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. |