a girl, begin development in the womb with a small bulge called a genital tubercle. If the developing foetus is destined to become male, the fetal testes will produce the male hormone testosterone and the genital tubercle will develop into a penis. If, on the other hand, the fetus is destined to become a female, the fetal ovary will not produce any hormones and instead the genital tubercle will develop into the clitoris.

If an animal must mate to reproduce, the entire future of its species depends on having sex. The most obviously beneficial adaptation for such a species is, therefore, pleasurable sex. While it's difficult to ask them if they enjoy doing the deed, a quick look at their behavior shows that, at the very least, most mammals and birds experience sexual pleasure. When it comes to the question of whether or not animals experience sexual pleasure, the answer is simple: most animals wouldn't take time out for sex if it didn't feel good. They certainly don't choose to have sex in order to make babies as they are not capable of understanding reproduction.

But establishing whether sexual pleasure ever actually tips over into orgasm is hard. Whether they regularly have them during normal copulation is much less certain; most animal sex is very brief and often quite violent. Most other vertebrates use external fertilisation; the female deposits her eggs and the male squirts them with sperm.