Tiny microbe turns tropical butterfly into male killer. (University of Exeter)

They are so beautiful, these man-eaters.

In Nairobi, the heart of Kenya, within what scientists call a hybrid zone for butterflies, two subspecies of the African queen have no males. That's because the females are infected with a bacterium known as Spiroplasma ixodeti that kills 100 percent of their male offspring. The eggs don't hatch.

What's worse, at least for the males, is that the female eggs hatch normally, and their doomed brothers are among their first meals. The Daryl Hall and John Oates lyrics say it best: "Oh-oh here she comes. Watch out, boy, she'll chew you up. ... She's a man-eater."

Don't blame the lovely cannibal butterflies, said Richard ffrench-Constant, a professor of molecular natural history at the University of Exeter in England and the author of a study released Tuesday. Blame nature. "The sisters eating their dead brothers is just a byproduct of the males dying in their eggs," he said very matter-of-factly. "Many caterpillars eat their own eggs after hatching, so it's probably just a by-product of that."

Spiroplasma ixodeti is one of many bacteria that commit manslaughter, killing males before their first breaths. It's so common in nature that scientists have considered putting the tiny microbes to good use, such as infecting female mosquitoes to control the Zika virus.

Although the microbe is fairly widespread, it had not been found to have an effect on the pretty yellow-winged African queen butterfly that exists throughout East Africa. The new research, published in the journal Proceedings of the Royal Society B, is the first to discover its lethal effects on males hatched by the two subspecies in Nairobi, an odd zone where every night is ladies' night. The researchers discovered that these butterflies have an odd chromosome that may make them susceptible to the microbe's man-killing ways.

With no males, one might naturally wonder, how do the African queens manage to procreate? The answer, the author said, is immigration. Males from elsewhere travel into the realm for sex in a world of females. But the joke's on them.

"Mating in the hybrid zone is presumably fun," ffrench-Constant said, "but his mating won't make any male heirs."

"The reason this system is so interesting is that it's a smoking gun for speciation," he explained. "We can see the subspecies splitting apart." The agent of this change is the baby-killer microbe. The subspecies live side-by-side but, without males, can never interbreed and so are becoming more and more genetically distinct.

"We tend to think of new species coming about due to environmental changes but here it's clearly the microbe that is driving these two subspecies apart," he said. "This means that no males are made in the hybrid zone, and that mating success in the zone is effectively zero, thereby creating a barrier with a new species on either side," ffrench-Constant said.

The study was led by David Smith, who worked at the Natural History Museum at Eton College, when the work was carried out. The research took 13 years of field work by teams of scientists from England, Germany and Nairobi. Ian Gordon, a researcher at the National Museums of Kenya in Nairobi, carefully recorded the sex and color pattern of butterflies around Nairobi.

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