Bacteria Make Female Butterflies Promiscuous, Scientists Say
The common eggfly (Hypolimnas bolina) is often infected with a male-killing germ known as Wolbachia. Scientists now find male deaths can trigger a vicious circle of increasing female promiscuity and resulting male sexual exhaustion.
Credit: Sylvain Charlat

A germ that kills males triggers a vicious cycle of increasing female promiscuity and male sexual exhaustion in a species of butterfly, scientists report.

Male-killing bacteria known as Wolbachia are extremely widespread in insects, found in more than one-fifth of species. The germs can turn males to females and cause infected females to reproduce without males.

Scientists had assumed these bacteria would profoundly alter the natural mating patterns of their hosts, but only had scant evidence of what these changes would entail in the wild.

Evolutionary biologist Sylvain Charlat at University College London and his colleagues investigated the common eggfly Hypolimnas bolina. This butterfly is found in locations ranging from Madagascar to Asia, and from Australia and to Easter Island.

The bacteria infects Pacific Island and Southeast Asian populations of the butterfly [image], getting transmitted from mother to son and killing males before eggs hatch. Each island's butterflies are affected by Wolbachia differently, leading to different ratios of males to females. The male population can range as low as one male to every 100 females in some areas.

Over the course of three years, the scientists inspected the butterflies' sex ratio in 20 different locales, including Vietnam, Australia and 18 different islands, including Borneo, New Guinea, Vanuatu and Tahiti. They also investigated female mating frequency and the size of the male sperm package.

Some research sites were easily reachable by airplane, but the scientists relied on private sailboats to get to the more remote spots. While butterflies were common at some locations, they were rare at others, requiring days and days of hiking to find spots for collection. "People were generally very curious about what I was doing, and amused when they knew it was all about sex in butterflies," Charlat said

The researchers expected that the fewer male butterflies there were, the less sex females likely would have with males. Surprisingly, female promiscuity actually rose.

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

The actual mechanism behind how the females detect sperm package size remains a mystery so far.

The fact that the Wolbachia bacteria are widespread in insects could mean, Charlat speculated, that this phenomenon might also be widespread in insects in nature.

Charlat and his colleagues reported their findings in the Feb. 6 issue of the journal Current Biology.