A bacterial infection that kills male great eggflys causes the female butterflies to become rampantly promiscuous, British researchers report.

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SYDNEY: A bacterial infection that kills male butterflies can make their female counterparts rampantly promiscuous, according to British researchers.

"We've shown that butterfly mating patterns are strongly determined by the killer bacteria," said lead author Sylvain Charlat, of the Department of Biology at University College London (UCL). "Contrary to expectation, we also find that female promiscuity actually rises when male numbers are reduced.

"Greater numbers of female partners leads to fatigue in males [and] they start producing smaller sperm packages. Unfortunately, the female butterflies instinctively know that the packages are smaller and that their chances of having been impregnated after mating are lower than usual. This just makes them more rampant!"

The male-killing bacteria of genus Wolbachia are transmitted from mother to son and actually kill the son before the embryo hatches into a caterpillar. Only female offspring of female carriers of the bacteria can survive, and widespread infection can lead to the male population being as low as one male to every hundred females in some areas.

According to the study, published in the journal Current Biology, male-killing bacteria are common in many insect populations. In some tropical butterflies, said the team, the entire mating system is determined by bacterial infection.

"It's amazing that the numbers of male butterflies can get so low and yet the population is still sustainable and stable," said co-author Greg Hurst, also of UCL. "You don't need many male butterflies to continue the population successfully. This is partly because the decision to mate is mainly under female control and because males have a high mating capacity."

The researchers assessed the sex ratio in 20 populations of the butterfly Hypolimnas bolina - the great eggfly - and combined this data with female mating frequency and the size of the male sperm package per copulation to find how female promiscuity was affected by the sex ratio. They found that the size of the sperm package was key to female promiscuity, but that promiscuity only rose up to the point where males became so rare that female virginity rates rose as well.

The male-killing bacterial infection that affects the great eggfly was first identified in 1920, but has not received much attention until now. According to the team, the new findings are significant for the scientific community because they demonstrate how a species' mating system can be determined by the abundance of a parasite.