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Estrus, And The Evolution Of ‘Mean Girl’ Behavior Like Slut Shaming

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An evolutionary shift in the great ape line may illuminate the roots of some mating- and dating-arena viciousness among modern women.

Humans are great apes. We share about 98.8% of our DNA with chimpanzees and bonobos, who together make up the pan species. Humans don’t directly descend from pans, but we do share a common ancestor.

Female chimps and bonobos eagerly and frenetically mate with many males in a single day when they are in the fertile phase of their menstrual cycles. Males know that a female is fertile (a/k/a “in estrus”) because her genital area becomes swollen and rosy, and she is not at all shy about waggling it around in their direction.

Gorillas are also great apes. They, too, have rosy “shows” of estrus. Even so, because each breeding-age female is dominated by a single male, a female gorilla’s estrus does not generally spark a community-wide sex party.

Humans, however, have no rosy rump estrus shows. Speculating about why humans have “lost” estrus, many researchers have thought in terms of what the benefit to women might have been both from revealing and from concealing their ovulation status from men. Their answers for the most part have had to do with the idea that having a blatant estrus and lots of sex while fertile greatly increased the chances of getting pregnant. “Losing” estrus, on the other hand, may have kept males closer at hand to provide and protect. For example, had a man wanted to impregnate a specific woman and yet not known her ovulation status, he would have had to have sex with her throughout her monthly cycle. Familiarity, family feeling, and protection of her children might have been the result.

Athena Aktipis is an associate professor of psychology and a cooperation theorist at Arizona State University. Skeptical of the dominance of the long-held idea that estrus disappeared in order to hide ovulation status from men, she decided to take a fresh approach to the topic. Might not the evolutionary change that resulted in lost estrus have also been driven by a need to conceal ovulation status from other women?

In the modern world, competition among women for mates can be fierce. Recognizing that, Aktipis decided to explore whether struggles among ancestral women for mates and reproductive success may have been a key environmental driver of the evolutionary change. She led a study that included researchers at ASU as well as at the University of Oklahoma, the University of Exeter, and the University of California, Los Angeles. The team’s paper was published in the January 25 issue of the journal Nature Human Behavior.

Aktipis calls her idea that intrasexual aggression was a way of life for females and that it drove the evolutionary change the “female rivalry hypothesis.” She and her colleagues devised an innovative way to test it that did not require participation by living women. (Testing the hypothesis on living women was impossible. For starters, if any of the ancestral aggression was physical, the researchers would have had to ask women to be physically violent with each other. Also, gathering a control group of women with blatant estrus shows would have been impossible. Estrus has not been apparent in women in recorded history.)

The researchers created a computational model of behavior.

The Model

Aktipis and colleagues designed a model that was based on the ancestral world’s community life, in which small groups of people (100 – 250) lived non-monogamously in the wild.

Although the model was not a game, the hardware and software together created a look that resembled a 2-D game console from the Pac-Man era. Autonomous male and female “agents,” 100 of each, were represented by large, colored dots that were randomly distributed on the screen during any simulation’s set-up. As the simulations ran, the dots moved about according to rules, most of which had to do mating, pre-mating, and post-mating behavior. Male and female agents could form pairs, mate, conceive, and invest in offspring together. There was a fundamental trade-off for both males and females between time spent searching for a mate and time invested in care of children.

For males, typical characteristics and behaviors were:

Mate value. This was a general measure encompassing all of the qualities (like attractiveness, strength, good health, social standing, and youth) that might make a mate appealing.

Promiscuity. All males wanted badly to mate. Some were not at all promiscuous. Others had higher promiscuity ratings. Some had very high ratings. If a male had a low promiscuity rating, after mating with a woman he might stay around to mate again, and so might also provide and protect. Males with high ratings generally immediately wandered off to mate with others.

For females, typical characteristics and behaviors were:

Female mate value relied heavily on physical attractiveness and incorporated ovulation status.

