Competition for mates among prehistoric female women may have contributed to ‘hidden ovulation’ – a lack of significant physical clues that a woman is fertile, experts say.
Using computer models, U.S. researchers have found evidence that hidden ovulation in humans – uncommon in the animal kingdom – has evolved to enable females to hide their fertility status from other females.
This would have helped to avoid female conflict, perhaps driven by aggression against potential competitors for men.
Previously, scientists thought that women evolved to hide ovulation from men to encourage them to look after children.
The new research shows that the origin of hidden ovulation may have been more female-oriented than previously thought.
Human females evolved to hide physical signs of their ovulation – meaning males are not the pointer
“The study of human evolution tended to look at things from a male perspective,” said senior study author Athena Aktipis, an associate professor of psychology at Arizona State University in the United States.
‘Even adaptations specific to women – such as their social behavior and hidden ovulation – have been seen in terms of how men shape them.
“Our calculation model shows that female sociality is much more than securing male investment.”
Human females are considered hidden ovulation because there is no outward physiological sign, either for a woman herself or for others, that ovulation is taking place.
As a result, women rely on useful tools such as mapping, test strips, programs or portable technology to identify periods of fertility.
In contrast, some animals, such as baboons, undergo obvious physical changes during an ovulation period – especially swelling of the perineal skin.
Gradually during the evolution of man it is likely that the fertility of women became increasingly difficult to detect from the point of view of an observer.
For almost half a century, the evolution of hidden ovulation in female women has been explained by a theory called the male investment hypothesis.
Human women rely on tools like mapping, test strip apps, smart monitors and portable technology to identify periods of fertility
In essence, the theory suggests that hidden ovulation was helpful in ensuring male partners to raise and support children.
This hypothesis has been the predominant explanation for hidden ovulation for decades, although it has undergone few empirical tests and has so far not been formally modeled.
But female primates do not only interact with males – they interact with each other, sometimes collaboratively and sometimes in conflict.
“I have been amazed at the male investment hypothesis for years, and because you can not reason with an oral hypothesis, I started working on testing it,” Aktipis said.
At the same time, Aktipis is working on ‘female sociality’ – a term to describe female individuals in an animal population who tend to associate in groups.
“It struck me that females could have been aggressive towards other females that had ovulatory cues, which could then have created an advantage for concealing ovulation.”
Sexual swelling in a baboon. The skin around the perineum of a female baboon generally shows cyclic changes in size, color and firmness during a menstrual cycle
This theory, called ‘the female rival hypothesis’, is now an alternative and convincing argument about how the hidden ovulation developed.
Ovulatory cues would have made women more conspicuous if potential love for a male partner was opposed.
Evolutionary adaptations in humans occur on the time scale of many generations, making it difficult to test whether traits can develop.
Aktipis and colleagues thus tested the hypothesis of female rivalry using computational modeling, which allows researchers to test ideas that are difficult to test in the real world.
In agent-based arithmetic models, an ‘agent’ represents an individual whose behavior can be programmed and analyzed.
Each agent follows a specific set of rules and can interact with other agents and with the environment.
In the model developed to test the hypothesis of female rivalry, male and female agents followed rules that determined their movement, reproductive behavior, and attractiveness.
The male agents showed divergence in terms of their promiscuity – promiscuous men did not team up with women to raise subsequent children, while male agents who were not promiscuous kept on sharing resources and supporting future children.
Female drugs had physical indications indicating when they were ovulating, or that ovulation was hidden.
The female agents can also act aggressively towards each other.
The female and male agents interacted with each other and were given opportunities to form parenting partnerships.
The model supported the female competition hypothesis by showing that women who concealed ovulation performed better.
They had more children, avoided aggression between women and women and managed to form parental relationships with men.
“Work in the social sciences tends to assume that male cognition and behavior are the standard,” said Jaimie Arona Krems, first study author, assistant professor of psychology at Oklahoma State University.
‘But females always face some unique challenges, especially in their interactions with other females.
‘This work is due in part to the idea of taking this idea seriously.
“If we do, I think we will learn more, not only about the female mind, but also about the human mind.”
The research team also used the model to test the hypothesis of male investment by performing scenarios that do not allow women to act aggressively towards each other.
But in this scenario, there was no clear benefit to concealing ovulation, which in turn suggests that hidden ovulation developed as a result of interactions with other females.
“This work is a necessary shift in thinking about how human women have evolved,” Aktipis said.
“Female sociality and other adaptations are not just about securing male investment, although it has long been the underlying assumption about the purpose of female social behavior.”
The study was published in Nature Human Behavior.