We’ve all been there (though some of us longer ago than others): the cruising bar, fraternity party or other gathering place where men vastly outnumber women. As the men trip all over themselves trying to make their competitors look like losers and themselves like desirable partners, women get the upper hand: they have their pick of partners, and can crush the already-sensitive egos of the men with the back of their manicured hand.

If you assumed that this kind of female-over-male dominance was a freak result of humans’ peculiar mating habits, biologists in Germany have some monkeys they’d like you to meet. The higher the percentage of males in troops of lemurs, macaques and other primates, they report in the journal PLoS ONE, the more dominant over males the females are.

That there are any circumstances in which female primates lord it over males in a social hierarchy may come as a surprise, but it's actually not that uncommon. Although in most species females rank below the males (which means most males win aggressive encounters), in the lemurs of Madagascar the females are dominant, in bonobos the males and females are roughly equal in dominance, and among macaques females are weakly dominant, with “the most dominant females rank[ing] above approximately a third of  the males,” says biologist Charlotte Hemelrijk of the University of Groningen, who led the new study.

There are two competing hypotheses for how this female dominance develops. One holds that dominance is inborn; you are more likely to be dominant if you are born big and strong, or if you inherit it from your mother, and that’s that. The alternative holds that there is a “winner-loser effect.” Primates have chance encounters, and if they win they are more likely to win again, while if they lose they are more likely to lose the next time; it's a snowball effect.

The reason is that the outcome alters an individual’s fighting ability. Winning raises, and losing lowers, self-confidence, which can be self-fulfilling (animals filled with swagger are more likely to win the next time, too). As Hemelrijk puts it, if an individual monkey wins an aggressive interaction, “the monkey’s self-confidence grows and it also wins other aggressive interactions. It’s a self-reinforcing effect.” Also, losing is so traumatic that it raises an animal’s levels of corticosteroids (stress hormones) and lowers its levels of testosterone; that makes for a wimpy monkey more likely to lose its next encounter.

So imagine what happens in troops with many more males than females. The males are always mixing it up, playing one-upmanship in the drive to be the alpha male. That provides many chances for males to lose and hence to feel bad about themselves and have a losing mix of testosterone and stress hormones. The females take advantage of this. “In groups with more males, males are more often defeated by other males,” says Hemelrijk. “Consequently, high-ranking females may be victorious over these losers. Furthermore, the presence of more males in the group leads to more interactions between males and females, causing more chance winnings by females. Through a self-reinforcing effect, these females will go on to win more frequently and grow more dominant.”

In other words, the large number of losing males in a group with a preponderance of males makes them more likely to lose a fight with another male, and therefore with a female; the female gains confidence (and higher testosterone levels), enabling her to go on to lord it over more males. As a result, say the scientists, “high ranking females may beat low ranking males and rank above them.” In contrast, in less-aggressive primate groups, such as the egalitarian societies of macaques, the presence of many more males than females does not lead to female dominance over males: the males don’t fight enough to produce enough losers for the females to lord it over.

The scientists were particularly struck by their finding that whether females dominate males has little to do with the difference in their sizes, or what’s called sexual dimorphism. That is “unexpected,” they say, because size seems to explain male dominance in species where males are way bigger than females, such as gorillas. But when it comes to whether females can be the top bananas, the relative sizes of males and females matters less than the percentage of each sex in the group.

Says Hemelrijk, “It would not surprise me if [similar mechanisms] play a role in the development of dominance between the sexes among human beings, too.” Keep it in mind next time you find yourself in a group where the sex ratio veers far from 50-50.