Scientists and philosophers have puzzled for more than 2,000 years over the biological point of the female climax.
Why do women have orgasms?
While in men the function is clear — to transfer sperm — scientists and philosophers have puzzled for more than 2,000 years over the biological point of the female climax, which is not required for conception.
This week researchers came up with a new theory: an ancient ancestor needed orgasms to induce ovulation, thus releasing eggs for fertilisation.
"Female orgasm seems to be a happy afterthought of our evolutionary past," said a statement from Yale University, which took part in the study published this week in the Journal of Experimental Zoology.
"Human female orgasm is an evolutionary holdover from induced ovulation," study co-author and Yale biologist Gunter Wagner told AFP.
The team used published research to create an evolutionary model of ovulation in animals.
In many mammals, they found, release of the hormones prolactin and oxytocin is linked with ovulation, while in humans it accompanies the female orgasm.
"We interpret this evidence to suggest that the physiological changes caused by human female orgasm are homologous (similar) to those that cause ovulation in other species," they concluded.
Whether or not other animals have orgasms remains largely a mystery, though it is known that males induce ovulation in some mammal species, including rabbits and cats.
The model showed that in an ancient common mammalian forebear, which lived some 60-65 million years ago, females needed male stimulation to induce ovulation.
This trait was later replaced in some species by cyclical, or spontaneous, ovulation as exists humans today.
This likely happened around the dawn of the primates — the group of animals we humans belong to.
"Female orgasm did not evolve in the human lineage, but may have acquired additional roles" after it was no longer required for ovulation, such as pair bonding, said the study.
As ovulation became spontaneous, the layout of the female genitalia also changed, it added.
The clitoris "relocated from its ancestral position inside the copulatory canal," where it remains in many animals today, but not in humans or monkeys.
"This may help explain why many women do not have orgasm during intercourse but are able to reach orgasm during masturbation or during clitoral stimulation during intercourse," said a statement.
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