Male-killer makes female butterflies promiscuous

17:50 05 February 2007
NewScientist.com news service
Roxanne Khamsi

Female butterflies become sexually promiscuous in the presence of a bacterium that fatally targets the male offspring of their species, a new study shows. However, the few males that survive become fatigued by the increased sexual demands of the females, and so release fewer sperm in each mating.

The unexpected findings could shed light on how the insect species can survive when there are only a few males available, the researchers say.

Greg Hurst at University College London in the UK, and colleagues, captured and studied Hypolimnas bolina butterflies from various islands in South Asia and the Pacific.

On some of the islands these butterflies suffer from a type of Wolbachia bacteria that specifically kills their male embryos. Other islands, meanwhile, remain free of the bacteria.

Sperm balls

About 25 female butterflies were captured from each of the various islands, which were dissected in search of empty “sperm packages”. Female butterflies receive a tiny ball of sperm each time they mate and store its empty casing inside them for the rest of their life.

By counting the empty sperm packages inside the insects, Hurst’s team knew how many males each female had mated with.

The researchers found that on the islands with abundant males, female butterflies mated only once between the time they hatched from their cocoon and died one month later. By comparison, the females living on islands infested by the killer microbes mated about five times before dying.

Lucky few

The fact that female butterflies could even find mates in the presence of male-killing Wolbachia stunned scientists. In some islands there was only one male for every 40 females. One would expect this type of sex imbalance to leave females deprived of a mate, explains Hurst.

“To our knowledge we’ve never heard of female promiscuity being caused by fewer males,” he says.

The finding is important, he adds, because it explains how these insect populations can survive even with hardly any males: “This means that the bacteria will not cause butterfly populations to crash – you can live with just a few males.”

But even though females on islands with male-killing bacteria mate often, the sperm packages they receive are smaller than usual. This is because the male butterflies on the islands become fatigued: “The size of the packages can go down by 50% in volume if the males are really tired,” says Hurst.

Journal reference: Current Biology (DOI: 10.1016/j.cub.2006.11.068)

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