This study was carried out on Hypolimnas bolina butterflies in Pacific Island and South-East Asian populations. The islands provide an ideal location because every island is differently affected by the male-killling bacteria so that each has a different ratio of males to females.
The researchers assessed the natural sex ratio in 20 populations and combined this data with female mating frequency and the size of the male sperm package (Spermatophore) per copulation to find how female promiscuity was affected by the sex ratio. They found that the size of the Spermatophore was key to female promiscuity. However, female promiscuity only rises up to the point where males become so rare that female virginity rates rise.
The male-killling phenomenon in this species was first identified in 1920 by Hubert Simmonds but has not received much attention until now. This finding is significant for the scientific community because it demonstrates how a species' mating system can be determined by the frequency of a parasite.