Death key to sex in butterflies

The Current Biology study looked at the Hypolimnas bolina species, common to the Pacific and SE Asia.

The team discovered as the bacteria caused male populations to fall, females mated more frequently to boost their chances of becoming impregnated.

The study has revealed the bacteria's powerful effect on mating systems.

The Wolbachia bacteria are passed from mother to son in some species of tropical butterfly, and kill the embryo before it hatches. The bacteria are so effective, some islands can be left with one male to every 100 females.

Theoretically, an excess of females should lead to an increase of mating opportunities for males and a decrease in the average number of matings per female, as males become increasingly rare.

"This just makes them more rampant!"
Dr Sylvain Charlat, UCL

Dr Sylvain Charlat, of the University College London, UK, who led the study said: "Contrary to expectation, we find that female promiscuity actually rises when male numbers are reduced.

"Greater numbers of female partners leads to fatigue in males. They start producing smaller sperm packages."

The females instinctively know their chances of impregnation are lower than usual.

"This just makes them more rampant!" Dr Charlat added.

This continues until the lack of male partners finally becomes limiting and results in rising female virginity rates within a population.

50/1 ratio

The team began its research after unearthing old papers from the 1920s on both the male-killing phenomenon and the gender-ratios of butterflies on the islands, but the work had never been followed up.

Dr Greg Hurst, a senior author on the study, said: "As the male butterflies got less common, females mated more, when it should be harder to find a male."

While most female butterflies are expected to mate once or twice in a lifetime, the team found females on the islands were mating three to five times.

Dr Hurst added: "The remarkable thing is the species can survive with a ratio of 50 females to every one male."

While the male-killing bacteria are found in lots of insects, including the British ladybird, scientists do not know what effect the microbes would have on sexual habits in these other species unless population numbers reached a frequency similar to those of H. bolina.