Origin of the female orgasm
Female orgasm has been described as a happy afterthought of evolution — unnecessary, but fun! Now, a new study in mammals shows that the female orgasm actually helps stimulate ovulation.
The role of female orgasm plays no obvious role in human reproduction, and has intrigued scholars as far back as Aristotle. Numerous theories have tried to explain an origin for the reflex, but most theories have concentrated on human and primate biology.
Now scientists at Yale and the Cincinnati Children's Hospital have examined its evolution across different species. Their study appears in the journal JEZ-Molecular and Developmental Evolution.
"Prior studies have tended to focus on evidence from human biology, and the modification of a trait rather than its evolutionary origin."
Gunter Wagner PhD, the Alison Richard Professor of Ecology and Evolutionary biology, member of Systems Biology Institute, Yale University, and Adjunct Professor of Obstetrics and Gynecology, Wayne State University.
Instead, Wagner and Mihaela Pavlicev of the Center for Prevention of Preterm Birth at Cincinnati Children's Hospital, propose that the original trait evolved into human female orgasm from its original function of inducing ovulation.
As there is no association between having, or not having, an orgasm and the number of children women can give birth to, scientists focused on a specific physiological result simultaneous to female orgasm — the release of the hormones prolactin and oxytocin. Prolactin is released from the front of the pituitary gland and stimulates a mother's milk production after childbirth. Oxytocin, sometimes referred to as the love hormone, is released by both males and females during sexual intercourse when orgasm is achieved. Scientists looked for the release of these two hormones in other mammals, finding that in many placental mammals, orgasm also plays a role in ovulation - or production of the egg.
In spite of the enormous diversity of reproductive biology in mammals, some essential characteristics can be traced throughout all mammalian evolution. The female ovarian cycle in humans, for instance, is not dependent upon sexual activity. However, in other mammal species, ovulation is only induced by males. Scientists' analysis of this fact supports that male-induced ovulation evolved first, with spontaneous female ovulation evolving later.
"Homologous traits in different species are often difficult to identify, as they can change substantially over the course of evolution. We think that the "hormonal surge" characterizes a trait we know as "female orgasm" in humans. This insight enables us to trace the evolution of the trait across species."
Mihaela Pavlicev PhD, 1Center for Prevention of Preterm Birth, Perinatal Institute, Cincinnati Children’s Hospital Medical
Center, and Department of Pediatrics, University of Cincinnati College of Medicine, Cincinnati, Ohio, USA
The evolutionary explanation of female orgasm has been difficult to come by. The orgasm in women does not obviously contribute to the reproductive success, and surprisingly unreliably accompanies heterosexual intercourse. Two types of explanations have been proposed: one insisting on extant adaptive roles in reproduction, another explaining female orgasm as a byproduct of selection on male orgasm, which is crucial for sperm transfer.We emphasize that these explanations tend to focus on evidence from human biology and thus address the modification of a trait rather than its evolutionary origin. To trace the trait through evolution requires identifying its homologue
in other species, which may have limited similarity with the human trait. Human female orgasm is associated with an endocrine surge similar to the copulatory surges in species with induced ovulation. We suggest that the homolog of human orgasm is the reflex that, ancestrally, induced ovulation. This reflex became superfluous with the evolution of spontaneous ovulation, potentially freeing female orgasm for other roles. This is supported by phylogenetic evidence showing that induced ovulation is ancestral, while spontaneous ovulation is derived within eutherians. In addition, the comparative anatomy of female reproductive tract shows that evolution of spontaneous ovulation is correlated with increasing distance of clitoris from the copulatory canal. In summary, we suggest that the female orgasm-like trait may have been adaptive, however for a different role, namely for inducing ovulation. With the evolution of spontaneous ovulation, orgasm was freed to gain secondary roles, which may explain its maintenance, but not its origin. J. Exp. Zool.
(Mol. Dev. Evol.) 00B:1–12, 2016. C 2016 Wiley Periodicals, Inc.
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