Unlike many animals, it is not obvious with human women when they are fertile. What are the evolutionary benefits of hidden ovulation? With the help of a computer model, researchers tested various hypotheses. Accordingly, the most likely explanation is that openly exposed fertility led to aggressiveness in early human women, which is why the visibly fertile women had to fear physical violence. Women whose ovulation was undetectable escaped these conflicts and thus had better chances of reproduction.
Many female animals show significant physical changes around ovulation. In female baboons, for example, the genital area swells considerably during the fertile days and thus signals the willingness to mate. In human women, on the other hand, ovulation usually takes place unnoticed. A common explanation for this up to now has been that hidden ovulation had the evolutionary benefit of strengthening couple relationships: since the man could not be sure exactly when the woman was fertile, he had to spend many nights with her in order to increase his chances of reproduction. This so-called “male investment” hypothesis has been quoted regularly, but has not yet been empirically verified.
Interactions between women
A team led by Jaimie Arona Krems from Oklahoma State University has now developed and tested an alternative hypothesis. “Previous research has largely focused on the benefits women had of hiding their ovulation from men,” the researchers say. “In contrast, we suggest that women may have benefited from hiding their ovulation from other women.”
Arizona State University co-author Athena Aktipis explains, “I pondered the male investment hypothesis for years, and because a verbal hypothesis cannot be argued, I started working on how to test it. “At the same time, she was exploring female sociality. “It struck me that women could be aggressive towards other women who show evidence of ovulation, which would then be an advantage in hiding ovulation.”
Computer simulation for female and male behavior
To test this hypothesis, the researchers developed what is known as agent-based modeling. In this type of modeling, the individuals are represented by so-called agents, whose behavior can be programmed and analyzed. Each individual follows a certain set of rules and can interact with other individuals and with the environment. In the model by Krems and colleagues, the simulated women and men followed rules regarding their movement, their attractiveness and their reproductive behavior.
The male individuals in the model differed in terms of their promiscuity. Promiscuous males did not enter into solid partnerships with females to help raise future children, while males who were not promiscuous stayed around to share resources and support future children. The simulated women either had physical signs showing when they were ovulating, or ovulation was hidden. They could also become aggressive towards one another. In the course of the simulation, the women and men interacted with each other and had the opportunity to reproduce and form parental partnerships.
Not just men as standard
In fact, the model showed: women with hidden ovulation fared better. They had more children, avoided conflicts with other women, and were more successful in forming parenting relationships with men. The model could thus support the new hypothesis of female rivalry. In addition, Krems and colleagues also tested the male investment hypothesis by suppressing aggressive interactions between women in their model. In this case, women with hidden ovulation had little advantage over those whose fertile days were clearly visible. In the team’s view, female interactions are therefore the more likely explanation – even if the model cannot answer with any certainty whether that was actually the reason why hidden ovulation evolved.
“Social science work has tended to regard male cognition and male behavior as the standard. But women continue to face some unique challenges – especially in their interactions with other women. This work is based on taking this idea seriously. If we do that, I think we will learn more, not just about the female psyche, but about the human psyche, ”says Krems.
Source: Jaimie Arona Krems (Oklahoma State University) et al., Nature Human Behavior, doi: 10.1038 / s41562-020-01038-9