Females did not have promiscuity ratings, but they roamed in search of mates and were free to mate promiscuously before, during, and after pregnancy.

At the start of each simulation, no females were pregnant. They could get pregnant only by mating during their ovulation-determined fertility windows.

All females were either “revealers” (with estrus shows) or “concealers” (without estrus shows). For the revealers, attractiveness ratings varied according to their menstrual cycles. Having a show of estrus boosted attractiveness by 25% until the fertility period ended.

Some simulations called for aggression in which females would always attack any nearby female who was more attractive than they. Other simulations called for females to attack any nearby female who was ovulating. The paper in Nature Human Behavior never specified whether “attacks” were overtly physical or were indirect assaults like “mean girl” gossip, though in a Zoom interview Aktipis clarified, “Because they would have involved no immediate physical risks, covert attacks on reputation may have been less costly to the perpetrator while being just as costly as physical injury to the victim’s value as a mate.”

Either way, according to the rules of the computational model, when a female was attacked her mate value was temporarily reduced.

Females healed slowly.

The Experiments

The researchers created three experiments.

Experiment 1 served as the control condition. No females were programmed to be aggressive. Without that female-to-female rivalry, it was also a test of the “male investment hypotheses.” Experiment 1 produced only ambiguous support for that hypothesis. Researchers tweaked the mate value for revealers during periods when they were not ovulating. As they did, revealers gained and lost a slight advantage over concealers in lifetime reproductive success.

Experiment 2 tested one aspect of the female rivalry hypothesis. In it, females attacked any nearby female who was more physically attractive than they. Given that a revealer’s attractiveness score rose by 25% while she was ovulating, more revealers were attacked than concealers. In Experiment 2 simulations, concealers had a small but significant increase in lifetime reproductive success — and similar increase in male investment in her children. In a follow-up email, Aktipis explained that the increased investment on the part of the males may have been due to the fact that “concealers were less likely to be the target of aggression and so were less likely to have their mate value damaged.”

Experiment 3 tested what Aktipis flagged in follow-up email correspondence as the key aspect of the female rivalry hypothesis. In it, concealers and revealers both attacked revealers who were clearly in estrus. They did not attack based on simple attractiveness, though revealers’ attractiveness was higher during estrus. Being attacked diminished a revealer’s mate value. As was true in Experiment 2, victims were slow to heal. Concealers were never attacked. In general, a concealer enjoyed a surprisingly large increase in lifetime reproductive success and a higher investment from mates in her children.

The increase in males’ paternal investment for children of concealers that was found in Experiment 3 had also been noted in Experiment 2. Yet, according to the study’s design, a male’s likelihood of providing and protecting was tied only to his promiscuity rating. By definition, males who were not promiscuous stayed with their mates and promiscuous males wandered. Asked why in both Experiment 2 and Experiment 3 females’ lifetime reproductive success seemed to be tied to male parental investment, Aktipis flagged the open question about male paternal investment as a possible area for future research. She also offered that the “investing” males were not necessarily exhibiting devotion to their children. They were staying in the family because of the mate value of the female. The children who benefitted from their presence were not necessarily their offspring.

What Would Charles Darwin Say?

From 1831-1836 a very young Charles Darwin traveled around the Atlantic and Pacific oceans on the HMS Beagle as the ship’s official naturalist. Whenever the Beagle was in port, Darwin made overland treks. He observed animal behavior and collected thousands of fossils, which he carefully observed and catalogued over the next twenty-odd years. In 1859 he published On the Origin of Species by Means of Natural Selection, a work based on the evidence of evolution that was apparent in his fossils. In On the Origin of Species, he pronounced that all organisms are primarily driven by a biological imperative to successfully reproduce. Anything that helps a plant or animal bear hardy offspring would most probably be favored by evolution and would not disappear.

