Male-killing bacteria trigger a cycle of increasing male fatigue and female promiscuity.

Sex-ratio distorters are found in numerous species and can reach high frequencies within populations. Here, we address the compelling, but poorly tested, hypothesis that the sex ratio bias caused by such elements profoundly alters their host's mating system. We compare aspects of female and male reproductive biology between island populations of the butterfly Hypolimnas bolina that show varying degrees of female bias, because of a male-killing Wolbachia infection. Contrary to expectation, female bias leads to an increase in female mating frequency, up to a point where male mating capacity becomes limiting. We show that increased female mating frequency can be explained as a facultative response to the depleted male mating resources in female biased populations. In other words, this system is one where male-killing bacteria trigger a vicious circle of increasing male fatigue and female promiscuity.