Competition for partners between prehistoric human women may have contributed to ‘hidden ovulation’ – a lack of remarkable physical evidence that a woman is fertile, experts say.
Using computer models, American researchers found evidence that hidden ovulation in humans – which is uncommon in the animal kingdom – has evolved to allow females to hide their fertility status from other females.
This would have helped avoid conflict between women, perhaps driven by aggression towards potential male partner rivals.
Previously, scientists thought women evolved to hide ovulation from men to encourage them to help care for children.
The new research shows that the origin of a hidden ovulation may be much more focused on women than previously thought.
Human females have evolved to hide physical signs of when they are ovulating – meaning males are no wiser
“The study of human evolution tends to look at things from a male perspective,” said senior study author Athena Aktipis, associate professor of psychology at Arizona State University in the USA.
Even adaptations specific to women – such as their social behaviors and hidden ovulation – have been viewed in terms of how men shape them.
“Our computational model shows that female sociality is about much more than securing male investments.”
Human women are believed to have hidden ovulation, because there is no outward physiological sign, neither for a woman herself, nor for others, that ovulation is taking place.
As a result, women rely on useful tools such as charts, test strips, apps or wearable technology to identify periods of fertility.
In contrast, some animals, such as baboons, undergo marked physical changes during an ovulation period, particularly swelling of the perineal skin.
Gradually during human evolution, it is likely that female fertility became increasingly difficult to detect from an observer’s point of view.
For nearly half a century, the evolution of hidden ovulation in human women has been explained by a theory called the male investment hypothesis.
Human women rely on tools such as charts, test strip apps, smart monitors, and wearable technology to identify periods of fertility
Essentially, the theory suggests that concealed ovulation was helpful in helping male partners raise and support children.
This hypothesis has been the predominant explanation for hidden ovulation for decades, although it has undergone little empirical testing and has not been formally modeled to date.
But female primates don’t just interact with males – they communicate with each other, sometimes collaborate, and sometimes they are in conflict.
“I puzzled over the male investment hypothesis for years, and since you can’t argue with a verbal hypothesis, I started testing it,” Aktipis said.
At the same time, Aktipis was working on ‘female sociality’ – a term to describe female individuals in an animal population who tend to associate in groups.
“I noticed that females could be aggressive towards other females showing ovulatory signals, which would then provide an advantage in hiding ovulation.”
Sexual swelling in a baboon. In general, the skin around a female baboon’s perineum shows cyclic changes in size, color, and firmness throughout a menstrual cycle
This theory, called “the female rivalry hypothesis,” is now an alternative and compelling argument about how hidden ovulation evolved.
Ovulatory signals would make women stand out more as potential love competitors for a male partner.
Evolutionary adaptations in humans occur on the timescale of many generations, making it difficult to test whether and how traits can evolve.
Aktipis and colleagues therefore tested the hypothesis of female rivalry using computer modeling, which allows researchers to test ideas that are difficult to test in the real world.
In agent-based computer models, an ‘agent’ represents a person whose behavior can be programmed and analyzed.
Each agent follows a specific set of rules and can communicate with other agents and the environment.
In the model developed to test the female rivalry hypothesis, male and female agents followed rules that govern their movement, reproductive behavior, and attractiveness.
The male agents varied in terms of their promiscuity – promiscuous men did not work with women to raise subsequent children, while male agents who were not promiscuous hung out to share resources and support future children.
Female officers had physical cues that indicate when they were ovulating or if ovulation was hidden.
The female agents can also behave aggressively towards each other.
The female and male agents interacted with each other and had the opportunity to procreate and form parenting partnerships.
The model supported the female rivalry hypothesis by showing that women who concealed ovulation fared better, the team found.
They had more children, avoided female-female aggression, and managed to establish parenting relationships with men.
“Work in the social sciences tends to assume that male cognition and behavior is the standard,” said study first author Jaimie Arona Krems, assistant professor of psychology at Oklahoma State University.
But women regularly face some unique challenges, especially in their interactions with other women.
This work is partly the result of taking that idea seriously.
“If we do that, I think we will learn more, not just about the feminine mind, but about the human mind as well.”
The research team also used the model to test the male investment hypothesis, by running scenarios that did not allow women to behave aggressively towards each other.
But there was no apparent benefit of hiding ovulation in this scenario, again suggesting that hidden ovulation evolved due to interactions with other females.
“This work represents a necessary shift in thinking about how human females have evolved,” Aktipis said.
“Female sociality and other adaptations are not just about securing male investments, even if that has long been the underlying assumption about the purpose of female social behavior.”
The study is published in Nature Human Behavior.