Aktipis and her team ran 10,000 simulations on their computational model, always with an eye to reproductive success. Statistically analyzing the results, they concluded that women’s aggressive behavior as embodied in their “female rivalry hypothesis” may have improved reproductive success for the species and therefore been a key driver of women’s evolutionary change away from shows of estrus. However, Aktipis was careful to point out in a follow-up email that other drivers may have contributed to the change, as well, including the long-held “male investment hypothesis.”

Is Estrus Entirely Gone?

Many scientists would argue that estrus never disappeared entirely.  Looking for subtle clues about ovulation status, their suspicion has fallen particularly on qualities of voice, smell, and physical appearance.

In 1969 a study from the University of California, Los Angeles showed that women’s voices become more high-pitched during ovulation. In 2000 researchers in Utah found that both men and women have receptors for sex hormones in their vocal chords. A 2017 study from the University of Zadar in Croatia identified vocal changes in women throughout the menstrual cycle.

Looking for changes in physical appearance, in 2017 researchers from the University of Newcastle showed men and women photos of women’s faces that were taken when the women were ovulating. Both men and women judged the photos taken during ovulation to be more appealing.

The search for estrus-indicating odors has produced several sets of intriguing results. For a study published in 1975 in the British journal Advances in the Biosciences, an independent panel of judges compared vaginal odors taken from women across their menstrual cycles. Those of pre-ovulatory and ovulating women scored as more appealing. In 2004 evolutionary psychologists in Finland determined that men found the T-shirt odors of ovulating women to be more attractive. In 2019 a team at Israel’s University of Haifa found that, as far as men were concerned, a woman’s body odors during ovulation increased her social standing.

There have also been studies of the mythical, non-odiferous scents — pheromones. Again and again, scientists have tried to determine whether they play a part in signaling a woman’s fertility status. Nothing definitive has ever emerged. The sensory “vomeronasal” organs that, in mammals, are the receptors for pheromones are only present in some humans, and may be vestigial in the rest. The hundreds of millions of dollars spent on perfume branding notwithstanding (“Raw Chemistry!” “True Sexiness!” “Xist Oil for Breaking Out of the Friend Zone!”), if human pheromones have a come-hither effect on anyone, it may be very slight.

‘Lost’ Estrus in the Modern World

The study by Aktipis and colleagues showing how female rivalry may have contributed to the loss of estrus is part of a growing body of research looking at female aggression and female sociality in general as forces in both the ancestral and modern worlds.

By now, many studies have shown “mean girl” behavior like that hypothesized by Aktipis for ancestral women to be a common ploy in modern dating and mating. She is not the first to claim evolutionary origins for cattiness. In a 2013 article in Philosophical Transactions of the Royal Society B, University of Ottawa psychologist Tracy Vaillancourt reviewed much of the research and promptly caught pushback from colleagues. For example, in a story written by Olga Khazan for The Atlantic, Durham University psychologist Anne Campbell and University of Notre Dame anthropologist Augustin Fuentes were both quoted as suggesting that, as common as verbal backstabbing may be among modern women of mating age, there’s no real evidence to suggest that it originated in humans’ ancestral past. They suggested to Khazan instead that its roots may lie in the modern social pressures that can discourage a woman from acting more directly and positively on her own behalf.

With no eyeglass that will show us the extent of female intrasexual attacks in humans’ ancestral past and to clarify whether it was overtly violent or covertly verbal, we’ll never know for sure how aggressive ancestral women were to each other. The social world of ancestral humans, lived out in communities of only 100 - 250 people, was probably far less complex than ours. Even so, it could have been impressively byzantine, including subtle signals and motivators that today’s psychologists and anthropologists have yet even to imagine.

Aktipis, Vailllancourt, and many of their colleagues seem to suggest that at least some forms of verbal aggression among women are hard-wired. Are they correct? We could make better guesses if behaviors had gotten fossilized in the same way that bones do.


The paper’s first author was Jaimie Arona Krems. Other team members were Scott Claessens, Melissa R. Fales, Marco Campenni, and Martie G. Haselton.

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