Sex-role-reversed mating systems in which females compete for males and mal
es may be choosy are usually associated with males investing more than fema
les in offspring. We report that sex-role reversal may also be caused by se
lfish genetic elements which distort the sex ratio towards females. Some po
pulations of the butterflies Acraea encedon and Acraea encedana are extreme
ly female biased because over 90% of females are infected with a Wolbachia
bacterium that is maternally inherited and kills male embryos. Many females
in these populations are virgins suggesting that their reproductive succes
s may be limited by access to males. These females form lekking swarms at l
andmarks in which females exhibit behaviours which we interpret as function
ing to solicit matings from males. The hypothesis that female A. encedon sw
arm in order to mate is supported by the finding that, in release-recapture
experiments, mated females tend to leave the swarm while unmated females r
emained. This behaviour is a sex-role-reversed form of a common mating syst
em in insects in which males form lekking swarms at landmarks and compete f
or females. Female lekking swarms are absent from less female-biased popula
tions and here the butterflies are instead associated with resources in the
form of the larval food plant.