Competition for couples between prehistoric human women may have contributed to “hidden ovulation,” the lack of remarkable physical clues that a woman is fertile, experts say.
Using computer models, American researchers found evidence that hidden ovulation in humans, something unusual in the animal kingdom, evolved to allow women to hide their fertility status from other women.
This would have helped avoid female conflicts, perhaps driven by aggression against potential rivals for male peers.
Previously, scientists have thought that women evolved to hide men’s ovulation to encourage them to help care for children.
New research shows that the origin of occult ovulation could have actually been much more female-oriented than previously thought.
Human females have evolved to hide physical signs of when they ovulate; that is, men are not the wisest
“The study of human evolution has tended to look at things from a male perspective,” said study lead author Athena Aktipis, an associate professor of psychology at Arizona State University in the United States. .
“Even specific adaptations of females, such as their social behavior and hidden ovulation, have been seen in terms of how males form them.
“Our computational model shows that female sociality is much more than securing male investment.”
Human females are thought to have concealed ovulation because there is no outward physiological sign, for either a woman or other people, that ovulation will occur.
As a result, women rely on useful aids such as graphics, test strips, applications, or usable technology to identify fertility periods.
In contrast, some animals, such as baboons, undergo obvious physical changes during a period of ovulation, especially inflammation 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 nearly half a century, the evolution of hidden ovulation in human females has been explained by a theory called the male inversion hypothesis.
Human females rely on aids such as graphics, test strip applications, smart monitors, and wearable technology to identify fertility periods.
Basically, the theory suggests that occult ovulation was useful in securing male couples to help 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 so far not been formally modeled.
But primates not only interact with males, but interact with each other, sometimes cooperating, and sometimes conflicting.
“I’ve been baffled about the male investment hypothesis for years and since it can’t be discussed with a verbal hypothesis, I started working on how to test it,” Aktipis said.
At the same time, Aktipis worked on “female sociality,” a term to describe female individuals in an animal population who tend to associate in groups.
“It struck me that women could have assaulted other women who showed signs of ovulation, which would lead to a benefit in hiding ovulation.”
Sexual swelling in a baboon. In general, the skin surrounding the perineum of a female baboon shows cyclical changes in size, color, and firmness throughout a menstrual cycle.
This theory, called the “female rivalry hypothesis,” is now an alternative and persuasive argument about how occult ovulation evolved.
Ovulatory indications would have made females more visible as possible love rivals for a male mate.
Evolutionary adaptations in humans occur on the time scale of many generations, making it difficult to test whether traits can evolve.
Therefore, Aktipis and colleagues tested the female rivalry hypothesis using computational modeling, which allows researchers to test ideas that would be difficult to test in the real world.
In agent-based computational 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 the environment.
In the model developed to test the female rivalry hypothesis, male and female agents followed rules governing their movement, reproductive behavior, and attractiveness.
Male agents varied in their promiscuity: promiscuous men did not partner with women to help raise later children, while non-promiscuous male agents stayed to share resources and support. to future children.
Female agents had physical clues that indicated when they were ovulating or ovulation was hidden.
Women could also behave aggressively with each other.
Female and male agents interacted with each other and had opportunities to procreate and form parent pairs.
The team found that the model supported the female rivalry hypothesis by showing that women who hid ovulation worked best.
They had more children, avoided aggression between women and women, and managed to establish kinship relationships with men.
“Work in the social sciences has tended to assume that male cognition and behavior are the defaults,” said the study’s author, Jaimie Arona Krems, an assistant professor of psychology at Oklahoma State University.
“But females recurringly face some unique challenges, especially in their interactions with other women.
“This work is the result, in part, of taking this idea seriously.
“When we do, 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 using scenarios that did not allow women to behave aggressively.
But there was no clear benefit of hiding ovulation in this scenario, suggesting again that hidden ovulation evolved due to interactions with other females.
“This work represents a necessary change in thinking about how human females have evolved,” Aktipis said.
“Female sociality and other adaptations are not just about securing male investment, although this has long been the underlying assumption about the purpose of female social behavior.”
The study has been published in Nature Human Behavior.