Competition for pairs between prehistoric human women could have contributed to “hidden ovulation” – the lack of any notable physical evidence that a woman is fertile, experts say.
Using computational models, American researchers have found evidence that hidden ovulation in humans – which is unusual in the animal kingdom – has evolved to allow women to hide their fertility from other females.
This would have helped to avoid female conflicts, probably caused by aggression towards potential rivals for male colleagues.
Previously, scientists believed that women evolved to hide ovulation from men to encourage them to help care for children.
New research shows that the origin of hidden ovulation could actually have been more female-oriented than previously thought.
Human females have evolved to hide the physical signs of ovulation – which means men are no wiser
“The study of human evolution tended to look at things from a male perspective,” said the study’s lead author, Athena Aktipis, an associate professor of psychology at Arizona State University in the United States.
Even women’s specific adaptations – such as their social behavior and hidden ovulation – have been viewed in terms of how men shape them.
“Our calculation model shows that female sociality is much more than securing male investment.”
It is considered that human females have hidden ovulation, because there is no external physiological sign, either for the woman herself or for other people, that ovulation occurs.
As a result, women rely on useful aids, such as charts, test strips, applications or wearable technology to identify periods of fertility.
In contrast, some animals, such as baboons, undergo obvious physical changes during a period of ovulation – in particular, swelling of the perineal skin.
Gradually, during human evolution, female fertility is likely to become increasingly difficult to detect from the observer’s point of view.
For almost half a century, the evolution of hidden ovulation in human women has been explained by a theory called the male investment hypothesis.
Human females rely on aids such as diagrams, test strip applications, smart monitors and wearable technology to identify fertility periods.
In essence, the theory suggests that hidden ovulation has been helpful in securing male partners to help 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 not been formally modeled so far.
But female primates do not only interact with males – they interact with each other, sometimes cooperating and sometimes involving conflicts.
“I’ve been puzzled by the male investment hypothesis for years, and because you can’t argue with a verbal hypothesis, I’ve started working on how to test it,” Aktipis said.
At the same time, Aktipis was working on “female sociality” – a term used to describe female individuals in a population of animals that tend to associate in groups.
“It struck me that women could have assaulted other females who showed ovulatory signs, which would create a benefit to hide ovulation.”
Sexual swelling in a baboon. In general, the skin surrounding the perineum of a female baboon shows cyclic changes in size, color, and firmness during a menstrual cycle.
This theory, called the “female rivalry hypothesis,” is now an alternative and convincing argument for how hidden ovulation has evolved.
Ovulatory cues would have made women more obvious as potential loving rivals for a man.
Evolutionary adaptations in humans take place on the time scale of many generations, which makes it difficult to test whether or how traits might evolve.
Therefore, Aktipis and colleagues tested the female rivalry hypothesis using computer modeling, which allows researchers to test ideas that would be difficult to test in the real world.
In agent-based computational models, an “agent” is a person whose behavior can be programmed and analyzed.
Each agent follows a specific set of rules and can interact with other agents and the environment.
In the model developed to test the female rivalry hypothesis, male and female agents followed the rules governing their movement, reproductive behavior, and attractiveness.
Male agents varied in their promiscuity – promiscuous males did not associate with females to help raise offspring, while non-promiscuous male agents blocked out to share resources and to support future children.
Female agents had either physical clues that indicated when they were ovulating, or ovulation was hidden.
Female agencies could also behave aggressively towards each other.
Female and male agents interacted with each other and had opportunities to procreate and form parental partnerships.
The model supported the hypothesis of female rivalry, showing that women who hid ovulation did better, the team found.
They had several children, avoided female-female aggression and managed to form parental relationships with men.
“Working in the social sciences has tended to assume that male knowledge and behavior is implicit,” said study author Jaimie Arona Krems, an assistant professor of psychology at Oklahoma State University.
“But women repeatedly face some unique challenges, especially in their interactions with other women.
“This work is the result, in part, of taking the idea seriously.
“When we do that, I think we’ll learn more, not just about the female mind, but about the human mind.”
The research team also used the model to test the male investment hypothesis, creating scenarios that did not allow women to behave aggressively with each other.
But there was no clear benefit from hiding ovulation in this scenario, again suggesting that hidden ovulation evolved due to interactions with other females.
“This work is a necessary change in thinking about how human females have evolved,” said Aktipis.
“Women’s sociality and other adaptations are not just about ensuring male investment, even though this has long been the underlying assumption about the purpose of female social behavior.”
The study was published in Nature Human Behavior.